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Review article, environmental and health impacts of air pollution: a review.
- 1 Delphis S.A., Kifisia, Greece
- 2 Laboratory of Hygiene and Environmental Protection, Faculty of Medicine, Democritus University of Thrace, Alexandroupolis, Greece
- 3 Centre Hospitalier Universitaire Vaudois (CHUV), Service de Médicine Interne, Lausanne, Switzerland
- 4 School of Social and Political Sciences, University of Glasgow, Glasgow, United Kingdom
One of our era's greatest scourges is air pollution, on account not only of its impact on climate change but also its impact on public and individual health due to increasing morbidity and mortality. There are many pollutants that are major factors in disease in humans. Among them, Particulate Matter (PM), particles of variable but very small diameter, penetrate the respiratory system via inhalation, causing respiratory and cardiovascular diseases, reproductive and central nervous system dysfunctions, and cancer. Despite the fact that ozone in the stratosphere plays a protective role against ultraviolet irradiation, it is harmful when in high concentration at ground level, also affecting the respiratory and cardiovascular system. Furthermore, nitrogen oxide, sulfur dioxide, Volatile Organic Compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) are all considered air pollutants that are harmful to humans. Carbon monoxide can even provoke direct poisoning when breathed in at high levels. Heavy metals such as lead, when absorbed into the human body, can lead to direct poisoning or chronic intoxication, depending on exposure. Diseases occurring from the aforementioned substances include principally respiratory problems such as Chronic Obstructive Pulmonary Disease (COPD), asthma, bronchiolitis, and also lung cancer, cardiovascular events, central nervous system dysfunctions, and cutaneous diseases. Last but not least, climate change resulting from environmental pollution affects the geographical distribution of many infectious diseases, as do natural disasters. The only way to tackle this problem is through public awareness coupled with a multidisciplinary approach by scientific experts; national and international organizations must address the emergence of this threat and propose sustainable solutions.
Approach to the Problem
The interactions between humans and their physical surroundings have been extensively studied, as multiple human activities influence the environment. The environment is a coupling of the biotic (living organisms and microorganisms) and the abiotic (hydrosphere, lithosphere, and atmosphere).
Pollution is defined as the introduction into the environment of substances harmful to humans and other living organisms. Pollutants are harmful solids, liquids, or gases produced in higher than usual concentrations that reduce the quality of our environment.
Human activities have an adverse effect on the environment by polluting the water we drink, the air we breathe, and the soil in which plants grow. Although the industrial revolution was a great success in terms of technology, society, and the provision of multiple services, it also introduced the production of huge quantities of pollutants emitted into the air that are harmful to human health. Without any doubt, the global environmental pollution is considered an international public health issue with multiple facets. Social, economic, and legislative concerns and lifestyle habits are related to this major problem. Clearly, urbanization and industrialization are reaching unprecedented and upsetting proportions worldwide in our era. Anthropogenic air pollution is one of the biggest public health hazards worldwide, given that it accounts for about 9 million deaths per year ( 1 ).
Without a doubt, all of the aforementioned are closely associated with climate change, and in the event of danger, the consequences can be severe for mankind ( 2 ). Climate changes and the effects of global planetary warming seriously affect multiple ecosystems, causing problems such as food safety issues, ice and iceberg melting, animal extinction, and damage to plants ( 3 , 4 ).
Air pollution has various health effects. The health of susceptible and sensitive individuals can be impacted even on low air pollution days. Short-term exposure to air pollutants is closely related to COPD (Chronic Obstructive Pulmonary Disease), cough, shortness of breath, wheezing, asthma, respiratory disease, and high rates of hospitalization (a measurement of morbidity).
The long-term effects associated with air pollution are chronic asthma, pulmonary insufficiency, cardiovascular diseases, and cardiovascular mortality. According to a Swedish cohort study, diabetes seems to be induced after long-term air pollution exposure ( 5 ). Moreover, air pollution seems to have various malign health effects in early human life, such as respiratory, cardiovascular, mental, and perinatal disorders ( 3 ), leading to infant mortality or chronic disease in adult age ( 6 ).
National reports have mentioned the increased risk of morbidity and mortality ( 1 ). These studies were conducted in many places around the world and show a correlation between daily ranges of particulate matter (PM) concentration and daily mortality. Climate shifts and global planetary warming ( 3 ) could aggravate the situation. Besides, increased hospitalization (an index of morbidity) has been registered among the elderly and susceptible individuals for specific reasons. Fine and ultrafine particulate matter seems to be associated with more serious illnesses ( 6 ), as it can invade the deepest parts of the airways and more easily reach the bloodstream.
Air pollution mainly affects those living in large urban areas, where road emissions contribute the most to the degradation of air quality. There is also a danger of industrial accidents, where the spread of a toxic fog can be fatal to the populations of the surrounding areas. The dispersion of pollutants is determined by many parameters, most notably atmospheric stability and wind ( 6 ).
In developing countries ( 7 ), the problem is more serious due to overpopulation and uncontrolled urbanization along with the development of industrialization. This leads to poor air quality, especially in countries with social disparities and a lack of information on sustainable management of the environment. The use of fuels such as wood fuel or solid fuel for domestic needs due to low incomes exposes people to bad-quality, polluted air at home. It is of note that three billion people around the world are using the above sources of energy for their daily heating and cooking needs ( 8 ). In developing countries, the women of the household seem to carry the highest risk for disease development due to their longer duration exposure to the indoor air pollution ( 8 , 9 ). Due to its fast industrial development and overpopulation, China is one of the Asian countries confronting serious air pollution problems ( 10 , 11 ). The lung cancer mortality observed in China is associated with fine particles ( 12 ). As stated already, long-term exposure is associated with deleterious effects on the cardiovascular system ( 3 , 5 ). However, it is interesting to note that cardiovascular diseases have mostly been observed in developed and high-income countries rather than in the developing low-income countries exposed highly to air pollution ( 13 ). Extreme air pollution is recorded in India, where the air quality reaches hazardous levels. New Delhi is one of the more polluted cities in India. Flights in and out of New Delhi International Airport are often canceled due to the reduced visibility associated with air pollution. Pollution is occurring both in urban and rural areas in India due to the fast industrialization, urbanization, and rise in use of motorcycle transportation. Nevertheless, biomass combustion associated with heating and cooking needs and practices is a major source of household air pollution in India and in Nepal ( 14 , 15 ). There is spatial heterogeneity in India, as areas with diverse climatological conditions and population and education levels generate different indoor air qualities, with higher PM 2.5 observed in North Indian states (557–601 μg/m 3 ) compared to the Southern States (183–214 μg/m 3 ) ( 16 , 17 ). The cold climate of the North Indian areas may be the main reason for this, as longer periods at home and more heating are necessary compared to in the tropical climate of Southern India. Household air pollution in India is associated with major health effects, especially in women and young children, who stay indoors for longer periods. Chronic obstructive respiratory disease (CORD) and lung cancer are mostly observed in women, while acute lower respiratory disease is seen in young children under 5 years of age ( 18 ).
Accumulation of air pollution, especially sulfur dioxide and smoke, reaching 1,500 mg/m3, resulted in an increase in the number of deaths (4,000 deaths) in December 1952 in London and in 1963 in New York City (400 deaths) ( 19 ). An association of pollution with mortality was reported on the basis of monitoring of outdoor pollution in six US metropolitan cities ( 20 ). In every case, it seems that mortality was closely related to the levels of fine, inhalable, and sulfate particles more than with the levels of total particulate pollution, aerosol acidity, sulfur dioxide, or nitrogen dioxide ( 20 ).
Furthermore, extremely high levels of pollution are reported in Mexico City and Rio de Janeiro, followed by Milan, Ankara, Melbourne, Tokyo, and Moscow ( 19 ).
Based on the magnitude of the public health impact, it is certain that different kinds of interventions should be taken into account. Success and effectiveness in controlling air pollution, specifically at the local level, have been reported. Adequate technological means are applied considering the source and the nature of the emission as well as its impact on health and the environment. The importance of point sources and non-point sources of air pollution control is reported by Schwela and Köth-Jahr ( 21 ). Without a doubt, a detailed emission inventory must record all sources in a given area. Beyond considering the above sources and their nature, topography and meteorology should also be considered, as stated previously. Assessment of the control policies and methods is often extrapolated from the local to the regional and then to the global scale. Air pollution may be dispersed and transported from one region to another area located far away. Air pollution management means the reduction to acceptable levels or possible elimination of air pollutants whose presence in the air affects our health or the environmental ecosystem. Private and governmental entities and authorities implement actions to ensure the air quality ( 22 ). Air quality standards and guidelines were adopted for the different pollutants by the WHO and EPA as a tool for the management of air quality ( 1 , 23 ). These standards have to be compared to the emissions inventory standards by causal analysis and dispersion modeling in order to reveal the problematic areas ( 24 ). Inventories are generally based on a combination of direct measurements and emissions modeling ( 24 ).
As an example, we state here the control measures at the source through the use of catalytic converters in cars. These are devices that turn the pollutants and toxic gases produced from combustion engines into less-toxic pollutants by catalysis through redox reactions ( 25 ). In Greece, the use of private cars was restricted by tracking their license plates in order to reduce traffic congestion during rush hour ( 25 ).
Concerning industrial emissions, collectors and closed systems can keep the air pollution to the minimal standards imposed by legislation ( 26 ).
Current strategies to improve air quality require an estimation of the economic value of the benefits gained from proposed programs. These proposed programs by public authorities, and directives are issued with guidelines to be respected.
In Europe, air quality limit values AQLVs (Air Quality Limit Values) are issued for setting off planning claims ( 27 ). In the USA, the NAAQS (National Ambient Air Quality Standards) establish the national air quality limit values ( 27 ). While both standards and directives are based on different mechanisms, significant success has been achieved in the reduction of overall emissions and associated health and environmental effects ( 27 ). The European Directive identifies geographical areas of risk exposure as monitoring/assessment zones to record the emission sources and levels of air pollution ( 27 ), whereas the USA establishes global geographical air quality criteria according to the severity of their air quality problem and records all sources of the pollutants and their precursors ( 27 ).
In this vein, funds have been financing, directly or indirectly, projects related to air quality along with the technical infrastructure to maintain good air quality. These plans focus on an inventory of databases from air quality environmental planning awareness campaigns. Moreover, pollution measures of air emissions may be taken for vehicles, machines, and industries in urban areas.
Technological innovation can only be successful if it is able to meet the needs of society. In this sense, technology must reflect the decision-making practices and procedures of those involved in risk assessment and evaluation and act as a facilitator in providing information and assessments to enable decision makers to make the best decisions possible. Summarizing the aforementioned in order to design an effective air quality control strategy, several aspects must be considered: environmental factors and ambient air quality conditions, engineering factors and air pollutant characteristics, and finally, economic operating costs for technological improvement and administrative and legal costs. Considering the economic factor, competitiveness through neoliberal concepts is offering a solution to environmental problems ( 22 ).
The development of environmental governance, along with technological progress, has initiated the deployment of a dialogue. Environmental politics has created objections and points of opposition between different political parties, scientists, media, and governmental and non-governmental organizations ( 22 ). Radical environmental activism actions and movements have been created ( 22 ). The rise of the new information and communication technologies (ICTs) are many times examined as to whether and in which way they have influenced means of communication and social movements such as activism ( 28 ). Since the 1990s, the term “digital activism” has been used increasingly and in many different disciplines ( 29 ). Nowadays, multiple digital technologies can be used to produce a digital activism outcome on environmental issues. More specifically, devices with online capabilities such as computers or mobile phones are being used as a way to pursue change in political and social affairs ( 30 ).
In the present paper, we focus on the sources of environmental pollution in relation to public health and propose some solutions and interventions that may be of interest to environmental legislators and decision makers.
Sources of Exposure
It is known that the majority of environmental pollutants are emitted through large-scale human activities such as the use of industrial machinery, power-producing stations, combustion engines, and cars. Because these activities are performed at such a large scale, they are by far the major contributors to air pollution, with cars estimated to be responsible for approximately 80% of today's pollution ( 31 ). Some other human activities are also influencing our environment to a lesser extent, such as field cultivation techniques, gas stations, fuel tanks heaters, and cleaning procedures ( 32 ), as well as several natural sources, such as volcanic and soil eruptions and forest fires.
The classification of air pollutants is based mainly on the sources producing pollution. Therefore, it is worth mentioning the four main sources, following the classification system: Major sources, Area sources, Mobile sources, and Natural sources.
Major sources include the emission of pollutants from power stations, refineries, and petrochemicals, the chemical and fertilizer industries, metallurgical and other industrial plants, and, finally, municipal incineration.
Indoor area sources include domestic cleaning activities, dry cleaners, printing shops, and petrol stations.
Mobile sources include automobiles, cars, railways, airways, and other types of vehicles.
Finally, natural sources include, as stated previously, physical disasters ( 33 ) such as forest fire, volcanic erosion, dust storms, and agricultural burning.
However, many classification systems have been proposed. Another type of classification is a grouping according to the recipient of the pollution, as follows:
Air pollution is determined as the presence of pollutants in the air in large quantities for long periods. Air pollutants are dispersed particles, hydrocarbons, CO, CO 2 , NO, NO 2 , SO 3 , etc.
Water pollution is organic and inorganic charge and biological charge ( 10 ) at high levels that affect the water quality ( 34 , 35 ).
Soil pollution occurs through the release of chemicals or the disposal of wastes, such as heavy metals, hydrocarbons, and pesticides.
Air pollution can influence the quality of soil and water bodies by polluting precipitation, falling into water and soil environments ( 34 , 36 ). Notably, the chemistry of the soil can be amended due to acid precipitation by affecting plants, cultures, and water quality ( 37 ). Moreover, movement of heavy metals is favored by soil acidity, and metals are so then moving into the watery environment. It is known that heavy metals such as aluminum are noxious to wildlife and fishes. Soil quality seems to be of importance, as soils with low calcium carbonate levels are at increased jeopardy from acid rain. Over and above rain, snow and particulate matter drip into watery ' bodies ( 36 , 38 ).
Lastly, pollution is classified following type of origin:
Radioactive and nuclear pollution , releasing radioactive and nuclear pollutants into water, air, and soil during nuclear explosions and accidents, from nuclear weapons, and through handling or disposal of radioactive sewage.
Radioactive materials can contaminate surface water bodies and, being noxious to the environment, plants, animals, and humans. It is known that several radioactive substances such as radium and uranium concentrate in the bones and can cause cancers ( 38 , 39 ).
Noise pollution is produced by machines, vehicles, traffic noises, and musical installations that are harmful to our hearing.
The World Health Organization introduced the term DALYs. The DALYs for a disease or health condition is defined as the sum of the Years of Life Lost (YLL) due to premature mortality in the population and the Years Lost due to Disability (YLD) for people living with the health condition or its consequences ( 39 ). In Europe, air pollution is the main cause of disability-adjusted life years lost (DALYs), followed by noise pollution. The potential relationships of noise and air pollution with health have been studied ( 40 ). The study found that DALYs related to noise were more important than those related to air pollution, as the effects of environmental noise on cardiovascular disease were independent of air pollution ( 40 ). Environmental noise should be counted as an independent public health risk ( 40 ).
Environmental pollution occurs when changes in the physical, chemical, or biological constituents of the environment (air masses, temperature, climate, etc.) are produced.
Pollutants harm our environment either by increasing levels above normal or by introducing harmful toxic substances. Primary pollutants are directly produced from the above sources, and secondary pollutants are emitted as by-products of the primary ones. Pollutants can be biodegradable or non-biodegradable and of natural origin or anthropogenic, as stated previously. Moreover, their origin can be a unique source (point-source) or dispersed sources.
Pollutants have differences in physical and chemical properties, explaining the discrepancy in their capacity for producing toxic effects. As an example, we state here that aerosol compounds ( 41 – 43 ) have a greater toxicity than gaseous compounds due to their tiny size (solid or liquid) in the atmosphere; they have a greater penetration capacity. Gaseous compounds are eliminated more easily by our respiratory system ( 41 ). These particles are able to damage lungs and can even enter the bloodstream ( 41 ), leading to the premature deaths of millions of people yearly. Moreover, the aerosol acidity ([H+]) seems to considerably enhance the production of secondary organic aerosols (SOA), but this last aspect is not supported by other scientific teams ( 38 ).
Climate and Pollution
Air pollution and climate change are closely related. Climate is the other side of the same coin that reduces the quality of our Earth ( 44 ). Pollutants such as black carbon, methane, tropospheric ozone, and aerosols affect the amount of incoming sunlight. As a result, the temperature of the Earth is increasing, resulting in the melting of ice, icebergs, and glaciers.
In this vein, climatic changes will affect the incidence and prevalence of both residual and imported infections in Europe. Climate and weather affect the duration, timing, and intensity of outbreaks strongly and change the map of infectious diseases in the globe ( 45 ). Mosquito-transmitted parasitic or viral diseases are extremely climate-sensitive, as warming firstly shortens the pathogen incubation period and secondly shifts the geographic map of the vector. Similarly, water-warming following climate changes leads to a high incidence of waterborne infections. Recently, in Europe, eradicated diseases seem to be emerging due to the migration of population, for example, cholera, poliomyelitis, tick-borne encephalitis, and malaria ( 46 ).
The spread of epidemics is associated with natural climate disasters and storms, which seem to occur more frequently nowadays ( 47 ). Malnutrition and disequilibration of the immune system are also associated with the emerging infections affecting public health ( 48 ).
The Chikungunya virus “took the airplane” from the Indian Ocean to Europe, as outbreaks of the disease were registered in Italy ( 49 ) as well as autochthonous cases in France ( 50 ).
An increase in cryptosporidiosis in the United Kingdom and in the Czech Republic seems to have occurred following flooding ( 36 , 51 ).
As stated previously, aerosols compounds are tiny in size and considerably affect the climate. They are able to dissipate sunlight (the albedo phenomenon) by dispersing a quarter of the sun's rays back to space and have cooled the global temperature over the last 30 years ( 52 ).
Air Pollutants
The World Health Organization (WHO) reports on six major air pollutants, namely particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Air pollution can have a disastrous effect on all components of the environment, including groundwater, soil, and air. Additionally, it poses a serious threat to living organisms. In this vein, our interest is mainly to focus on these pollutants, as they are related to more extensive and severe problems in human health and environmental impact. Acid rain, global warming, the greenhouse effect, and climate changes have an important ecological impact on air pollution ( 53 ).
Particulate Matter (PM) and Health
Studies have shown a relationship between particulate matter (PM) and adverse health effects, focusing on either short-term (acute) or long-term (chronic) PM exposure.
Particulate matter (PM) is usually formed in the atmosphere as a result of chemical reactions between the different pollutants. The penetration of particles is closely dependent on their size ( 53 ). Particulate Matter (PM) was defined as a term for particles by the United States Environmental Protection Agency ( 54 ). Particulate matter (PM) pollution includes particles with diameters of 10 micrometers (μm) or smaller, called PM 10 , and extremely fine particles with diameters that are generally 2.5 micrometers (μm) and smaller.
Particulate matter contains tiny liquid or solid droplets that can be inhaled and cause serious health effects ( 55 ). Particles <10 μm in diameter (PM 10 ) after inhalation can invade the lungs and even reach the bloodstream. Fine particles, PM 2.5 , pose a greater risk to health ( 6 , 56 ) ( Table 1 ).
Table 1 . Penetrability according to particle size.
Multiple epidemiological studies have been performed on the health effects of PM. A positive relation was shown between both short-term and long-term exposures of PM 2.5 and acute nasopharyngitis ( 56 ). In addition, long-term exposure to PM for years was found to be related to cardiovascular diseases and infant mortality.
Those studies depend on PM 2.5 monitors and are restricted in terms of study area or city area due to a lack of spatially resolved daily PM 2.5 concentration data and, in this way, are not representative of the entire population. Following a recent epidemiological study by the Department of Environmental Health at Harvard School of Public Health (Boston, MA) ( 57 ), it was reported that, as PM 2.5 concentrations vary spatially, an exposure error (Berkson error) seems to be produced, and the relative magnitudes of the short- and long-term effects are not yet completely elucidated. The team developed a PM 2.5 exposure model based on remote sensing data for assessing short- and long-term human exposures ( 57 ). This model permits spatial resolution in short-term effects plus the assessment of long-term effects in the whole population.
Moreover, respiratory diseases and affection of the immune system are registered as long-term chronic effects ( 58 ). It is worth noting that people with asthma, pneumonia, diabetes, and respiratory and cardiovascular diseases are especially susceptible and vulnerable to the effects of PM. PM 2.5 , followed by PM 10 , are strongly associated with diverse respiratory system diseases ( 59 ), as their size permits them to pierce interior spaces ( 60 ). The particles produce toxic effects according to their chemical and physical properties. The components of PM 10 and PM 2.5 can be organic (polycyclic aromatic hydrocarbons, dioxins, benzene, 1-3 butadiene) or inorganic (carbon, chlorides, nitrates, sulfates, metals) in nature ( 55 ).
Particulate Matter (PM) is divided into four main categories according to type and size ( 61 ) ( Table 2 ).
Table 2 . Types and sizes of particulate Matter (PM).
Gas contaminants include PM in aerial masses.
Particulate contaminants include contaminants such as smog, soot, tobacco smoke, oil smoke, fly ash, and cement dust.
Biological Contaminants are microorganisms (bacteria, viruses, fungi, mold, and bacterial spores), cat allergens, house dust and allergens, and pollen.
Types of Dust include suspended atmospheric dust, settling dust, and heavy dust.
Finally, another fact is that the half-lives of PM 10 and PM 2.5 particles in the atmosphere is extended due to their tiny dimensions; this permits their long-lasting suspension in the atmosphere and even their transfer and spread to distant destinations where people and the environment may be exposed to the same magnitude of pollution ( 53 ). They are able to change the nutrient balance in watery ecosystems, damage forests and crops, and acidify water bodies.
As stated, PM 2.5 , due to their tiny size, are causing more serious health effects. These aforementioned fine particles are the main cause of the “haze” formation in different metropolitan areas ( 12 , 13 , 61 ).
Ozone Impact in the Atmosphere
Ozone (O 3 ) is a gas formed from oxygen under high voltage electric discharge ( 62 ). It is a strong oxidant, 52% stronger than chlorine. It arises in the stratosphere, but it could also arise following chain reactions of photochemical smog in the troposphere ( 63 ).
Ozone can travel to distant areas from its initial source, moving with air masses ( 64 ). It is surprising that ozone levels over cities are low in contrast to the increased amounts occuring in urban areas, which could become harmful for cultures, forests, and vegetation ( 65 ) as it is reducing carbon assimilation ( 66 ). Ozone reduces growth and yield ( 47 , 48 ) and affects the plant microflora due to its antimicrobial capacity ( 67 , 68 ). In this regard, ozone acts upon other natural ecosystems, with microflora ( 69 , 70 ) and animal species changing their species composition ( 71 ). Ozone increases DNA damage in epidermal keratinocytes and leads to impaired cellular function ( 72 ).
Ground-level ozone (GLO) is generated through a chemical reaction between oxides of nitrogen and VOCs emitted from natural sources and/or following anthropogenic activities.
Ozone uptake usually occurs by inhalation. Ozone affects the upper layers of the skin and the tear ducts ( 73 ). A study of short-term exposure of mice to high levels of ozone showed malondialdehyde formation in the upper skin (epidermis) but also depletion in vitamins C and E. It is likely that ozone levels are not interfering with the skin barrier function and integrity to predispose to skin disease ( 74 ).
Due to the low water-solubility of ozone, inhaled ozone has the capacity to penetrate deeply into the lungs ( 75 ).
Toxic effects induced by ozone are registered in urban areas all over the world, causing biochemical, morphologic, functional, and immunological disorders ( 76 ).
The European project (APHEA2) focuses on the acute effects of ambient ozone concentrations on mortality ( 77 ). Daily ozone concentrations compared to the daily number of deaths were reported from different European cities for a 3-year period. During the warm period of the year, an observed increase in ozone concentration was associated with an increase in the daily number of deaths (0.33%), in the number of respiratory deaths (1.13%), and in the number of cardiovascular deaths (0.45%). No effect was observed during wintertime.
Carbon Monoxide (CO)
Carbon monoxide is produced by fossil fuel when combustion is incomplete. The symptoms of poisoning due to inhaling carbon monoxide include headache, dizziness, weakness, nausea, vomiting, and, finally, loss of consciousness.
The affinity of carbon monoxide to hemoglobin is much greater than that of oxygen. In this vein, serious poisoning may occur in people exposed to high levels of carbon monoxide for a long period of time. Due to the loss of oxygen as a result of the competitive binding of carbon monoxide, hypoxia, ischemia, and cardiovascular disease are observed.
Carbon monoxide affects the greenhouses gases that are tightly connected to global warming and climate. This should lead to an increase in soil and water temperatures, and extreme weather conditions or storms may occur ( 68 ).
However, in laboratory and field experiments, it has been seen to produce increased plant growth ( 78 ).
Nitrogen Oxide (NO 2 )
Nitrogen oxide is a traffic-related pollutant, as it is emitted from automobile motor engines ( 79 , 80 ). It is an irritant of the respiratory system as it penetrates deep in the lung, inducing respiratory diseases, coughing, wheezing, dyspnea, bronchospasm, and even pulmonary edema when inhaled at high levels. It seems that concentrations over 0.2 ppm produce these adverse effects in humans, while concentrations higher than 2.0 ppm affect T-lymphocytes, particularly the CD8+ cells and NK cells that produce our immune response ( 81 ).It is reported that long-term exposure to high levels of nitrogen dioxide can be responsible for chronic lung disease. Long-term exposure to NO 2 can impair the sense of smell ( 81 ).
However, systems other than respiratory ones can be involved, as symptoms such as eye, throat, and nose irritation have been registered ( 81 ).
High levels of nitrogen dioxide are deleterious to crops and vegetation, as they have been observed to reduce crop yield and plant growth efficiency. Moreover, NO 2 can reduce visibility and discolor fabrics ( 81 ).
Sulfur Dioxide (SO 2 )
Sulfur dioxide is a harmful gas that is emitted mainly from fossil fuel consumption or industrial activities. The annual standard for SO 2 is 0.03 ppm ( 82 ). It affects human, animal, and plant life. Susceptible people as those with lung disease, old people, and children, who present a higher risk of damage. The major health problems associated with sulfur dioxide emissions in industrialized areas are respiratory irritation, bronchitis, mucus production, and bronchospasm, as it is a sensory irritant and penetrates deep into the lung converted into bisulfite and interacting with sensory receptors, causing bronchoconstriction. Moreover, skin redness, damage to the eyes (lacrimation and corneal opacity) and mucous membranes, and worsening of pre-existing cardiovascular disease have been observed ( 81 ).
Environmental adverse effects, such as acidification of soil and acid rain, seem to be associated with sulfur dioxide emissions ( 83 ).
Lead is a heavy metal used in different industrial plants and emitted from some petrol motor engines, batteries, radiators, waste incinerators, and waste waters ( 84 ).
Moreover, major sources of lead pollution in the air are metals, ore, and piston-engine aircraft. Lead poisoning is a threat to public health due to its deleterious effects upon humans, animals, and the environment, especially in the developing countries.
Exposure to lead can occur through inhalation, ingestion, and dermal absorption. Trans- placental transport of lead was also reported, as lead passes through the placenta unencumbered ( 85 ). The younger the fetus is, the more harmful the toxic effects. Lead toxicity affects the fetal nervous system; edema or swelling of the brain is observed ( 86 ). Lead, when inhaled, accumulates in the blood, soft tissue, liver, lung, bones, and cardiovascular, nervous, and reproductive systems. Moreover, loss of concentration and memory, as well as muscle and joint pain, were observed in adults ( 85 , 86 ).
Children and newborns ( 87 ) are extremely susceptible even to minimal doses of lead, as it is a neurotoxicant and causes learning disabilities, impairment of memory, hyperactivity, and even mental retardation.
Elevated amounts of lead in the environment are harmful to plants and crop growth. Neurological effects are observed in vertebrates and animals in association with high lead levels ( 88 ).
Polycyclic Aromatic Hydrocarbons(PAHs)
The distribution of PAHs is ubiquitous in the environment, as the atmosphere is the most important means of their dispersal. They are found in coal and in tar sediments. Moreover, they are generated through incomplete combustion of organic matter as in the cases of forest fires, incineration, and engines ( 89 ). PAH compounds, such as benzopyrene, acenaphthylene, anthracene, and fluoranthene are recognized as toxic, mutagenic, and carcinogenic substances. They are an important risk factor for lung cancer ( 89 ).
Volatile Organic Compounds(VOCs)
Volatile organic compounds (VOCs), such as toluene, benzene, ethylbenzene, and xylene ( 90 ), have been found to be associated with cancer in humans ( 91 ). The use of new products and materials has actually resulted in increased concentrations of VOCs. VOCs pollute indoor air ( 90 ) and may have adverse effects on human health ( 91 ). Short-term and long-term adverse effects on human health are observed. VOCs are responsible for indoor air smells. Short-term exposure is found to cause irritation of eyes, nose, throat, and mucosal membranes, while those of long duration exposure include toxic reactions ( 92 ). Predictable assessment of the toxic effects of complex VOC mixtures is difficult to estimate, as these pollutants can have synergic, antagonistic, or indifferent effects ( 91 , 93 ).
Dioxins originate from industrial processes but also come from natural processes, such as forest fires and volcanic eruptions. They accumulate in foods such as meat and dairy products, fish and shellfish, and especially in the fatty tissue of animals ( 94 ).
Short-period exhibition to high dioxin concentrations may result in dark spots and lesions on the skin ( 94 ). Long-term exposure to dioxins can cause developmental problems, impairment of the immune, endocrine and nervous systems, reproductive infertility, and cancer ( 94 ).
Without any doubt, fossil fuel consumption is responsible for a sizeable part of air contamination. This contamination may be anthropogenic, as in agricultural and industrial processes or transportation, while contamination from natural sources is also possible. Interestingly, it is of note that the air quality standards established through the European Air Quality Directive are somewhat looser than the WHO guidelines, which are stricter ( 95 ).
Effect of Air Pollution on Health
The most common air pollutants are ground-level ozone and Particulates Matter (PM). Air pollution is distinguished into two main types:
Outdoor pollution is the ambient air pollution.
Indoor pollution is the pollution generated by household combustion of fuels.
People exposed to high concentrations of air pollutants experience disease symptoms and states of greater and lesser seriousness. These effects are grouped into short- and long-term effects affecting health.
Susceptible populations that need to be aware of health protection measures include old people, children, and people with diabetes and predisposing heart or lung disease, especially asthma.
As extensively stated previously, according to a recent epidemiological study from Harvard School of Public Health, the relative magnitudes of the short- and long-term effects have not been completely clarified ( 57 ) due to the different epidemiological methodologies and to the exposure errors. New models are proposed for assessing short- and long-term human exposure data more successfully ( 57 ). Thus, in the present section, we report the more common short- and long-term health effects but also general concerns for both types of effects, as these effects are often dependent on environmental conditions, dose, and individual susceptibility.
Short-term effects are temporary and range from simple discomfort, such as irritation of the eyes, nose, skin, throat, wheezing, coughing and chest tightness, and breathing difficulties, to more serious states, such as asthma, pneumonia, bronchitis, and lung and heart problems. Short-term exposure to air pollution can also cause headaches, nausea, and dizziness.
These problems can be aggravated by extended long-term exposure to the pollutants, which is harmful to the neurological, reproductive, and respiratory systems and causes cancer and even, rarely, deaths.
The long-term effects are chronic, lasting for years or the whole life and can even lead to death. Furthermore, the toxicity of several air pollutants may also induce a variety of cancers in the long term ( 96 ).
As stated already, respiratory disorders are closely associated with the inhalation of air pollutants. These pollutants will invade through the airways and will accumulate at the cells. Damage to target cells should be related to the pollutant component involved and its source and dose. Health effects are also closely dependent on country, area, season, and time. An extended exposure duration to the pollutant should incline to long-term health effects in relation also to the above factors.
Particulate Matter (PMs), dust, benzene, and O 3 cause serious damage to the respiratory system ( 97 ). Moreover, there is a supplementary risk in case of existing respiratory disease such as asthma ( 98 ). Long-term effects are more frequent in people with a predisposing disease state. When the trachea is contaminated by pollutants, voice alterations may be remarked after acute exposure. Chronic obstructive pulmonary disease (COPD) may be induced following air pollution, increasing morbidity and mortality ( 99 ). Long-term effects from traffic, industrial air pollution, and combustion of fuels are the major factors for COPD risk ( 99 ).
Multiple cardiovascular effects have been observed after exposure to air pollutants ( 100 ). Changes occurred in blood cells after long-term exposure may affect cardiac functionality. Coronary arteriosclerosis was reported following long-term exposure to traffic emissions ( 101 ), while short-term exposure is related to hypertension, stroke, myocardial infracts, and heart insufficiency. Ventricle hypertrophy is reported to occur in humans after long-time exposure to nitrogen oxide (NO 2 ) ( 102 , 103 ).
Neurological effects have been observed in adults and children after extended-term exposure to air pollutants.
Psychological complications, autism, retinopathy, fetal growth, and low birth weight seem to be related to long-term air pollution ( 83 ). The etiologic agent of the neurodegenerative diseases (Alzheimer's and Parkinson's) is not yet known, although it is believed that extended exposure to air pollution seems to be a factor. Specifically, pesticides and metals are cited as etiological factors, together with diet. The mechanisms in the development of neurodegenerative disease include oxidative stress, protein aggregation, inflammation, and mitochondrial impairment in neurons ( 104 ) ( Figure 1 ).
Figure 1 . Impact of air pollutants on the brain.
Brain inflammation was observed in dogs living in a highly polluted area in Mexico for a long period ( 105 ). In human adults, markers of systemic inflammation (IL-6 and fibrinogen) were found to be increased as an immediate response to PNC on the IL-6 level, possibly leading to the production of acute-phase proteins ( 106 ). The progression of atherosclerosis and oxidative stress seem to be the mechanisms involved in the neurological disturbances caused by long-term air pollution. Inflammation comes secondary to the oxidative stress and seems to be involved in the impairment of developmental maturation, affecting multiple organs ( 105 , 107 ). Similarly, other factors seem to be involved in the developmental maturation, which define the vulnerability to long-term air pollution. These include birthweight, maternal smoking, genetic background and socioeconomic environment, as well as education level.
However, diet, starting from breast-feeding, is another determinant factor. Diet is the main source of antioxidants, which play a key role in our protection against air pollutants ( 108 ). Antioxidants are free radical scavengers and limit the interaction of free radicals in the brain ( 108 ). Similarly, genetic background may result in a differential susceptibility toward the oxidative stress pathway ( 60 ). For example, antioxidant supplementation with vitamins C and E appears to modulate the effect of ozone in asthmatic children homozygous for the GSTM1 null allele ( 61 ). Inflammatory cytokines released in the periphery (e.g., respiratory epithelia) upregulate the innate immune Toll-like receptor 2. Such activation and the subsequent events leading to neurodegeneration have recently been observed in lung lavage in mice exposed to ambient Los Angeles (CA, USA) particulate matter ( 61 ). In children, neurodevelopmental morbidities were observed after lead exposure. These children developed aggressive and delinquent behavior, reduced intelligence, learning difficulties, and hyperactivity ( 109 ). No level of lead exposure seems to be “safe,” and the scientific community has asked the Centers for Disease Control and Prevention (CDC) to reduce the current screening guideline of 10 μg/dl ( 109 ).
It is important to state that impact on the immune system, causing dysfunction and neuroinflammation ( 104 ), is related to poor air quality. Yet, increases in serum levels of immunoglobulins (IgA, IgM) and the complement component C3 are observed ( 106 ). Another issue is that antigen presentation is affected by air pollutants, as there is an upregulation of costimulatory molecules such as CD80 and CD86 on macrophages ( 110 ).
As is known, skin is our shield against ultraviolet radiation (UVR) and other pollutants, as it is the most exterior layer of our body. Traffic-related pollutants, such as PAHs, VOCs, oxides, and PM, may cause pigmented spots on our skin ( 111 ). On the one hand, as already stated, when pollutants penetrate through the skin or are inhaled, damage to the organs is observed, as some of these pollutants are mutagenic and carcinogenic, and, specifically, they affect the liver and lung. On the other hand, air pollutants (and those in the troposphere) reduce the adverse effects of ultraviolet radiation UVR in polluted urban areas ( 111 ). Air pollutants absorbed by the human skin may contribute to skin aging, psoriasis, acne, urticaria, eczema, and atopic dermatitis ( 111 ), usually caused by exposure to oxides and photochemical smoke ( 111 ). Exposure to PM and cigarette smoking act as skin-aging agents, causing spots, dyschromia, and wrinkles. Lastly, pollutants have been associated with skin cancer ( 111 ).
Higher morbidity is reported to fetuses and children when exposed to the above dangers. Impairment in fetal growth, low birth weight, and autism have been reported ( 112 ).
Another exterior organ that may be affected is the eye. Contamination usually comes from suspended pollutants and may result in asymptomatic eye outcomes, irritation ( 112 ), retinopathy, or dry eye syndrome ( 113 , 114 ).
Environmental Impact of Air Pollution
Air pollution is harming not only human health but also the environment ( 115 ) in which we live. The most important environmental effects are as follows.
Acid rain is wet (rain, fog, snow) or dry (particulates and gas) precipitation containing toxic amounts of nitric and sulfuric acids. They are able to acidify the water and soil environments, damage trees and plantations, and even damage buildings and outdoor sculptures, constructions, and statues.
Haze is produced when fine particles are dispersed in the air and reduce the transparency of the atmosphere. It is caused by gas emissions in the air coming from industrial facilities, power plants, automobiles, and trucks.
Ozone , as discussed previously, occurs both at ground level and in the upper level (stratosphere) of the Earth's atmosphere. Stratospheric ozone is protecting us from the Sun's harmful ultraviolet (UV) rays. In contrast, ground-level ozone is harmful to human health and is a pollutant. Unfortunately, stratospheric ozone is gradually damaged by ozone-depleting substances (i.e., chemicals, pesticides, and aerosols). If this protecting stratospheric ozone layer is thinned, then UV radiation can reach our Earth, with harmful effects for human life (skin cancer) ( 116 ) and crops ( 117 ). In plants, ozone penetrates through the stomata, inducing them to close, which blocks CO 2 transfer and induces a reduction in photosynthesis ( 118 ).
Global climate change is an important issue that concerns mankind. As is known, the “greenhouse effect” keeps the Earth's temperature stable. Unhappily, anthropogenic activities have destroyed this protecting temperature effect by producing large amounts of greenhouse gases, and global warming is mounting, with harmful effects on human health, animals, forests, wildlife, agriculture, and the water environment. A report states that global warming is adding to the health risks of poor people ( 119 ).
People living in poorly constructed buildings in warm-climate countries are at high risk for heat-related health problems as temperatures mount ( 119 ).
Wildlife is burdened by toxic pollutants coming from the air, soil, or the water ecosystem and, in this way, animals can develop health problems when exposed to high levels of pollutants. Reproductive failure and birth effects have been reported.
Eutrophication is occurring when elevated concentrations of nutrients (especially nitrogen) stimulate the blooming of aquatic algae, which can cause a disequilibration in the diversity of fish and their deaths.
Without a doubt, there is a critical concentration of pollution that an ecosystem can tolerate without being destroyed, which is associated with the ecosystem's capacity to neutralize acidity. The Canada Acid Rain Program established this load at 20 kg/ha/yr ( 120 ).
Hence, air pollution has deleterious effects on both soil and water ( 121 ). Concerning PM as an air pollutant, its impact on crop yield and food productivity has been reported. Its impact on watery bodies is associated with the survival of living organisms and fishes and their productivity potential ( 121 ).
An impairment in photosynthetic rhythm and metabolism is observed in plants exposed to the effects of ozone ( 121 ).
Sulfur and nitrogen oxides are involved in the formation of acid rain and are harmful to plants and marine organisms.
Last but not least, as mentioned above, the toxicity associated with lead and other metals is the main threat to our ecosystems (air, water, and soil) and living creatures ( 121 ).
In 2018, during the first WHO Global Conference on Air Pollution and Health, the WHO's General Director, Dr. Tedros Adhanom Ghebreyesus, called air pollution a “silent public health emergency” and “the new tobacco” ( 122 ).
Undoubtedly, children are particularly vulnerable to air pollution, especially during their development. Air pollution has adverse effects on our lives in many different respects.
Diseases associated with air pollution have not only an important economic impact but also a societal impact due to absences from productive work and school.
Despite the difficulty of eradicating the problem of anthropogenic environmental pollution, a successful solution could be envisaged as a tight collaboration of authorities, bodies, and doctors to regularize the situation. Governments should spread sufficient information and educate people and should involve professionals in these issues so as to control the emergence of the problem successfully.
Technologies to reduce air pollution at the source must be established and should be used in all industries and power plants. The Kyoto Protocol of 1997 set as a major target the reduction of GHG emissions to below 5% by 2012 ( 123 ). This was followed by the Copenhagen summit, 2009 ( 124 ), and then the Durban summit of 2011 ( 125 ), where it was decided to keep to the same line of action. The Kyoto protocol and the subsequent ones were ratified by many countries. Among the pioneers who adopted this important protocol for the world's environmental and climate “health” was China ( 3 ). As is known, China is a fast-developing economy and its GDP (Gross Domestic Product) is expected to be very high by 2050, which is defined as the year of dissolution of the protocol for the decrease in gas emissions.
A more recent international agreement of crucial importance for climate change is the Paris Agreement of 2015, issued by the UNFCCC (United Nations Climate Change Committee). This latest agreement was ratified by a plethora of UN (United Nations) countries as well as the countries of the European Union ( 126 ). In this vein, parties should promote actions and measures to enhance numerous aspects around the subject. Boosting education, training, public awareness, and public participation are some of the relevant actions for maximizing the opportunities to achieve the targets and goals on the crucial matter of climate change and environmental pollution ( 126 ). Without any doubt, technological improvements makes our world easier and it seems difficult to reduce the harmful impact caused by gas emissions, we could limit its use by seeking reliable approaches.
Synopsizing, a global prevention policy should be designed in order to combat anthropogenic air pollution as a complement to the correct handling of the adverse health effects associated with air pollution. Sustainable development practices should be applied, together with information coming from research in order to handle the problem effectively.
At this point, international cooperation in terms of research, development, administration policy, monitoring, and politics is vital for effective pollution control. Legislation concerning air pollution must be aligned and updated, and policy makers should propose the design of a powerful tool of environmental and health protection. As a result, the main proposal of this essay is that we should focus on fostering local structures to promote experience and practice and extrapolate these to the international level through developing effective policies for sustainable management of ecosystems.
Author Contributions
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
Conflict of Interest
IM is employed by the company Delphis S.A.
The remaining authors declare that the present review paper was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Keywords: air pollution, environment, health, public health, gas emission, policy
Citation: Manisalidis I, Stavropoulou E, Stavropoulos A and Bezirtzoglou E (2020) Environmental and Health Impacts of Air Pollution: A Review. Front. Public Health 8:14. doi: 10.3389/fpubh.2020.00014
Received: 17 October 2019; Accepted: 17 January 2020; Published: 20 February 2020.
Reviewed by:
Copyright © 2020 Manisalidis, Stavropoulou, Stavropoulos and Bezirtzoglou. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Ioannis Manisalidis, giannismanisal@gmail.com ; Elisavet Stavropoulou, elisabeth.stavropoulou@gmail.com
† These authors have contributed equally to this work
This article is part of the Research Topic
Environment and Health
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- Air Research
Research on Health Effects from Air Pollution
Decades of research have shown that air pollutants such as ozone and particulate matter (PM) increase the amount and seriousness of lung and heart disease and other health problems. More investigation is needed to further understand the role poor air quality plays in causing detrimental effects to health and increased disease, especially in vulnerable populations. Children, the elderly, and people living in areas with high levels of air pollution are especially susceptible.
Results from these investigations are used to support the nation's air quality standards under the Clean Air Act and contribute to improvements in public health.
Research is being conducted in the following areas:
Health Effects of Air Pollutants on Vulnerable Populations
Long-Term and Short-Term Effects from Exposure to Air Pollutants
- Multipollutant Exposures and Changes in Environmental Conditions
- Leveraging Big Data for Innovations in Health Science
Health Effects of Wildfire Smoke
Public Health Intervention and Communications Strategies
Integrated Science Assessments for Air Pollutants
Research has shown that some people are more susceptible than others to air pollutants. These groups include children, pregnant women, older adults, and individuals with pre-existing heart and lung disease. People in low socioeconomic neighborhoods and communities may be more vulnerable to air pollution because of many factors. Proximity to industrial sources of air pollution, underlying health problems, poor nutrition, stress, and other factors can contribute to increased health impacts in these communities.
There is a need for greater understanding of the factors that may influence whether a population or age group is at increased risk of health effects from air pollution. In addition, advances to analytical approaches used to study the health effects from air pollution will improve exposure estimates for healthy and at-risk groups.
The research by EPA scientists and others inform the required reviews of the primary National Ambient Air Quality Standards (NAAQS), which is done with the development of Integrated Science Assessments (ISAs). These ISAs are mandated by Congress every five years to assess the current state of the science on criteria air pollutants and determine if the standards provide adequate protection to public health.
Research is focused on addressing four areas:
- Identifying and characterizing whether there are key reproductive factors and critical stages of development that are impacted by air pollution exposures;
- Determining the role of acute and chronic sociodemographic factors in air pollution health disparities;
- Understanding how diet modifies responses to air pollution;
- Evaluating long-term lifestyle and chronic disease effects on air pollution-induced respiratory and cardiovascular responses
A multi-disciplinary team of investigators is coordinating epidemiological, human observational, and basic toxicological research to assess the effects of air pollution in at-risk populations and develop strategies to protect these populations, particularly those with pre-existing disease. The results from these products will improve risk assessments by clarifying the role of modifying factors such as psychosocial stress (e.g. noise) and diet, and determining the impact of individual susceptibility on the relationship between air pollutant exposures and health.
Related Links:
- Healthy Heart Research
- Integrated Science Assessments
- Criteria Air Pollutants
People can experience exposure to varying concentrations of air pollution. Poor air quality can impact individuals for a short period of time during the day, or more frequently during a given day. Exposure to pollutants can also occur over multiple days, weeks or months due to seasonal air pollution, such as increased ozone during the summer or particulate matter from woodstoves during the winter.
The health impact of air pollution exposure depends on the duration and concentrations, and the health status of the affected populations. Studies are needed to increase knowledge of the exposure duration and the possible cumulative increase in risk.
The research is focused on three main areas:
- Short-term peak exposures, such as wildfires, traffic-related sources, or other episodic events;
- Intermittent and cumulative exposures;
- Mechanisms underlying the exposure risks
Researchers are evaluating the health responses of intermittent multiple days versus one-day air pollution exposure in controlled human exposure, animal, and in vitro models and associated cellular and molecular mechanisms. They are employing population-based models and electronic health records to assess the health effects of short-term and long-term exposures and identifying populations at greatest risk of health effects. The work is improving our understanding of the possible cumulative effects of multiple short-term peak exposures and the relationship of these exposures to longer-term exposures and risks.
Multipollutant Exposures and Changes in Environmental Conditions
EPA research is providing information to understand how individuals may respond to two or more pollutants or mixtures and how environmental conditions may impact air quality. While risk estimates for exposure to individual criteria air pollutants such as PM and ozone are well established, the acute and cumulative effects of combinations of pollutants is not well understood. In addition, research is needed to determine how changes in the environment affect both pollutant formation and subsequent responsiveness to exposures in healthy and susceptible individuals.
The research is focused on three specific questions:
- What is the role of temperature and photochemical aging on the health impact of wildfire smoke and air pollution mixtures?
- What is the effect of changing environmental conditions (i.e., temperature and humidity) on responsiveness to air pollution?
- Does prior pollutant exposure modify responsiveness to subsequent exposures?
The integrated, multi-disciplinary research includes:
- Epidemiologic analyses of environmental influences on morbidity and mortality in populations,
- Simulations of changing environmental conditions in multi-pollutant formation in atmospheric chamber studies coupled with clinical and toxicological assessments in healthy and at-risk populations,
- Evaluation of pre-exposure as a modifying effect on subsequent exposures
The results are revealing how changes in environmental conditions affect pollutant formation and subsequent health impact in at-risk populations. The research findings are informing EPA’s Integrated Science Assessments for criteria air pollutants and assisting with future regulatory decisions on the National Ambient Air Quality Standards (NAAQS).
Leveraging Big Data for Innovations in Health Science
EPA is at the cutting edge of health science, using electronic health records, novel data systems, tissue-like advanced cellular models, molecular approaches, and animal models to evaluate the health impacts of air pollution. Researchers are using these powerful new techniques to identify factors that may increase sensitivity and vulnerability to air pollution effects.
The research is building capacity for future risk assessment and regulatory analyses that go beyond traditional lines of evidence to more clearly define populations and lifestages at increased risk of health effects from air pollution.
To continue to protect public health from poor air quality, researchers must consider new epidemiological, toxicological and clinical approaches to understand the health risks of poor air quality and the biological mechanisms responsible for these risks. At the center of these new research approaches is an explosion of data availability and methodological approaches for handling large clinical and molecular datasets, also known as "big data."
While data of increasing size, depth, and complexity have accelerated research for many industries and scientific fields, big data is sometimes less recognized for the impacts it is having on environmental health studies. Increasingly, researchers are able to examine vulnerable populations with unprecedented precision and detail while also evaluating hundreds of thousands of molecular biomarkers in order to understand biological mechanisms associated with exposure.
Larger and more intense wildfires are creating the potential for greater smoke production and chronic exposures in the United States, particularly in the West. Wildfires increase air pollution in surrounding areas and can affect regional air quality.
The health effects of wildfire smoke can range from eye and respiratory tract irritation to more serious disorders, including reduced lung function, exacerbation of asthma and heart failure, and premature death. Children, pregnant women, and the elderly are especially vulnerable to smoke exposure. Emissions from wildfires are known to cause increased visits to hospitals and clinics by those exposed to smoke.
It is important to more fully understand the human health effects associated with short- and long-term exposures to smoke from wildfires as well as prescribed fires, together referred to as wildland fires. EPA is conducting research to advance understanding of the health effects from different types of fires as well as combustion phases. Researchers want to know:
- What is the full extent of health effects from smoke exposure?
- Who is most at risk?
- Are there differences in health effects from different wildfire fuel types or combustion phases (burning versus flaming)?
- What strategies and approaches are most effective in protecting public health?
- What are the environmental, social and economic impacts of wildfire emissions?
Related Links
- Wildland Fire Research
- Smoke-Ready Toolbox for Wildfires
- Smoke Sense Project and App
Many communities throughout the United States face challenges in providing advice to residents about how best to protect their health when they are exposed to elevated concentrations of air pollutants from motor vehicle and industrial emissions and other sources of combustion, including wildland fire smoke.
Researchers are studying intervention strategies to reduce the health impacts from exposure to air pollution as well as ways to effectively communicate these health risks. To translate the science for use in public health communication and community empowerment, EPA is collaborating with other federal agencies, such as the Centers for Disease Control and Prevention (CDC) and the National Heart, Lung, and Blood Institute (NHLBI), and state and local agencies and tribes. The objectives are to identify ways to lower air pollution exposure or mitigate the biological responses at individual, community or ecosystem levels, and ultimately evaluate whether such interventions have benefits as measured by indicators of health, well-being or economics.
Studies are evaluating the interactions between behavior and social and economic factors to more thoroughly understand how these factors may influence health and well-being outcomes, which can inform effective and consistent health risk messaging.
- Healthy Heart Initiative and Research
EPA sets National Ambient Air Quality Standards (NAAQS) for six principal criteria air pollutants—nitrogen oxides, sulfur oxides, particulate matter, carbon monoxide, ozone and lead—all of which have been shown to be harmful to public health and the environment.
The Agency’s Integrated Science Assessments (ISAs) form the scientific foundation for the review of the NAAQS standards by providing the primary (human health-based) assessments and secondary (welfare-based, e.g. ecology, visibility, materials) assessments. The ISAs are assessments of the state of the science on the criteria pollutants. They are conducted as mandated under the Clean Air Act.
- Original research article
- Open Access
- Published: 01 June 2018
Outdoor air pollution and respiratory health: a bibliometric analysis of publications in peer-reviewed journals (1900 – 2017)
- Waleed M. Sweileh 1 ,
- Samah W. Al-Jabi 2 ,
- Sa’ed H. Zyoud 2 &
- Ansam F. Sawalha 1
Multidisciplinary Respiratory Medicine volume 13 , Article number: 15 ( 2018 ) Cite this article
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Outdoor air pollution is a major threat to global public health that needs responsible participation of researchers at all levels. Assessing research output is an important step in highlighting national and international contribution and collaboration in a certain field. Therefore, the aim of this study was to analyze globally-published literature in outdoor air pollution – related respiratory health.
Outdoor air pollution documents related to respiratory health were retrieved from Scopus database. The study period was up to 2017. Mapping of author keywords was carried out using VOSviewer 1.6.6.
Search query yielded 3635 documents with an h -index of 137. There was a dramatic increase in the number of publications in the last decade of the study period. The most frequently encountered author keywords were: air pollution (835 occurrences), asthma (502 occurrences), particulate matter (198 occurrences), and children (203 occurrences). The United States of America ranked first (1082; 29.8%) followed by the United Kingdom (279; 7.7%) and Italy (198; 5.4%). Annual research productivity stratified by income and population size indicated that China ranked first (22.2) followed by the USA (18.8). Analysis of regional distribution of publications indicated that the Mediterranean, African, and South-East Asia regions had the least contribution. Harvard University (92; 2.5%) was the most active institution/organization followed the US Environmental Protection Agency (89; 2.4%). International collaboration was restricted to three regions: Northern America, Europe, and Asia. The top ten preferred journals were in the field of environmental health and respiratory health. Environmental Health Perspective was the most preferred journal for publishing documents in outdoor pollution in relation to respiratory health.
Research on the impact of outdoor air pollution on respiratory health had accelerated lately and is receiving a lot of interest. Global research networks that include countries with high level of pollution and limited resources are highly needed to create public opinion in favor of minimizing outdoor air pollution and investing in green technologies.
Outdoor air pollution is defined as the presence of one or more substances in the atmospheric air at concentrations and duration above the natural limits [ 1 ]. Such substances include ozone [O3], airborne lead [Pb], carbon monoxide [CO], sulphur oxides [SOx] and nitrogen oxides [NOx] [ 2 ]. Recently, air pollution with particulate matters (PM), especially those with less than 2.5 μm, has been the focus of most outdoor air pollution studies due to its ability to penetrate the lung tissue and induce local and systemic effects [ 2 ].
Air pollution has been described as one of the “great killers of our age” and as “major threat to health” due to its tremendous and various health effects on humans of all ages and in both genders [ 3 , 4 ]. In 2014, the World Health Organization (WHO) estimated that 92% of the world population was living in places with less than optimum outdoor air quality. Furthermore, WHO reported that in 2012, outdoor air pollution caused around 3 million deaths worldwide and 6.5 million deaths (11.6% of all global deaths) were associated with indoor and outdoor air pollution together [ 5 ].
Air pollution was linked to cancer, respiratory diseases, negative pregnancy outcomes, infertility, cardiovascular diseases, stroke, cognitive decline, and other adverse medical conditions [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. Nearly 90% of air-pollution-related deaths occur in low- and middle-income countries, with nearly 2 out of 3 occurring in South-East Asia and Western Pacific regions. The problem of outdoor pollution is not a new one, but the rapid urbanization, particularly in Asia, made the problem of air pollution more visible and its health burden more tangible [ 14 , 15 , 16 , 17 ].
Bibliometric analysis is the application of statistical methods on published literature to analyze publication trends with time and to shed light on influential researchers, countries, and institutions in the field. In the past decade, at least seven bibliometric studies on air pollution were published [ 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. However, none of the published bibliometric studies have shed light on the air pollution - related respiratory health. Therefore, in the current study, we aim to analyze literature pertaining to outdoor air pollution and respiratory health. The size of the literature and research productivity in this field is a good indicator of national and international efforts to improve air quality and to decrease the health and economic burden of air pollution. Furthermore, the quality of the air we breathe is the responsibility of everyone including researchers and academics. This study comes in line with perceived personal responsibility toward better air quality.
Search strategy
This study aimed to analyze the documents about outdoor air pollution – related respiratory health. Scopus database was used to retrieve relevant documents because of its advantages over other databases [ 25 , 26 , 27 , 28 ]. The search strategy developed for this study consisted of nine steps (Additional file 1 ). The first six steps utilized various keywords and search queries to retrieve the maximum number of documents. Keywords included in search queries were those found in recent relevant systematic reviews [ 6 , 12 , 13 , 29 , 30 ]. The combined result of search queries underwent a filtration process by adding exclusion and limitation components (steps 7 – 9).
False positive results were minimized by using title search. Therefore, all retrieved documents have the keywords of interest. Despite that, false positive results need to be searched by reviewing the retrieved documents. The review process was carried out on a sample of 200 documents chosen based on the number of citation. The review process was carried out by the authors (W.S and A.J) and approved by a third author (A.S). Keywords of the irrelevant documents (false positive results) were used in the exclusion step. A complete list of irrelevant keywords is written in Additional file 1 . The exclusion of false positive results is not enough to confirm the validity of the search strategy. Therefore, the authors compared two different methods of data collection. In the first one, we collected data regarding research output for each of the most active authors as obtained through the search strategy, whilst in the second one, the research output of each of the most active authors was extracted and reviewed by exploring the author profile as presented by Scopus. The extent of agreement between the two methods is measured by interclass correlation coefficient using SPSS [ 31 , 32 , 33 , 34 , 35 ]. An excellent agreement between the two methods with an interclass correlation above 95% and a p less than 5% is indicative of high validity of the search strategy. In the current study, the interclass correlation was 0.98.7% and p was 0.001.
The retrieved data were also sorted based on the number of different country affiliations per article to calculate international collaboration. Documents with authors from different countries represent international or inter-country collaboration while documents in which all authors have one country affiliation represent intra-country collaboration. It should be emphasized that Scopus has the function which can separate documents with intra or inter – country collaboration. Therefore, the calculation of international collaboration was extracted from data provided by Scopus.
Bibliometric analysis versus systematic reviews
It should be emphasized that the bibliometric analysis is not the same as systematic reviews. In contrast to systematic reviews, bibliometric analysis focuses on quantitative and qualitative aspects of all documents retrieved from one electronic large database. In bibliometric analysis, the investigated research question is the volume of research published, how this volume of literature evolved with time, what major topics were of high interest, and the scientific impact of literature in a particular subject. However, in systematic reviews, a complete and exhaustive summary of current literature obtained from several electronic databases and relevant to a research question is provided.
In bibliometric analysis, only one large and well – known database, such as Scopus, is used. Therefore, the retrieved documents will not include any duplicates. On the other hand, duplicate documents might appear in systematic reviews because several databases are used to retrieve the required documents.
Data analysis and visualization
In this study, Hirsch-index ( h -index) was used as a measure of impact of publications [ 36 ]. Graphs were created using Statistical Package for Social Sciences (SPSS). Hirsch - index is defined as the number of articles (n) that have received at least n citations [ 36 ]. VOSviewer software was used to create visualization maps while ArcMap 10.1 was used to create geographical distribution of the retrieved documents [ 37 , 38 , 39 ]. For VOSviewer mapping of most frequent author keywords, a minimum occurrence of 10 was used as a cut-off point for inclusion of the keyword in mapping analysis. Analysis also included distribution of publications based on World Health Organization (WHO) regions.
Types and growth of publications
The search strategy yielded 3635 documents. The earliest document in this field was published in 1943 in American Journal of Epidemiology [ 40 ]. The analysis of the types of documents showed that research articles (2935, 80.7%) were the most common type followed by review articles (359; 9.9%). The remaining documents (341; 9.4%) were conference papers, letters, editorials, short surveys, and notes. English (2923, 80.4%) was the primary language of documents followed by French (156; 4.3%) and German (124; 3.4%). The subject areas of the documents were medicine (2772; 76.3%) followed by environmental science (1038; 28.6%) and biochemistry/ genetics/ molecular biology (317; 8.7%) with the possibility of overlap among different subject areas. The growth of publications showed a dramatic increase in the past decade. Figure 1 shows the annual growth of publications. There was a 72% increase in number of publications in 2017 compared to that in 2008.
Annual growth of publications in Outdoor air pollution and respiratory health (1900 – 2017)
Author keywords
Analysis of author keywords showed that the most frequently encountered author keywords were: air pollution (835 occurrences), asthma (502 occurrences), particulate matter (198 occurrences), and children (203 occurrences) (Fig. 2a ). Further mapping of types of pollutants most commonly encountered in author keywords showed that particulate matter (198 occurrences), ozone (192 occurrences), nitrogen oxide (95 occurrences), PM10 (75 occurrences), PM2.5 (57 occurrences), and Sulfur dioxide (54 occurrences), were the most frequently encountered author keywords (Fig. 2b ).
Most frequent author keywords encountered in the retrieved documents ( a ) and most frequently encountered types of outdoor air pollutants encountered in the retrieved documents ( b )
Active journals
Table 1 shows the top ten journals that were involved in publishing the retrieved documents. Environmental Health Perspective was the most active journal (153; 4.2%) followed by Environmental Research (112; 3.1%) and American Journal of Respiratory and Critical Care Medicine (100; 2.8%). The top ten active journals included four in the field of environmental health, four in the field of respiratory health, one in allergy/immunology, and the last one in toxicology field.
Authorship analysis
The number of different author names who participated in publishing documents was 11,014; giving an average of 3.0 authors per document. Table 2 lists the top ten active authors with their affiliations. The top active authors were mainly from Western and Northern Europe, particularly from the Netherlands, Italy, and the United Kingdom [ 21 ]. Prof. Brunekreef, B. from the Netherlands was the most active researcher in this field with 77 (2.2%) documents. Authors with a minimum productivity of 20 publications were also visualized using network visualization map that showed research networking among active authors (Fig. 3 ). The map showed that active authors with minimum productivity of 20 publications existed in four clusters. The largest cluster consisted of eight authors (dark red color). The second cluster consisted of seven authors (green). The third cluster consisted of six authors (blue). The fourth cluster consisted of four researchers (dark yellow). Authors with minimum productivity of 20 publications who are not shown in the map are usually those who did not exist within a research network that has prominent productivity.
Network visualization map of authors with minimum productivity of 20 publications in the studied field and exist within a collaborative research group
Active countries
Researchers from 92 different countries contributed to the retrieved documents. Table 3 lists the top ten countries actively involved in air pollution – related respiratory health. Researchers from the USA participated in publishing 1082 (29.8%) documents. The top 10 list included countries from Northern America, Western Europe, and Asia. Researchers from these top ten countries participated in publishing 2630 (72.3%) documents. Figure 4 shows worldwide geographical distribution of retrieved documents. Regional distribution of retrieved documents indicated that the regions of Americas, Europe, and Western pacific had the highest percentage of contribution while Mediterranean, Africa, and South-East Asia regions had the least contribution (Fig. 5 ).
Geographical distribution of published research in outdoor air pollution and respiratory health (1900 – 2017)
Geographical distribution of published research in outdoor air pollution and respiratory health (1900 – 2017) based on WHO world region. WP: Western Pacific; EM: Eastern Mediterranean; E: Europe; SEA: South Eastern Asia; AM: Americas; AF: Africa
International collaboration
International collaboration in air pollution – related respiratory health showed that there were three clusters. There was relatively adequate collaboration among countries within each cluster and there was adequate collaboration between countries in the two different clusters (Fig. 6 ). The first cluster consisted of 9 European countries shown in green color while the second cluster consisted of 9 countries in different regions in the world particularly those in Northern and Southern America, South East Asia, and Western Pacific regions. The third cluster consisted of one item, India with research connections with countries in both cluster number 1 and 2. International collaboration among countries in the Mediterranean region, Africa, or Eastern Europe and those in Northern America, Europe, or Asia did not show up in the map.
Network visualization map of research collaboration in outdoor air pollution and respiratory health (1900 – 2017)
Table 3 shows the extent and the percentage of intra and inter (international) country collaboration for the top active countries. In terms of quantity, the USA had the largest number of documents (276; 25.5%) with international authors. However, this quantity represents only 25.5% of total research productivity from the USA which means that approximately 75% of USA research production in this field was produced by authors from the USA without collaboration with international researchers. Japan had the least percentage (22.2%) of international collaboration while Sweden had the largest percentage (58.0%) of documents with international collaboration.
Active institutions
Harvard University ranked first in research output (92; 2.5%) and in the impact of publications ( h -index = 44). The US Environmental Protection Agency (EPA) ranked second in research output (89; 2.4%) and in the impact of publications ( h -index = 36). Table 4 shows the top ten active institutions/organizations. The list included seven academic institutions and three research centers. Six of the top active institutions were American institutions, three were European, and one was Canadian.
Citation analysis
The total number of citations received by documents was 101,113, with an average of 27.8 citations per document. Range of citations was [0 – 4294]. The h -index of the retrieved documents was 137. Table 5 shows the top ten highly cited articles. The article that received the highest number of citations (4294) was published in 2002 in Journal of American Medical Association (JAMA) and discussed the relation between lung cancer, cardiopulmonary mortality and air pollution [ 40 ].
Growth of publications
In this study, we analyzed global research output in outdoor air pollution – related respiratory health. The results showed a noticeable increase in the number of publications in the last decade of the study period. This indicates that the level of air pollution and its health consequences reached serious levels. In 2012, air pollution was responsible for 3 million deaths, representing 5.4% of the total global deaths. In the same year, about 25% were due to lung cancer deaths, 8% were due to chronic obstructive pulmonary disease (COPD) deaths, and about 17% of respiratory infection deaths were caused by outdoor air pollution [ 41 ]. A study indicated that the contribution of outdoor air pollution to global premature mortality could double by 2050 [ 42 ]. Another study concluded that outdoor air pollution contributes to the increase in global burden of COPD and that an increase of 10 μg/m (3) in PM10 produced significant increase in COPD deaths and exacerbations that can be substantially reduced by controlling air pollution [ 43 ]. A cohort Chinese study concluded that the risks of mortality and years of life lost were elevated corresponding to an increase in current ambient concentrations of the air pollutants [ 44 ].
The contribution of researchers from /ifferent scientific fields led to an acceleration in the growth of publications in this field. Scientists in the fields of the environment, respiratory health, public health, and even molecular biology/genetics contributed to the retrieved documents [ 45 , 46 , 47 , 48 , 49 ]. The fact that air pollution is a multidisciplinary field created a large number of readers from different scientific fields and thus leading to large number of citations, reflected in the relatively high h -index value of documents. For example, the h -index of literature in global carbapenem resistance was 102 and that for literature in resistant tuberculosis was 76 [ 50 , 51 ].
Active countries and institutions
Our results showed that China had the highest research productivity in terms of GDP per capita per year. In China, air pollution was previously estimated to contribute to 1.2 to 2 million deaths annually [ 52 ]. In its list of the world’s deadliest countries for air pollution, the WHO ranked China first followed by India, Russian Federation, Indonesia, Pakistan, Ukraine, Nigeria, Egypt, USA, and Bangladesh [ 53 ]. Out of the top 10 countries that have high total annual number of deaths from PM2.5 and PM10, only China and USA were among the top ten active countries in research output. The deadliest effects of air pollution in China led to the adoption of the Ambient Air Quality Standard in China in 2012 [ 54 ]. This system started a national Air Reporting System that now includes 945 sites in 190 cities.
The presence of active institutions and many high impact journals in the field of environmental health and respiratory medicine issued from the USA contributed to the leadership of USA in this field. Research output in any field is a function of money allocated to research as well as public health agendas of the country. The leadership of the USA was seen in several other scientific subjects [ 55 , 56 , 57 , 58 ]. The fact that English was the primary language of literature published in journals indexed in Scopus might have created some sort of bias toward English-speaking countries.
Outdoor air pollution is a global health concern and international collaboration in this field is necessary. In our study, the extent of international collaboration in research was relatively high, particularly within European countries and between USA and Asian countries. The WHO Collaborating Centre for Air Quality Management and Air Pollution Control (WHO CC) is working with member states in Europe and Asia to encourage collaboration in air quality programs through interaction and networking [ 59 ].
Highly cited documents
The top cited documents in the field was about the relationship between outdoor air pollution and lung cancer; and received a large number of citations suggestive of great importance. The International Agency for Research on Cancer [ 60 ], which is part of the WHO, has classified outdoor air pollution, as a whole, as a cancer-causing agent (carcinogen ) [ 60 ]. The International Agency for Research on Cancer (IARC) concluded that outdoor air pollution causes lung cancer and is associated with increased risk for bladder cancer . Urgent action to minimize level of outdoor air pollution and exposure of population to such carcinogenic pollutants is necessary, particularly in cities with high levels of outdoor air pollution [ 61 , 62 ].
Strength and limitations
It is the first to assess research activity in the field of outdoor air pollution – related respiratory health. Our study documented the accelerated increase in publications and the role of international collaboration. However, our study has a number of limitations. The Scopus database is a comprehensive and large database that includes different disciplines, but some peer – reviewed journals are not indexed in Scopus. This is particularly true for journals published from India, China, Indonesia, and other Asian and African countries where outdoor air pollution is a real public health problem. Therefore, documents published in un-indexed journals were not retrieved. Secondly, the results presented in this study reflect the search strategy implemented which is comprehensive in the subject but the presence of false positive and false negative results cannot be ruled out. This is true in all bibliometric studies [ 63 , 64 , 65 , 66 , 67 ]. Thirdly, when listing active authors and institutions, the authors depended on the outcome obtained from the Scopus. However, some authors might have more than one Scopus profile or might have their name written in different articles in different ways which will affect their research output and therefore their rank as well. Same applies to active institutions where the name of the institution might be written in different articles in different ways which will affect their research output and rank as well. Furthermore, the authors used the keyword “environment” in the search strategy in a strict way to avoid false positive results since not all environmental pollution could fit the scope of the current study which focused on outdoor air pollution and its impact on respiratory health. In this regard, the authors also avoided the use of the keyword “climate” in the search strategy to keep the research question focused on outdoor air pollution, particularly those produced by industry. Finally, it should be emphasized that the list of highly cited articles does not mean that these articles are the only influential ones in the field. The citation process is dynamic and sometimes high citation reflects self-citation rather than impact. There were many influential and highly cited articles in the field that were not listed in the highly cited article [ 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 ].
Conclusions
Growth of publications in outdoor air pollution – related respirator health is rapidly increasing. However, limited research output and international collaboration were seen in world regions such as the Middle East, Africa, and South-East Asia. International multidisciplinary research network, involving countries with high levels of air pollution and limited resources, are needed. Research in atmospheric pollution should also be directed toward prevention of air pollution problems by investing more in green technology.
The results presented in this study are indicative of how research activity is interacting with the urgent acceleration of the air pollution crisis at the global level. Furthermore, the research activity is indicative of the response rate adopted by certain countries to face this global problem in a responsible way. Pressure groups can use the research activity to enforce certain environmental and industrial agendas on politicians and political campaigns. Countries with high levels of outdoor air pollution, and therefore, poor air quality, need to get engaged in research pertaining to this field to provide health policymakers with baseline data for future action. Establishing research center for monitoring national air quality and level of air pollution is a step forward that needs to be adopted by all countries. Such centers could include scientists from different disciplines who can collaborate to convert research findings into national agendas and policies. At the national levels, different world countries need to adopt strict guidelines for air quality. Collaboration between industry and health authorities is needed to implement measures that could significantly reduce the levels of particulate matter. The outdoor air pollution is a global public health and therefore research networking between developed countries and developing countries with high levels of air pollution should be prioritized. The Chinese model in controlling air pollution and minimizing its health consequences could be of a global benefit. Finally, since the respiratory effects of air pollution are affecting children, there is a need to educate and increase the awareness of parents regarding this issue.
Abbreviations
International Agency for Research on Cancer
Institutional Review Board
World Health Organization
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The authors would like to thank An-Najah National University for giving the opportunities to this study.
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AS conceptualized and designed the study, coordinated the study and data analysis, interpreted the data, and did the final write-up of the manuscript. WS, SZ, SA; designed the method, data extraction, data analysis, data interpretation, and writing the manuscript. All authors read and approved the final manuscript.
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Sweileh, W.M., Al-Jabi, S.W., Zyoud, S.H. et al. Outdoor air pollution and respiratory health: a bibliometric analysis of publications in peer-reviewed journals (1900 – 2017). Multidiscip Respir Med 13 , 15 (2018). https://doi.org/10.1186/s40248-018-0128-5
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DOI : https://doi.org/10.1186/s40248-018-0128-5
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Air pollution research: visualization of research activity using density-equalizing mapping and scientometric benchmarking procedures
- Hanna Zell 1 ,
- David Quarcoo 1 ,
- Cristian Scutaru 1 , 2 ,
- Karin Vitzthum 1 , 2 ,
- Stefanie Uibel 1 ,
- Norman Schöffel 1 ,
- Stefanie Mache 1 ,
- David A Groneberg 1 &
- Michael F Spallek 1
Journal of Occupational Medicine and Toxicology volume 5 , Article number: 5 ( 2010 ) Cite this article
Due to constantly rising air pollution levels as well as an increasing awareness of the hazardousness of air pollutants, new laws and rules have recently been passed. Although there has been a large amount of research on this topic, bibliometric data is still to be collected. Thus this study provides a scientometric approach to the material published on this subject so far.
For this purpose, data retrieved from the "Web of Science" provided by the Thomson Scientific Institute was analyzed and visualized both with density-equalizing methods and classic data-processing methods such as tables and charts.
For the time span between 1955 and 2006, 26,253 items were listed and related to the topic of air pollution, published by 124 countries in 24 different languages. General citation activity has been constantly increasing since the beginning of the examined period. However, beginning with the year 1991, citation levels have been rising exponentially each year, reaching 39,220 citations in the year 2006. The United States, the UK and Germany were the three most productive countries in the area, with English and German ranked first and second in publishing languages, followed by French. An article published by Dockery, Pope, Xu et al. was both the most cited in total numbers and in average citation rate. J. Schwartz was able to claim the highest total number of citations on his publications, while D.W. Dockery has the highest citation rate per publication. As to the subject areas the items are assigned with, the most item were published in Environmental Sciences, followed by Meteorology & Atmospheric Sciences and Public, Environmental & Occupational Health. Nine out of the ten publishing journals with more than 300 entries dealt with environmental interests and one dealt with epidemiology.
Conclusions
Using the method of density-equalizing mapping and further common data processing procedures, it can be concluded that scientific work concerning air pollution and related topics enjoys unbrokenly growing scientific interest. This can be observed both in publication numbers and in citation activity.
Air pollution is defined as the emission of particulate toxic elements into the atmosphere by natural or anthropogenic sources [ 1 ]. These sources can be further differentiated into either mobile or stationary sources [ 2 ]. Anthropogenic air pollution commenced with human's systematic use of fire. Its historical development has been characterized by steadily increasing amounts of total emissions, the invention of new sources of pollution emission as well as the emission of pollutants that had not formerly been emitted by man-made sources. So far, this development has had the greatest impact on the air quality of so-called Mega-Cities (cities with over 10,000,000 inhabitants). Today the major sources of man-made air pollution are motorized street traffic (especially exhaust gases and tire abrasion), the burning of fuels, and larger factory emissions. Depending on the pollutant particles' size, they can be carried for distances of several thousand miles. With decreasing diameter, they are able to infiltrate finer lung structures [ 3 ].
The World Health Organization (WHO) estimates 2.4 million fatalities due to air pollution each year [ 4 ]. Since the breathing of polluted air may have severe health effects such as asthma, COPD or increased cardiovascular risks [ 5 , 6 ], most countries have strengthened laws to control the air quality in the past decade. Further, as polluted air is considered a super-regional problem, international conferences have recently developed different ways to improve and assure air quality employing global strategic perspectives [ 7 , 8 ].
Despite such enormous scientific and legislative efforts to measure and improve air quality levels, many people are still exposed to hazardously polluted breathing air on a daily basis [ 9 , 10 ]. Furthermore, there are currently no complete bibliometric analyses available on this topic.
The present scientometric compilation and analysis is intended to identify current scientific efforts with regard to air pollution, as well as to highlight research gaps requiring further attention.
All analyzed data were retrieved from Thomson Scientific's online-database "Web of Science". To provide a comparison data was also collected in some cases from "PubMed", the online database of the U.S. National Library of Medicine.
For the query, the terms "air pollution" and "air quality" were connected with the Boolean operator "OR" and entered in the search field "General Search".
The time frame was limited to the period between 1955 and 2006. For this purpose, the "Change Limits and Settings" function was adjusted initially and the query was made under this presetting. The limitation was performed since the years 2007-2009 were not finalized in the databases completely.
Results were also analyzed by means of citation. Therefore, the feature "Citation Report" was used to calculate the citation rate of both authors and citations per year of citation. To calculate the citation rates of the singular authors, results were first sorted by author. Afterwards, the ten most productive authors' publications were put under citation analysis. The average citation rate is the quotient of the total citation number divided by the publications listed for the author in question.
The "Web of Science" database provides several tools to analyze entries according to specified parameters [ 11 ]. The data set was analyzed by means of publication country, publication year, publishing author, publishing journals and published document type. Multiple distributions led to higher publication numbers when adding up results after analysis; for example, when a super regional publication was distributed to several countries. A common data processing program was used to display the results in tables, charts and diagrams.
Software using the method of density-equalizing mapping was employed to determine international correlations. This method resizes countries proportionally according to a predefined variable. In this study, the territory with the highest number of publications is depicted largest on the associated map. The basic principle was developed by Gastner and Newman [ 12 ].
Total number of published items
The overall number of items listed in the database served as a measure of both public interest and scientific productivity concerning the topic of air pollution. The comparison of results in "Web of Science" (26,253) and "PubMed" (28,565) indicated some differences. Entries in the "Web of Science" displayed a comparatively constant number over 25 years (1966-1990), following lower numbers in the prior decade (1955-1965). "PubMed" results, however, differed: Beginning with the year 1957 publication numbers increased up to a preliminary maximum in 1971. Subsequently, decreasing publication numbers equalized that of the "Web of Science" results again in 1978. After a period of relatively constant publication quantities (1978-1990), both databases showed an exponential increase in yearly published items. Despite recent points of nominal decline (2002 in "PubMed", 2005 in "Web of Science") the upward tendency remained consistent through 2006 (fig. 1 ).
Publications related to the topic air pollution 1955-2006 . Comparison of results in „Web of Science" and „PubMed".
Citations per year
Regarding the total citations per year (i.e. the overall citation activity for all entries), the numbers show a development similar to publication data. However, the increase since 1991 is marginally sharper than that of the publication numbers (fig. 2 ).
Citation per Year . Citations displayed in five-years-intervals.
Analysis of origin/language
Usage of the analysis tool "Countries/Territories" indicates that the USA holds the most publications by far. The UK and Germany ranked a distant second and third. The ten most publishing countries produced 76.44% of all the entries in the inquiry time frame. (fig. 3a ) Density-equalizing mapping demonstrates a huge contingent of publications provided by only a few countries' researchers (fig. 3b ).
Publication numbers . (a) Publications in totalnumbers, sorted by countries (b) Publications sorted by countries put into relation to each other (density-equalized).
As to the analysis by language, results showed a division consistent with the results obtained in the "country" analysis. The percentage of items published in English (96%) was much higher than the fraction published by Anglophone countries (fig. 4 ).
Publication languages . Languages used in publications.
Citation characteristics
Average citation rate (countries).
To obtain the average citation rate of single countries, the total number of citations for publications originated in each country was divided by the number of the said publications. In conclusion, Botswana achieved the highest rate with 191 citation/item, followed by Malta with 153.2/item. No other country achieved a citation rate higher than 30/item (fig. 5a ).
Citations per country . (a) Average citations per publications. (b) Average citations per publications (density-equalized). Threshold of 30 publications per country.
Inclusion of a threshold of at least 30 publication and density-equalizing calculations leads to a cartogram shown in figure 5b .
Average citation rate (publications and authors)
The average citation rate was calculated both for the most productive authors and the most cited publications. While the single author's citations had to be divided by his number of publications, the publications' total number of citation was divided by the number of years of citation activity for this item ("citations per year").
As for the publications' total citations, the most cited article was written by Dockery, DW, Pope, CA, Xu, XP et al. and published in the New England Journal of Medicine in 1993. The ten most cited entries in terms of total citation are shown in Table 1 , indicating author, title, publishing journal, publication date and the total citation number. Table 2 shows the ten items with the highest citation rate per year.
Examining the ten most productive authors' publications, J. Schwartz was cited most often in terms of total numbers (fig. 6 ), while D.W. Dockery achieved the highest citation rate ("citations per item") (fig. 7 ).
Citations per author/total . Citations of the ten most productive authors; citations given in total numbers.
Citations per author/item . Citations of the ten most productive authors; citations given in citations per item.
Analysis of assigned subject areas
Analysis of the subject areas assigned to the publications revealed only two involved medical issues in the first ten areas. The other eight areas dealt either with environmental questions or with matters related to engineering (fig. 8 ).
Subject areas . Publications' assignment to subjects areas; publications given in total numbers.
Publication type
The most commonly published document type was the article, followed by the meeting abstract. The ten most frequently used document types made up for 99.77% of all publications (fig. 9 ).
Document type . Publications displayed by published document type; publications given in total numbers.
Publishing journals
Out of the ten most publishing journals, nine dealt with environmental subject matters and one was dedicated to epidemiology (fig. 10 ). Items published in those journals added up to 27.40% of the entire amount of results.
Publications per journal . Number of publications in the ten most productive journals; publications given in total numbers.
Despite the fact that a similar search request in the medical database MEDLINE generated more results (28,565) than the one in "Web of Science", the entries were concerned more with environmental and technical interests than with medical subject matters. The first journal solely dedicated to toxicology (Inhalation Toxicology) was ranked thirteenth.
Air pollution has always been a subject of public concern. Particularly from the mid-twentieth century onwards, there has been a growing societal impetus to curtail and counteract the hazardous effects of air pollution. In recent decades, as anthropogenic air pollution has reached historically high levels, international public and scientific interest has intensified towards this topic. While natural, stationary emission cannot be significantly influenced, a major focus has been given to changes in man-made pollutant emissions. Heretofore, there has been no comprehensive analysis of data available on this topic. The present study sought to provide bibliometric data on research activity related to the subject air pollution, analyzed and displayed with both common and innovative data processing methods such as density-equalizing calculation. The examined time frame was limited to the period after 1955, since global research activity was relatively low before that date. The year of 2007 was excluded from the analysis because of incomplete data.
Mirroring public and scientific interest, general research activity on the topic of air pollution has increased steadily since the beginning of the analyzed period. However, a remarkable boost in both publication and citation activity can be observed at the beginning of the 1990s. On the one hand, this can be attributed to the availability of public internet access [ 13 ]. Before the early 1990s, access to such research data was restricted primarily to scientific institutions, larger companies and governmental organizations. On the other hand, authors' abstracts for the publications were only available since the year 1991 and authors of foreign languages were especially encouraged to write in English [ 14 ]. Thus, the major increase in citation activity observed since the beginning of the 1990s may be explained by an eased availability of contents both in the short form of the abstract as well as making summaries of publications in different languages available in English [ 15 ]. The increase in publication activity may be related to the easier access to the internet as a publication provider. Additionally, the massive simplification of data disposability in the shape of changing from paper to electronic devices might have encouraged authors to publish more of their findings.
Regarding the origin of publications, US researchers contribute by far the largest amount of scientific output related to air pollution. The second ranking of Great Britain, the third ranking of Germany and especially the People's Republic of China in fourth position is notable as well. Mega-Cities are defined as urban agglomeration areas with more than 10,000,000 inhabitants. They are characterized by social challenges such as poverty and crime but above all for producing massive environmental burdens especially in the field of air quality [ 16 ]. Considering the fact that three of the world's 26 mega-cities are located in China, four in directly neighbouring countries and another four in nearby countries (i.e. Japan, South-Korea and Bangladesh), it may be legitimate to contribute the large interest in air quality matters partly to those circumstances. Altogether it can be said that a minority of the world's countries contributes the majority of general research activity, as shown by density-equalizing methods [ 17 ]. However, the number of highly polluted areas (such as Mega-Cities) as well as the total amount of pollutants emitted should be connected to the high research effort of few countries.
English appears as the most common language in scientific releases; a finding that is concordant with papers' distribution to countries. While German plays a comparatively major role too, Chinese cannot be found among the most frequently used languages. This disproportion might lead to the assumption that English is used more commonly among Chinese researchers compared to European scientific publications. While French ranked third among the publications' languages, France's number of publications ranked seventh - past Italy and China whose languages do not appear at all among the most common ones. Russian also appears among the most frequently used languages, though Russia's publications ranked 27th in the international list. These facts could either be ascribed to political conditions and convictions, or to closer or broader affinity of languages. It is possible that states belonging to the former USSR still use Russian as their scientific language after the independency from the Russian government. It is also possible that Baltic and Eastern European States tend to rely more on research by their former authority (i.e. Russia) than Western European or American researchers and therefore use rather Russian than English in publications. Additionally, Slavic languages are linguistically closer related to Russian than to English. Older researchers might also have difficulties writing in a new language after using Russian for several decades.
While the total amount of publications was used as a distinguishing mark for research quantity, the average citation rate was introduced to evaluate the single nations' research quality [ 18 ]. In this regard Botswana ranked first with 191 citations per items followed closely by Malta with 153.2/item. Gambia ranking third could only unify 28.75 citations on every item published. As authors working in Botswana only published seven items in the whole time period investigated, the high citation rate appears exceptional. Malta unifying 1,532 citations on ten items is rather inconsequential as well. Closer attention to the results from the countries in question revealed that a world-wide asthma-study on children published in 1998 was distributed to all the 56 countries participating [ 19 ]. With 1,301 citations on that single item, it has a larger impact on the average citation rate, the lower the total publication number. Therefore the uncommonly high average citation rates for both Botswana and Malta must be credited with this international, highly-attended study. To avoid disproportionately high citation rates due to low total publication numbers, a borderline was drawn at ten publications. Given this condition and additionally taking the aforesaid study out of consideration, the Netherlands moved up to the first position, showing 19.08 citations per item followed by Sweden (17.43) and Switzerland (17.21).
Among the most cited articles, the aforementioned international asthma study is itself exceeded by an article by Dockery et al. in the New England Journal of Medicine, on the association between air pollution and mortality in six US cities [ 20 ]. This is one of the first studies pointing out the association between air pollution by particulates and sulfates and increased death rates due to pulmonary causes, deducting additional risks such as smoking beforehand.
As an author, D.W. Dockery features the highest average citation rate (125.54). However, in total citations, Dockery is only second to J. Schwartz.
With regard to "subject areas", the environmental sciences have produced the most results so far on the topic of air pollution, followed by the atmospheric sciences. Noticing that among the ten leading research fields there are only three concerned with health aspects, it can be said there is a rather significant lack of medical research on this subject. Given an estimated 2.4 million deaths yearly due to air pollution, it is rather surprising that medical research has lagged by this degree. Considering all the severe consequences polluted air has on public health, on international health conditions, and health care costs, it is justified to point out this obvious research gap, and recommend further scientific efforts in medicine in the future.
Since most of the publications are articles, it can be said that most of the scientific endeavours have been embossed by initiatives rather than by efforts in analysis and description of statistical coherences.
Among the ten most publishing journals, nine are occupied with environmental issues and one with epidemiology. This again demonstrates the existing deficit in medical interest.
However, it may be that medical interest in the field of air pollution and its consequences for human health may have developed only recently. Despite the fact that polluted air has always been a major threat on human health, there has not been any major supra-regional effort to change the polluting behaviour in the past 20 years. Although people have been suffering from polluted air for a along time, beginning several centuries ago, there has been no substantiated evidence for the connection between polluted air and deaths due to pulmonary diseases. New conclusions about coherences between certain air pollutants and their impact on public health have only been drawn recently, and therefore it is entirely possible that the medical research efforts will eventually catch up to and even exceed the scientific work already done in environmental fields.
Hereby given the first comprehensive analysis of scientometric data on the subject of air pollution, it can be said that scientific interest in this topic has steadily increased to the present day. However, there is to be noted a major research gap in terms of medical analysis. The major contingent of data originates from the USA and an even larger amount is written in the English language. Considering research quality as measured in terms of average citation, less productive countries such as the Netherlands and Sweden show the best results. A generally larger effort towards medical research into air pollution is strongly indicated at this time.
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This study was supported by a grant of EUGT e. V.
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Authors' contributions
HZ carried out the bibliometric investigations, participated in analyzing results and drafted the manuscript. CS participated in data research and performed the scientometric analysis. DQ, NS, SM participated in the design of the study. KV and SU carried out pilot data research. DAG participated in its design and data analyses. MS conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.
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Zell, H., Quarcoo, D., Scutaru, C. et al. Air pollution research: visualization of research activity using density-equalizing mapping and scientometric benchmarking procedures. J Occup Med Toxicol 5 , 5 (2010). https://doi.org/10.1186/1745-6673-5-5
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Received : 25 January 2010
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DOI : https://doi.org/10.1186/1745-6673-5-5
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