Air Pollution

This section looks at an air pollution definitioncauses of air pollution, effects of air pollutionair pollution solutions and air pollution facts. The latest air pollution news is included at the foot of the page.

Air pollution definition: the presence in or introduction into the air of substances (gaseous, liquid or solid) which have harmful effects.

Causes of air pollution

Air pollution is caused by harmful man-made and natural substances being introduced into the Earth's atmosphere. Air pollution deaths run into the millions every year but according to reports by the World Health Organisation, more deaths are linked to indoor sources of air pollution than to outdoor sources.

Air pollution types                     

Air pollution can be caused by liquid, gaseous or solid substances. These substances have harmful impacts on the natural world - including humans. Air pollution can be classified into indoor and outdoor sub-types.


Outdoor air pollution is a huge environmental health issue affecting the populations in developing and developed  countries across the globe. Outdoor air pollution in both urban and rural areas was estimated - by the World Health Organisation - to cause 3 million premature deaths worldwide per year in 2012; this death rate is caused by exposure to small airbourne particles 10 microns or less in diameter (1 micron = 1/1000th of a millimeter).


Indoor air pollution from smoke is a major health risk for around 3 billion people who cook and warm their dwellings with carbon-based fuels and coal. In 2012, around 4.3 million premature deaths were linked to domestic air pollution. Indoor air pollution disproportionately impacts low-middle-income countries.


Primary and secondary pollutants


Pollutants can be described as either primary or secondary. Primary pollutants usually derive from a distinct process. For example, ash emissions from a volcanic eruption.


Secondary pollutants are those which typically form as a result of primary pollutants interacting or reacting with other substances.


Man-made causes of air pollution

Fixed sources include power plant smoke stacks, factories and manufacturing facilities, incinerators of waste along with furnaces and other types of fossil fuel-burning heating devices.


In poor and developing countries, traditional biomass burning is the main generator of airbourne pollutants; traditional biomass includes crop waste, dung and wood.

Mobile sources of air pollution include automotive vehicles on land, sea and air.

Controlled burn practices in agriculture and forest management also make a significant contribution - 'controlled' or 'prescribed' burning is a technique used in forest management, land management and farming. Fire is a natural part of both forest and grassland ecology but it is also used in a 'controlled' way to stimulate the germination of some desirable forest trees, thus renewing the forest.


Waste deposited in landfills can generate methane. Methane is highly flammable and can form highly flammable mixtures with air. Methane can also act to displace oxygen in an enclosed space through a process known as asphyxiation.

Military weaponary such as nuclear weapons, toxic gases, germ warfare and rockets are all known contributors to air pollution.

Fumes from paint, varnish, aerosol sprays and other solvents.

Natural causes of air pollution

Dust from natural sources, usually large areas of land with little or no vegetation. The world's major deserts are key contributors.

Methane, emitted by the digestion of food by animals, for example cattle.

Radon gas from radioactive decay within the Earth's crust. It is considered to be a health hazard. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as the basement and it is the second most frequent cause of lung cancer, after cigarette smoking.


Smoke and carbon monoxide from wildfires.

Vegetation can release large amounts of Volatile organic compounds (VOCs) into the atmosphere. These VOCs react with primary man-made pollutants especially carbon, nitrogen and sulphur-based compounds — to produce hazes of secondary pollutants typically following a seasonal pattern. Poplar, oak and willow are some examples of vegetation that contribute to VOC production. The VOC production from these species can have a massive impact on local ozone levels.

Volcanic activity, which produces sulphur, chlorine, and ash particulates emits millions of tons of material into the air each year. The US Geological Survey estimate that between 180-440 million tons of carbon dioxide is released through volcanic activity each year. See also the aerosol pollution pages.

China's toxic smog - 2017 BBC report revealing fossil fuel and motor vehicle dependence is creating an ever-increasing problem in urban areas

The many different types of man-made air pollutants

Man-made air pollutants come in all shapes, sizes and chemical composition

Primary pollutants include:


Ammonia (NH3): Generated by agricultural processes, ammonia is a compound (a compound is a substance made up of two or more elements) with the formula NH3. Comprised of nitrogen and hydrogen, it is usually found in gaseous form with a strong, distinctively pungent smell. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a key component of foodstuffs and fertilisers. Ammonia is also a key component for the production of many pharmaceuticals. Although in wide use, ammonia is both caustic (able to burn organic matter through chemical action) and hazardous. In the atmosphere, ammonia reacts with oxides of nitrogen and sulfur to form secondary particles. Ammonia plays a key role in nitrogen runoff, an environmental threat with its own section on this site.


Carbon dioxide (CO2): Carbon dioxide is a natural component of the atmosphere, essential for plant life and given off by the human respiratory system. CO2 currently forms about 405 parts per million (ppm) of Earth's atmosphere, compared to about 280 ppm in pre-industrial times, and billions of metric tons of CO2 are emitted annually by burning of fossil fuels. CO2 increases in Earth's atmosphere has been accelerating and is a major contributor to greenhouse gas emissions contributing to global warming.


Carbon monoxide (CO): CO is a colorless, odorless, toxic yet non-irritating gas. It is a product of incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of carbon monoxide.


Chlorofluorocarbons (CFCs): Harmful to the ozone layer; emitted from products are currently banned from use. These are gases which are released from air conditioners, refrigerators, aerosol sprays, etc. On release into the air, CFCs rise to the stratosphere (the layer of atmosphere ranging between 10 and 50km above the surface of the Earth). Here they come in contact with other gases and damage the ozone layer. This allows harmful ultraviolet rays to reach the Earth's surface. This can lead to skin cancer, eye disease and can even cause damage to plants.


Nitrogen oxides (NOx): Nitrogen oxides, particularly nitrogen dioxide, are expelled from high temperature combustion, and are also produced during thunderstorms by electric discharge. They can be seen as a brown haze dome above or a plume downwind of cities. Nitrogen dioxide is a chemical compound with the formula NO2. It is one of several nitrogen oxides (compounds containing oxygen atoms). One of the most problemmatic air pollutants, this reddish-brown toxic gas has a characteristic sharp, biting odor.

Odours: Such as from garbage, sewage, and industrial processes.


Particulates: Also referred to as particulate matter (PM), atmospheric particulate matter, or fine particles, are tiny particles (typically with a size less than 10 micometers - or 1/100th of a millimeter) of solid or liquid matter. Some particulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes also generate significant amounts of aerosols. Increased levels of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer.


Radioactive pollutants: Produced by nuclear explosions, nuclear events, war explosives, and natural processes such as the radioactive decay of radon.


Sulfur oxides (SOx): Particularly sulphur dioxide, a chemical compound with the formula SO2. Sulphur dioxide is produced by volcanoes and in various industrial processes. Coal and petroleum often contain sulphur compounds, and their combustion generates sulphur dioxide. Further oxidation of sulphur dioxide (adding of additional oxygen), usually in the presence of a catalyst such as nitrogen dioxide - NO2, forms acid rain - sulphuric acid - H2SO4. This is one of the causes for concern over the environmental impact of the use of these fuels as power sources.


Toxic metals: Such as lead and mercury, especially their compounds.


Volatile organic compounds (VOC):  VOCs are a well-known outdoor air pollutant. They are categorized as either methane - CH4 or non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon VOCs are also significant greenhouse gases because of their role in creating ozone - O3 and prolonging the life of methane in the atmosphere. This effect varies depending on local air quality. 


Secondary pollutants include:


Ground level ozone formed from nitrogen oxides and VOCs: Ozone - O3 is a key constituent of the troposphere (the lowest layer of the Earth's atmosphere extending up to 10km above the surface of the planet). It is also an important constituent of certain regions of the stratosphere commonly known as the ozone layer. Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night. At abnormally high concentrations brought about by human activities (largely the combustion of fossil fuel), it is a pollutant, and a constituent of smog.


Particulates: Created from gaseous primary pollutants and compounds in photochemical smog. Smog is a kind of air pollution. Classic smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulfur dioxide. Modern smog does not usually come from coal but from vehicular and industrial emissions that are acted on in the atmosphere by ultraviolet light from the sun to form secondary pollutants that also combine with the primary emissions to form photochemical smog.

Focus on: environmental impact of cars

Cars and other vehicles are often behind the negative media environmental headlines. Is the bad press deserved?

Cars and diesel engines in particular are often identified as a leading cause of environmental damage. The truth is, automotive vehicles have a significant environmental impact both before and after they become used for their primary purpose.


Automotive production requires significant energy consumption. Materials including rubber, steel, glass, paint and plastics, batteries, electrical components and chemical inputs must be created, manufactured and transported before vehicles leave the production line.


At the end of a vehicle's life, all these materials must be scrapped or recycled. The motor industry has made improvements to the green credentials of vehicles in this regard. According to a National Geographic article, about 75% of a modern car can be recycled. The same article also claims that between 80 to 90% of a vehicle's environmental impact occurs when the vehicle is on the road. Fuel consumption and the resultant emissions created from the fuel combustion process are a major contributor to air pollution. In the US, vehicle emissions are believed to account for between a third and a half of air pollution. The way in which pollutants are delivered from vehicles i.e. at low (human inhalation) level and in typically populous areas means the health impacts are accentuated.


Petroleum products carry a heavy environmental cost (extraction, transportation and refinement) before they even make it to the garage forecourt. Pipelines destroy natural habitats and numerous high-profile oil spillages remind us that ecological disaster is never far away.


The combustion process and the resultant cocktail of chemicals that make up vehicle emissions is where most environmental focus falls. The emissions can form primary pollutants in their own right or go on to combine with other substances to form secondary pollutants.


The main outputs of petroleum combustion are nitrogen, water and carbon dioxide.


Included in the total emissions package cocktail are various environmental undesirables: carbon dioxide - CO2, carbon monoxide - CO, nitrogen oxides - NOx, particulate matter (PM) and unburned hydrocarbons - HC. Particulate matter containing soot and metals create smog. The small size of these particles makes them particularly dangerous for human health, particle diameters less than 2.5 micrometers can be inhaled deep into the lungs. Hydrocarbons can react with nitrogen oxides to form ground level ozone - another key ingredient of smog. Sulphur-containing diesel fuels can also create sulphur dioxide with well-documented health risks to the young and asthmatics in particular.


The cocktail doesn't stop there. Also included are heavy metals, compounds of mercury, manganese, chromium, cobalt and arsenic. Methanol, phosperous, benzene, acetaldehyde and butadiene. The list goes on. Many of these chemicals have recognised serious health consequences including strong causal links as a carcinogenic source (cancer causing).


The answer appears compelling. Vehicle emissions are an extremely serious pollution issue on many environmental fronts.

Effects of air pollution on health

Air pollution is a major risk factor for a number of health conditions and diseases including respiratory infections, heart disease, Chronic Obstructive Pulminary Disease, stroke and lung cancer. The health effects caused by air pollution can include difficulty in breathing, wheezing, coughing, asthma and worsening of existing respiratory and cardiac conditions.  Individual reactions to air pollutants depend on the type of pollutant a person is exposed to, the degree of exposure, and the individual's health status and genetics. The World Health Organisation estimated that every year, air pollution causes the premature death of some 7 million people worldwide. Children and infants are particularly at risk due to the immaturity of their respiratory organ systems. 

Cardiovascular disease

A 2007 review of research evidence found ambient air pollution exposure is a risk factor correlating with increased total death from cardiovascular health problems. Air pollution is also emerging as a risk factor for stroke, particularly in developing countries where pollutant levels are highest. Air pollution was also found to be associated with increased incidence and mortality from coronary stroke in a 2011 study. Air pollution is also associated with the constriction of blood vessels (vasoconstriction), low-grade inflammation and the clogging of arteries with fatty material (atherosclerosis). Other mechanisms such as autonomic nervous system imbalance have also been suggested. The autonomic nervous system is responsible for control of the bodily functions not consciously directed, such as breathing, hearthbeat and digestive processes.


Lung disease

Chronic obstructive pulmonary disease (COPD) includes diseases such as chronic bronchitis and emphysema. Emphysema is a long-term progressive disease of the lungs that primarily causes shortness of breath - affecting the air sacs (alveoli) of the lungs.


Research has demonstrated increased risk of developing asthma and COPD from increased exposure to traffic-related air pollution. Additionally, air pollution has been associated with increased hospitalization and mortality from asthma and COPD.


It is believed that much like cystic fibrosis, by living in a more urban environment serious health hazards become more apparent. Studies have shown that in urban areas patients suffer mucus hypersecretion, lower levels of lung function, and more self-diagnosis of chronic bronchitis and emphysema.



A review of evidence regarding whether ambient air pollution exposure is a risk factor for cancer in 2007 found solid data to conclude that long-term exposure to PM2.5 (fine particulates with a diameter of 2.5 micometers or less) increases the overall risk of non-accidental mortality. Exposure to PM2.5 was also associated with an increased risk of mortality from lung cancer and total cardiovascular mortality.


The review further noted that living close to busy traffic appears to be associated with elevated risks of three major outcomes:

1) increase in lung cancer deaths 2) increase in cardiovascular deaths and 3) increase in overall non-accidental deaths. 


In 2011, a large Danish epidemiological study (the study and analysis of patterns, causes and effects of health and disease conditions in pre-defined populations) found an increased risk of lung cancer for patients who lived in areas with high nitrogen oxide concentrations. In this study, the association was higher for non-smokers than smokers. An additional Danish study, also in 2011, likewise noted evidence of possible associations between air pollution and other forms of cancer, including cervical cancer and brain cancer.



In the United States, despite the passage of the Clean Air Act in 1970, in 2002 at least 146 million Americans were living in non-attainment areas—regions in which the concentration of certain air pollutants exceeded federal standards. These dangerous pollutants are known as the criteria pollutants, and include ozone, particulate matter, sulphur dioxide, nitrogen dioxide, carbon monoxide, and lead. Protective measures to ensure children's health are being taken in cities such as New Delhi, India where buses now use compressed natural gas to help eliminate the "pea-soup" smog. A recent study in Europe has found that exposure to ultrafine particles can increase blood pressure in children.



Ambient levels of air pollution have been associated with preterm birth and low birth weight. A 2014 World Health Organisation (WHO)  worldwide survey on maternal and perinatal health found a statistically significant association between low birth weights (LBW) and increased levels of exposure to PM2.5. Women in regions with greater than average PM2.5 levels had statistically significant higher odds of pregnancy resulting in a low-birth weight infant even when adjusted for country-related variables. The effect is thought to be from stimulating inflammation and increasing oxidative stress.


A study by the University of York found that in 2010 exposure to PM2.5 was strongly associated with 18% of preterm births globally, which was approximately 2.7 million premature births. The countries with the highest air pollution associated preterm births were in South and East Asia, the Middle East, North Africa, and West sub-Saharan Africa.


The source of PM 2.5 differs greatly by region. In South and East Asia, pregnant women are frequently exposed to indoor air pollution because of the wood and other biomass fuels used for cooking which are responsible for more than 80% of regional pollution. In the Middle East, North Africa and West sub-Saharan Africa, fine PM comes from natural sources, such as dust storms. The United States had an estimated 50,000 preterm births associated with exposure to PM2.5 in 2010.


Studies have looked at the relationship between air pollution and proximity to a highway with pregnancy outcomes in a Vancouver cohort of pregnant woman using addresses to estimate exposure during pregnancy. Exposure to NO, NO2, CO PM10 and PM2.5 were associated with infants born small for gestational age (SGA). Women living <50meters away from an expressway or highway were 26% more likely to give birth to a SGA infant.


"Clean" areas

Even in the areas with relatively low levels of air pollution, public health effects can be significant and costly, since a large number of people breathe in such pollutants. A 2005 scientific study for the British Columbia Lung Association showed that a small improvement in air quality (1% reduction of ambient PM2.5 and ozone concentrations) would produce $29 million in annual savings in the Metro Vancouver region in 2010. This finding is based on health valuation of lethal (death) and sub-lethal (illness) affects.


Central nervous system

Data is accumulating that air pollution exposure also affects the central nervous system.


In a June 2014 study conducted by researchers at the University of Rochester Medical Center, published in the journal Environmental Health Perspectives, it was discovered that early exposure to air pollution causes the same damaging changes in the brain such as autism and schizophrenia. The study also shows that air pollution also affected short-term memory, learning ability, and impulsivity. The findings add to the growing body of evidence that air pollution may play a role in autism, as well as in other neurodevelopmental disorders. Air pollution has a more significant negative effect on males than on females.

Air pollution solutions

Government actions to reduce air pollution

Most sources of outdoor air pollution are well beyond the control of individuals and demand action by cities, as well as national and international policymakers in sector like transport, energy waste management, buildings and agriculture.


There are many examples of successful policies in transport, urban planning, power generation and industry that reduce air pollution:


for industry: clean technologies that reduce industrial smokestack emissions; improved management of urban and agricultural waste, including capture of methane gas emitted from waste sites as an alternative to incineration (for use as biogas);

for transport: shifting to clean modes of power generation; prioritizing rapid urban transit, walking and cycling networks in cities as well as rail interurban freight and passenger travel; shifting to cleaner heavy duty diesel vehicles and low-emissions vehicles and fuels, including fuels with reduced sulfur content;

for urban planning: improving the energy efficiency of buildings and making cities more compact, and thus energy efficient;
for power generation: increased use of low-emissions fuels and renewable combustion-free power sources (like solar, wind or hydropower); co-generation of heat and power; and distributed energy generation (e.g. mini-grids and rooftop solar power generation);

for municipal and agricultural waste management: strategies for waste reduction, waste separation, recycling and reuse or waste reprocessing; as well as improved methods of biological waste management such as anaerobic waste digestion to produce biogas, are feasible, low cost alternatives to the open incineration of solid waste.


Where incineration is unavoidable, then combustion technologies with strict emission controls are critical.

Personal actions to reduce air pollution

A significant proprotion of all air pollution is caused by the combustion of fossil fuels - oil and coal in particular. The reduction of fossil fuel demand is a pivotal focal point for personal pollution reduction actions. 


An increasing drive towards renewable, clean energy sources is something we can all do through personal choice. Wind power, solar power and hydro power are all currently available as domestic sources of energy supply.


Motor vehicles driven by fossil fuels can be replaced with vehciles using electric motors either fully or in hybrid engines.


Diesel and petrol vehicles can be driven in more fuel efficient ways by the driver. Appropriate gear changing, brake application, judicious use of throttle can reduce emissions.


Transportation choices to reduce pollution footprint include switching from air transport to train where possible. Using public transport, avoiding automotive options entirely and walking or cycling.


Insulating premises can help reduce heating requirments and in turn (if heating is provided through fossil fuels), reduce the carbon footprint.


Ammonia and nitrogen pollution through industrial-scale farming practices (fertiliser and animal excretions) strongly suggests that sourcing foods from organic / chemical-free channels and reducing meat consumption would be doing the planet a great favour. 


The worlds forests, woods and trees act as the planet's lungs. They remove carbon dioxide from the atmosphere and release oxygen in a process called photosynthesis.


Choosing ethically-sourced wood and avoiding palm oil products are just two ways that you can help to ensure that trees remain plentiful and that you are doing your bit to help keep the air clean. 





Air pollution news

For the latest air pollution news stories and other environmental news, check out our news page


Air pollution news, including commentary and archival articles published in The New York Times.


Air pollution news, including commentary and archival articles published in The Independent.


Air pollution news, including commentary and archival articles published in ScienceDaily.


Pollution news, including commentary and archival articles published in the Guardian.







Air pollution facts

Indoor air pollution kills more people globally per annum (4 million) than outdoor air pollution (3 million)

According to a World Health Organisation (WHO) report, air pollution in 2012 caused the deaths of around 7 million people worldwide.

92% of the global population are living in a polluted air environment

According to the WHO, in 2014, 92% of the world's population were living in areas where the WHO air quality guidelines were not met.

72% of outdoor air pollution deaths linked to heart disease and 14% linked to lung cancer

WHO estimates that in 2012, some 72% of outdoor air pollution-related premature deaths were due to ischaemic heart disease and strokes, while 14% of deaths were due to chronic obstructive pulmonary disease or acute lower respiratory infections, and 14% of deaths were due to lung cancer.

20,00 litres a day

The average human will inhale 20,000 litres of air in a day.

405 parts per million

Current carbon dioxide levels are around the 405 parts per million mark (and rising). This compares with pre-industrial measures around 280 ppm.


Why does this matter? Because of the link between CO2 levels and global temperatures. See the global warming pages for more.

Farming is a massive pollution concern in Europe

A recent article by the Guardian states that: "Farming is the biggest single cause of the worst air pollution in Europe, as nitrogen compounds from fertilisers and animal waste drift over industrial regions.


When the nitrogen compounds are mixed with air already polluted from industry, they combine to form solid particles that can stick in the fine lung tissue of children and adults, causing breathing difficulties, impaired lungs and heart function, and eventually even premature death."

Air Pollution News -- ScienceDaily

Unexpected wildfire emission impacts air quality worldwide (Mon, 21 Sep 2020)
During wildfires, nitrous acid plays a leading role -- spiking to levels significantly higher than scientists expected, driving increased ozone pollution and harming air quality, according to a new study.
>> Read more

NASA monitors carbon monoxide from California wildfires (Tue, 15 Sep 2020)
The observations from Earth orbit show high-altitude concentrations of the gas that are more than 10 times typical amounts.
>> Read more

Volcanic ash may have a bigger impact on the climate than we thought (Fri, 11 Sep 2020)
Volcanic ash shuts down air traffic and can sicken people. But a new study suggests that it may also be more important for Earth's climate than scientists once thought.
>> Read more

Multiphase buffering by ammonia explains wide range of atmospheric aerosol acidity (Thu, 10 Sep 2020)
Anthropogenic ammonia emissions and the water content matter more than dry particle composition for the acidity of atmospheric aerosols in populated regions.
>> Read more

New insight on the impacts of Earth's biosphere on air quality (Wed, 09 Sep 2020)
A new study provides the first global satellite measurements of one of the most important chemicals affecting Earth's atmosphere.
>> Read more

Lockdown did not reduce 'most harmful' type of air pollution in Scotland (Tue, 08 Sep 2020)
The significant reduction in vehicle journeys during the COVID-19 lockdown did not reduce the level of toxic fine particles in Scotland's air, according to experts at the University of Stirling.
>> Read more

Pollination: Air pollution renders flower odors unattractive to moths (Fri, 04 Sep 2020)
Researchers showed that tobacco hawkmoths lost attraction to the scent of their preferred flowers when that scent had been altered by ozone. This oxidizing pollutant thus disturbs the chemical communication between a plant and its pollinator. However, when given the chance, hawkmoths quickly learn that an unpleasantly polluted scent may lead to nutritious nectar.
>> Read more

Asphalt adds to air pollution, especially on hot, sunny days (Wed, 02 Sep 2020)
Asphalt is a near-ubiquitous substance -- it's found in roads, on roofs and in driveways -- but its chemical emissions rarely figure into urban air quality management plans. A new study finds that asphalt is a significant source of air pollutants in urban areas, especially on hot and sunny days.
>> Read more

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