# Air pollution

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Air pollution is the presence of substances in the atmosphere that are harmful to humans and other living beings, or cause damage to the environment. Air pollution can be chemical, physical or biological. There are many different types of air pollutants, such as gases (including ozone, nitrogen oxides, sulfur dioxide, carbon monoxide, ammonia, and methane), particulates (such as soot), lead and biological molecules. Air pollution can cause diseases, allergies, and even death; it can also cause harm to animals and crops and damage the natural environment (for example, climate change, ozone depletion or habitat degradation) or built environment (for example, acid rain). Air pollution can occur naturally or be caused by human activities.
Air pollution causes around 7 or 8 million deaths each year. It is a significant risk factor for a number of pollution-related diseases, including heart disease, stroke, chronic obstructive pulmonary disease (COPD), asthma and lung cancer. It is the fourth-largest risk factor overall for human health as 99% of people are exposed to harmful levels of air pollution. Outdoor particulate pollution (PM2.5) is the largest cause of death (4.7 million), followed by indoor air pollution (3.1 million) and ozone (0.5 million).
The World Bank has estimated that welfare losses (premature deaths) and productivity losses (lost labour) caused by air pollution cost the world economy over $8 trillion per year. Air quality is closely related to the Earth's climate and ecosystems globally. Many of the contributors of local air pollution are also sources of greenhouse emission i.e., burning of fossil fuel.
Many different technologies and strategies are available for reducing air pollution. National air quality laws have often been highly effective, notably the 1956 Clean Air Act in Britain and the US Clean Air Act, introduced in 1963. Some of these efforts have been successful at the international level, such as the Montreal Protocol, which reduced the release of harmful ozone-depleting chemicals, while others, such as international action on climate change, have been less successful.

## Sources of air pollution

There are many different sources of air pollution. Some air pollutants (such as nitrogen oxides) originate mainly from human activities, while some (notably radon gas) come mostly from natural sources. However, many air pollutants (including dust and sulfur dioxide) come from a mixture of natural and human sources.

### Human sources

Most of the world's air pollution is from burning fossil fuels for industry, construction, transportation, and heating, although humans make air pollution in many other ways. For instance, nuclear weapons, toxic gases, germ warfare, and rocketry can cause air pollution.

#### Industry and construction

* Fossil fuel power plants and biomass power plants both have smoke stacks (see for example environmental impact of the coal industry)
* Oil and gas sites that have methane leaks
* Other industrial plants A 2014 study found that in China equipment-, machinery-, and devices-manufacturing and construction sectors contributed more than 50% of air pollutant emissions. This high emission is due to high emission intensity and high emission factors in its industrial structure.
* construction (including activities of building refurbishment), demolition, and renovation of buildings and other structures

#### Waste disposal

* waste incineration (incinerators as well as open and uncontrolled fires of mismanaged waste, making up about a fourth of municipal solid terrestrial waste)
* Waste deposition in landfills produces methane and open burning of waste releases harmful substances.

#### Transportation

* Mobile sources include cars, trucks, and other road vehicles; diesel trains; ships and other marine vessels; and aircraft, space rockets, and space debris. Vehicles with petrol and diesel engines produce about half of their emissions from their exhaust gas, and the other half from non-exhaust emissions (tyre and brake wear and erosion or disturbance of the road surface); electric vehicles produce no tailpipe emissions, but still produce the other emissions. Road vehicles make a significant amount of all air pollution (typically, for example, around a third to a half of all nitrogen dioxide emissions) and are a major driver of climate change.

#### Agriculture and farming

* Agriculture and forest management strategies using controlled burns. Practices like slash-and-burn in forests like the Amazon cause large air pollution with the deforestation. Controlled or prescribed burning is a practice used in forest management, agriculture, prairie restoration, and greenhouse gas reduction. Foresters can use controlled fire as a tool because fire is a natural feature of both forest and grassland ecology. Controlled burning encourages the sprouting of some desirable forest trees, resulting in a forest renewal.
* Agricultural emissions and emissions from meat production or livestock contribute substantially to air pollution For instance, methane is emitted by the digestion of food by cattle, causing ground-level ozone.
* Fertilized farmland may be a major source of nitrogen oxides.

#### Household sources

* burning of traditional biomass such as wood, crop waste and dung. In developing and poor countries, traditional biomass burning is the major source of air pollutants. It is also the main source of particulate pollution in urban areas of some developed countries including, the UK and Sydney, Australia.
* furnaces and other types of fuel-burning heating devices
* Fumes from paint, hair spray, varnish, aerosol sprays and other solvents. These can be substantial; emissions from these sources was estimated to account for almost half of pollution from volatile organic compounds in the Los Angeles basin in the 2010s.

### Natural sources

* Dust from natural sources, usually large areas of land with little or no vegetation.
* Radon gas from radioactive decay within the Earth's crust. Radon is a colorless, odorless, naturally occurring, radioactive noble gas that is formed from the decay of radium. 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. During periods of active wildfires, smoke from uncontrolled biomass combustion can make up almost 75% of all air pollution by concentration.
* Vegetation, in some regions, emits environmentally significant amounts of volatile organic compounds (VOCs) on warmer days. These VOCs react with human pollution sources – specifically, NO$_x$, SO$_2$, and organic carbon compounds – to produce a seasonal haze of secondary pollutants. Black gum, poplar, oak and willow are some examples of vegetation that can produce abundant VOCs. The VOC production from these species result in ozone levels up to eight times higher than the low-impact tree species.
* Volcanic eruptions, which produce mostly steam (about 79 percent), but also carbon dioxide (12 per cent), sulfur dioxide (6.5 percent), and small amounts of other pollutants, such as chlorine and ash particulates.

## Major pollutants

An air pollutant is a material in the air that can have many effects on humans and the ecosystem. The substance can be solid particles, liquid droplets, or gases, and often takes the form of an aerosol (solid particles or liquid droplets dispersed and carried by a gas). A pollutant can be of human or natural origin.
Pollutants are classified as primary or secondary. Primary pollutants are produced directly by a source and remain in the same chemical form after they have been emitted into the atmosphere. Examples include ash from a volcanic eruption, carbon monoxide gas from motor vehicle exhausts, and sulfur dioxide released from factories. Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants react or interact. Ground-level ozone is a prominent example of a secondary pollutant. Some pollutants may be both primary and secondary: they are both emitted directly and formed from other primary pollutants.

### Ammonia

Ammonia (NH$_3$) is emitted mainly by agricultural waste. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers. Although in wide use, ammonia is both caustic and hazardous. In the atmosphere, ammonia reacts with oxides of nitrogen and sulfur to form secondary particles.

### Carbon dioxide

Carbon dioxide (CO$_2$) is mainly emitted by the burning of fossil fuels. It is potentially lethal at very high concentrations (typically 100 times "normal" atmospheric levels). Although the World Health Organization recognizes CO$_2$ as a climate pollutant, it does not include the gas in its Air Quality Guidelines or set recommended targets for it. Workplace exposure limits exist in places like UK (5,000 ppm for long-term exposure and 15,000 ppm for short-term exposure). Natural disasters like the limnic eruption at Lake Nyos can result in a large sudden release as well.
CO$_2$ is sometimes called an air pollutant, because it is the main greenhouse gas responsible for climate change. This question of terminology has practical consequences, for example, in determining whether the U.S. Clean Air Act (which is designed to improve air quality) is deemed to regulate CO$_2$ emissions. The Inflation Reduction Act of 2022 amended the Clean Air Act to define CO$_2$ from fossil fuel burning explicitly as an air pollutant.

### Carbon monoxide

Carbon monoxide (CO) is a colorless, odorless, toxic gas. It is a product of combustion of fuel such as natural gas, coal or wood. In the past, emissions from vehicles were the main source of CO, but modern vehicles do not emit much CO. Now, wildfires and bonfires are the main source of outdoors CO. Indoors, CO is a larger problem and mainly comes from cooking and heating.

### Nitrogen oxides

Nitrogen oxides (NO$_x$), particularly nitrous oxide (NO), are mostly created by the burning of fossil fuels, and in lesser amounts by lightning. Nitrogen dioxide (NO$_2$) is formed from NO in a reaction with other atmospheric gases. NO and NO$_2$ can form acid rain, can form into a haze, and can cause nutrient pollution in water. NO$_2$ is a reddish-brown toxic gas with a strong odor, whereas NO is odorless and does not have a color.

### Particulate matter

Particulate matter (PM), also known as particle pollution, is a mix of microscopic solid particles or droplets suspended in a gas. Volcanoes, dust storms, forest and grassland fires, living plants, and sea spray are all sources of particles. Aerosols are produced by human activities such as the combustion of fossil fuels in cars, power plants, and industrial processes. Increased levels of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer; a definitive link between fine particulate pollution and higher death rates in urban areas was established by the Harvard Six Cities study, published in 1993. Particulates are related to respiratory infections and can be particularly harmful to those with conditions like asthma. Efforts to reduce particulate matter in the air may result in better health.

### Sulfur dioxide

Sulfur dioxide (SO$_2$) is produced by volcanoes and in various industrial processes. Coal and petroleum often contain sulfur compounds, and their combustion generates sulfur dioxide. High concentrations of SO2 in the air upon emissions generally also lead to the formation of other sulfur oxides (SOx). SOx can react with other compounds in the atmosphere to form small particles and contribute to particulate matter (PM) pollution [...] At high concentrations, gaseous SOx can harm plants by damaging foliage and decreasing growth. SO2 and other sulfur oxides can contribute to acid rain. Further oxidation of SO$_2$, usually in the presence of a catalyst such as NO$_2$, forms H$_2$SO$_4$, and thus acid rain is formed.

### Ground-level ozone

Ground-level ozone (O$_3$): Ozone is created when NOx and VOCs mix. Photochemical and chemical reactions involving it fuel many of the chemical activities that occur in the atmosphere during the day and night. It is a pollutant and a component of smog that is produced in large quantities as a result of human activities (mostly the combustion of fossil fuels). O$_3$ is largely produced by chemical reactions involving NO$_x$ gases (nitrogen oxides, especially from combustion) and volatile organic compounds in the presence of sunlight. Due to the influence of temperature and sunlight on this reaction, high ozone levels are most common on hot summer afternoons.

### Volatile organic compounds

Volatile organic compounds (VOC): VOCs are both indoor and outdoor air pollutants. They are categorized as either methane (CH$_4$) 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 and prolonging the life of methane in the atmosphere. This effect varies depending on local air quality. The aromatic NMVOCs benzene, toluene and xylene are suspected carcinogens and may lead to leukemia with prolonged exposure. 1,3-butadiene is another dangerous compound often associated with industrial use.

### Other pollutants

* Chlorofluorocarbons (CFCs): Emitted from goods that are now prohibited from use; harmful to the ozone layer. These are gases emitted by air conditioners, freezers, aerosol sprays, and other similar devices. CFCs reach the stratosphere after being released into the atmosphere. They interact with other gases here, causing harm to the ozone layer. UV rays are able to reach the Earth's surface as a result of this. This can result in skin cancer, eye problems, and even plant damage.
* Persistent organic pollutants, which can attach to particulates. Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental degradation due to chemical, biological, or photolytic processes. As a result, they've been discovered to survive in the environment, be capable of long-range transmission, bioaccumulate in human and animal tissue, biomagnify in food chains, and pose a major threat to human health and the ecosystem. The Stockholm Convention on Persistent Organic Pollutants identified pesticides and other persistent organic pollutants of concern. These include dioxins and furans which are unintentionally created by combustion of organics, like open burning of plastics, and are endocrine disruptors and mutagens.
* Persistent free radicals connected to airborne fine particles are linked to cardiopulmonary disease.
* Polycyclic aromatic hydrocarbons (PAHs): a group of aromatic compounds formed from the incomplete combustion of organic compounds including coal and oil and tobacco. Wood burning is another significant source of PAHs.
* Radioactive pollutants: Produced by nuclear explosions, nuclear events, war explosives, and natural processes such as the radioactive decay of radon.
* Toxic metals, such as lead and mercury, especially their compounds.
* Peroxyacetyl nitrate (C$_2$H$_3$NO$_5$): formed from NO$_x$ and VOCs, like ozone.
* Photochemical smog: particles are formed from gaseous primary contaminants and chemicals. Smog is a type of pollution that occurs in the atmosphere. Smog is caused by a huge volume of coal being burned in a certain region, resulting in a mixture of smoke and sulfur dioxide. Modern smog is usually caused by automotive and industrial emissions, which are acted on in the atmosphere by UV light from the sun to produce secondary pollutants, which then combine with the primary emissions to generate photochemical smog.
* Microplastics from construction, cosmetics, clothing, and many other sources.
There are many other chemicals classed as hazardous air pollutants. Some of these are regulated in the USA under the Clean Air Act and in Europe under numerous directives (including the Air "Framework" Directive, 96/62/EC, on ambient air quality assessment and management, Directive 98/24/EC, on risks related to chemical agents at work, and Directive 2004/107/EC covering heavy metals and polycyclic aromatic hydrocarbons in ambient air).
* Before flue-gas desulfurization was installed, the emissions from this power plant in New Mexico contained excessive amounts of sulfur dioxide.
* Thermal oxidisers are air pollution abatement options for hazardous air pollutants (HAPs), volatile organic compounds (VOCs), and odorous emissions.
* This video provides an overview of a NASA study on the human fingerprint on global air quality.

## Exposure

There is an important difference between air pollution emissions and exposure. Emissions are the quantity of pollutants released from a particular source; exposure is the quantity of a pollutant breathed in by people or other living things. Where air pollution laws may seek to limit the emissions of a particular pollutant, or the concentration in a particular environment, the health impacts depend on people's exposure to that pollutant. Based on the 2021 WHO air quality standards, 99% of the world population is exposed to harmful air pollution.
The risk of air pollution is determined by the pollutant's hazard and the amount of exposure to that pollutant. Air pollution exposure can be measured for a person, a group, such as a neighborhood or a country's children, or an entire population. For example, one would want to determine a geographic area's exposure to a dangerous air pollution, taking into account the various microenvironments and age groups. This can be calculated as an inhalation exposure. This would account for daily exposure in various settings, e.g. different indoor micro-environments and outdoor locations. The exposure needs to include different ages and other demographic groups, especially infants, children, pregnant women, and other sensitive subpopulations.
For each specific time that the subgroup is in the setting and engaged in particular activities, the exposure to an air pollutant must integrate the concentrations of the air pollutant with regard to the time spent in each setting and the respective inhalation rates for each subgroup, playing, cooking, reading, working, spending time in traffic, etc. A little child's inhaling rate, for example, will be lower than that of an adult. A young person engaging in strenuous exercise will have a faster rate of breathing than a child engaged in sedentary activity. The daily exposure must therefore include the amount of time spent in each micro-environmental setting as well as the kind of activities performed there. The air pollutant concentration in each microactivity/microenvironmental setting is summed to indicate the exposure.
For some pollutants such as black carbon, traffic related exposures may dominate total exposure despite short exposure times since high concentrations coincide with proximity to major roads or participation in (motorized) traffic. A large portion of total daily exposure occurs as short peaks of high concentrations, but it remains unclear how to define peaks and determine their frequency and health impact.
In 2021, the WHO halved its recommended guideline limit for tiny particles from burning fossil fuels. The new limit for nitrogen dioxide (NO$_2$) is 75% lower. Growing evidence that air pollution—even when experienced at very low levels—hurts human health, led the WHO to revise its guideline (from 10 μg/m$^3$ to 5 μg/m$^3$) for what it considers a safe level of exposure of particulate pollution, bringing most of the world—97.3 percent of the global population—into the unsafe zone.

## Indoor air quality

A lack of ventilation indoors concentrates air pollution where people often spend the majority of their time. Indoor air pollution can pose a significant health risk. According to EPA reports, the concentrations of many air pollutants can be two to five times higher in indoor air than in outdoor air. Indoor air pollutants can be up to 100 times higher in some cases than they are inside.

### Cooking fuel

As of 2023, more than 2.3 billion people in developing countries rely on burning polluting biomass fuels such as wood, dry dung, coal, or kerosene for cooking, which causes harmful household air pollution. Health effects are concentrated among women, who are likely to be responsible for cooking, and young children. The World Health Organization (WHO) estimates that cooking-related pollution causes 3.8 million annual deaths. The Global Burden of Disease study estimated the number of deaths in 2021 at 3.1 million. The problem is closely related to energy poverty and cooking.

### Other indoor air pollution

Indoor contaminants that can cause pollution include asbestos, biologic agents, building materials, radon, tobacco smoke, and wood stoves, gas ranges, or other heating systems.
Radon (Rn) gas, a carcinogen, is exuded from the Earth in certain locations and trapped inside houses. Building materials including carpeting and plywood emit formaldehyde (H-CHO) gas. Paint and solvents give off volatile organic compounds (VOCs) as they dry. Lead paint can degenerate into dust and be inhaled.
Intentional air pollution is introduced with the use of air fresheners, incense, and other scented items. Controlled wood fires in cook stoves and fireplaces can add significant amounts of harmful smoke particulates into the air, inside and out. Indoor pollution fatalities may be caused by using pesticides and other chemical sprays indoors without proper ventilation. Also the kitchen in a modern produce harmful particles and gases, with equipment like toasters being one of the worst sources.
Carbon monoxide poisoning and fatalities are often caused by faulty vents and chimneys, or by the burning of charcoal indoors or in a confined space, such as a tent. Chronic carbon monoxide poisoning can result even from poorly-adjusted pilot lights. Traps are built into all domestic plumbing to keep sewer gas and hydrogen sulfide, out of interiors. Clothing emits tetrachloroethylene, or other dry cleaning fluids, for days after dry cleaning.
Though its use has now been banned in many countries, the extensive use of asbestos in industrial and domestic environments in the past has left a potentially very dangerous material in many localities. Asbestosis is a chronic inflammatory medical condition affecting the tissue of the lungs. It occurs after long-term, heavy exposure to asbestos from asbestos-containing materials in structures. Those with asbestosis have severe dyspnea (shortness of breath) and are at an increased risk regarding several different types of lung cancer. Care should be taken to distinguish between several forms of related diseases. They may be defined as asbestosis, lung cancer, and peritoneal mesothelioma (generally a very rare form of cancer, when more widespread it is almost always associated with prolonged exposure to asbestos).
Biological sources of air pollution are also found indoors, as gases and airborne particulates. Pets produce dander, people produce dust from minute skin flakes and decomposed hair, dust mites in bedding, carpeting and furniture produce enzymes and micrometre-sized fecal droppings, inhabitants emit methane, mold forms on walls and generates mycotoxins and spores, air conditioning systems can incubate Legionnaires' disease and mold, and houseplants, soil and surrounding gardens can produce pollen, dust, and mold. Indoors, the lack of air circulation allows these airborne pollutants to accumulate more than they would otherwise occur in nature.

## Health effects

Air pollution is an important risk factor for various diseases, such as COPD (a common lung disease), stroke, heart disease, lung cancer and pneumonia. Indoor air pollution is also associated with cataract. Air pollution has further been linked to brain disorders, such as dementia, depression, anxiety and psychosis. Diseases that develop from persistent exposure to air pollution are environmental health diseases, which develop when a health environment is not maintained.
Pollutants strongly linked to negative health effects include particulate matter, carbon monoxide, nitrogen dioxide (NO$_2$), ozone (O$_3$), and sulphur dioxide (SO$_2$). Fine particulates are especially damaging, as they can enter the bloodstream via the lungs and reach other organs. Air pollution causes disease by driving inflammation and oxidative stress, suppressing the immune system and by damaging DNA.
Even at very low levels (under the WHO recommended levels), fine particulates can continue to cause harm. Children are highly vulnerable to air pollution as they breathe at a higher rate than adults. Because they are shorter, they breathe in air closer to the ground, which usually has higher pollution levels (for instance, from vehicle exhaust or dust). People living in poverty, babies and older people are also disproportionately affected by air pollution; pregnancy is also more risky when exposed to air pollution.

### Mortality

Estimates of deaths due to air pollution vary. The 2024 Global Burden of Disease Study estimates that air pollution contributed to 8.1 million deaths in 2021, which is more than 1 in 8 deaths. Outdoor particulate pollution (PM2.5) was the largest cause of death (4.7 million), followed by indoor air pollution (3.1 million) and ozone (0.5 million).
The WHO estimates that 6.7 million people die from air pollution each year, 4.2 million due to outdoor air pollution. Roughly 68% of outdoor air pollution-related premature deaths were due to ischaemic heart disease and stroke, 14% due to COPD and 14% due to lung infections (lower respiratory tract infections).
A study published in 2019 estimated that, for 2015, the number was around 8.8 million, with 5.5 million of these premature deaths due to air pollution from human sources. The global mean loss of life expectancy from air pollution in 2015 was 2.9 years, substantially more than, for example, 0.3 years from all forms of direct violence.

#### By region

India and China have the higher number of deaths from air pollution. In India, it contributed to 2.1 million deaths in 2021, whereas China saw 2.4 million deaths. In some countries, more than 20% of deaths were attributed to air pollution, for instance in Nepal, Bangladesh, Laos and North Korea. Air pollution deaths are high in middle-income countries due to industry and in low-income countries due to the use of solid fuels for cooking.
Annual premature European deaths from air pollution are estimated at 416,000 to 800,000. The UK saw some 17,000 deaths in 2021 due to air pollution. Nigeria, Indonesia and Pakistan each saw over 200,000 deaths resulting from air pollution.
Eliminating energy-related emissions in the United States would prevent 46,900–59,400 premature deaths each year and provide $537–$678 billion in benefits from avoided PM$_{2.5}$-related illness and death. A 2023 study on sulfur dioxide emissions by coal power plants (coal PM$_{2.5}$) concluded that "exposure to coal PM$_{2.5}$ was associated with 2.1 times greater mortality risk than exposure to PM$_{2.5}$ from all sources." From 1999 to 2020, a total of 460,000 deaths in the US were attributed to coal PM$_{2.5}$.

#### By source

The largest cause of air pollution is fossil fuel combustion – mostly the production and use of cars, electricity production, and heating. There are estimated 4.5 million annual premature deaths worldwide due to pollutants released by high-emission power stations and vehicle exhausts.
A study concluded that PM$_{2.5}$ air pollution induced by the contemporary free trade and consumption by the 19 G20 nations causes two million premature deaths annually, suggesting that the average lifetime consumption of about ~28 people in these countries causes at least one premature death (average age ~67) while developing countries "cannot be expected" to implement or be able to implement countermeasures without external support or internationally coordinated efforts.

### Cardiovascular disease

There is strong evidence that air pollution increases the risk of cardiovascular disease, including stroke, high blood pressure, and ischemic heart disease. According to the Global Burden of Disease Study, air pollution is responsible for 27% of deaths from strokes worldwide and 28% of ischemic heart disease. The risks are highest in regions with higher air pollution (i.e. Asia), for elderly and for people who are overweight.
Air pollution is a leading risk factor for stroke, particularly in developing countries where pollutant levels are highest. A systematic analysis of 17 different risk factors in 188 countries found air pollution is associated with nearly one in three strokes (29%) worldwide (34% of strokes in developing countries versus 10% in developed countries). The mechanisms linking air pollution to increased cardiovascular mortality are not fully understood, but likely systemic inflammation and oxidative stress.

### Lung disease

Air pollution has been associated with increased hospitalization and mortality and COPD (chronic obstructive pulmonary disease). COPD is a common disease which causes breathing difficulties and is the fourth-largest cause of death globally. Almost half of COPD deaths are due to air pollution. Fine particles (PM$_{2.5}$) or NO$_2$ were associated with increased risk of developing COPD.
Air pollution is further associated with increased risk of asthma and worsening of symptoms, and this effect seems stronger in children. For adults, fine particles (PM$_{2.5}$) or NO$_2$ are most strongly linked to asthma onset. Short-term exposure to ozone makes asthma worse in children. There is limited evidence on (almost) fatal asthma attacks in children: ground-level ozone and PM$_{2.5}$ seem to increase its risk.
The risk of lung disease from air pollution is greatest for infants and young children, whose normal breathing is faster than that of older children and adults; the elderly; those who work outside or spend a lot of time outside; and those who have heart or lung disease comorbidities.

### Cancer

Around 265,000 lung cancer deaths were attributed globally in 2019 to exposure to fine particulate matter, PM$_{2.5}$, suspended in the air. Exposure to indoor air pollution, including radon, caused another 170,000 lung cancer deaths. Lung cancer was also more common among people exposed to NO$_2$ and black carbon.
Outdoor air pollution may increase risk of other types of cancer too, but the evidence is not as clear as for lung cancer. For instance, there may be a relationship between kidney cancer and PM$_{2.5}$ and NO$_2$ levels. Household air pollution, for instance from cooking with solid fuels, but also from radon in building material, has been associated with cervical, oral, and esophageal cancer.

### Pregnancy and children

Stillbirths, miscarriages and birth defects are all more likely when the mother is exposed to air pollution during pregnancy. Exposure to air pollution also raises the chance a baby has a low birth weight. The impacts might be due to pollutants directly impacting the placenta or fetus, or indirectly as via the mother's health (as air pollution can cause systemic inflammation and oxidative stress).
Over a third of preterm births were associated with air pollution in 2021 globally. It causes more than half a million newborn deaths, a quarter of overall deaths. 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 wood and other biomass fuels being 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.
Including older children, polluted air leads resulted in the death of over 700,000 children in 2021 (709,000 under 5 years of age and 16,600 aged 5–14 years). Children in low or middle income countries are exposed to higher levels of fine particulate matter than those in high income countries. Further health effects of air pollution on children include asthma, pneumonia and lower respiratory tract infections. There is possibly a link between exposure to air pollution during pregnancy and after birth and autism in children.

### Brain health

Air pollution is linked to various diseases of the brain. Indoor air pollution exposure during childhood may negatively affect cognitive function and neurodevelopment. Prenatal exposure may also affect neurodevelopment. It may also contribute to neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. There are various routes by which air pollution could cause brain damage: it can cause neuroinflammation, or secondary effects from lung inflammation. At the same time, it can cause cardiovascular disease which in itself raises the risk of brain diseases.
Exposure to air pollution may also drive mental health issues, such as depression and anxiety. In particular, air pollution from the use of solid fuels was associated with a higher depression risk. Depression risk and suicide was more strongly linked to finer particulate matter (PM2.5), compared to coarser particles (PM10). The association was strongest for people over the age of 65.
Problems with thinking (cognitive issues) are also associated with air pollution. In people over the age of 40, both NO$_x$ and PM$_{2.5}$ have been linked to general cognitive problems. PM$_{2.5}$ was also associated with reduced verbal fluency (for instance, number of animals one can list in a minute) and worse executive functions (like attention and working memory). Similarly, children tended to fare worse in tests involving working memory when there was NO$_x$, PM$_{2.5}$, or PM$_{10}$ pollution.

## Agricultural effects

Various studies have estimated the impacts of air pollution on agriculture, especially ozone. A 2020 study showed that ozone pollution in California may reduce yields of certain perennial crops such as table grapes by as much as 22% per year, translating into economic damages of more than $1 billion per year. After air pollutants enter the agricultural environment, they not only directly affect agricultural production and quality, but also enter agricultural waters and soil. The COVID-19 induced lockdown served as a natural experiment to expose the close links between air quality and surface greenness. In India, the lockdown induced improvement in air quality, enhanced surface greenness and photosynthetic activity, with the positive response of vegetation to reduce air pollution was dominant in croplands. On the other hand, agriculture in its traditional form is one of the primary contributors to the emission of trace gases like atmospheric ammonia.

## Economic effects

A World Bank study found that PM$_{2.5}$ pollution in 2019 cost the world economy over $8 trillion, which was over 6% of global GDP. They suggested prioritising reducing pollution from diesel fuelled vehicles and coal-fired power plants.$^{:ix}$
The problem is most acute in the developing world. "Children under age 5 in lower-income countries are more than 60 times as likely to die from exposure to air pollution as children in high-income countries." Choosing an appropriate Concentration Response Function and Value of a Statistical Life can be difficult for researchers in low and middle income countries.
A study published in 2022 found "a strong and significant connection between air pollution and construction site accidents" and that "a 10-ppb increase in NO$_2$ levels increases the likelihood of an accident by as much as 25%".

## Historical disasters

The world's worst short-term civilian pollution crisis was the 1984 Bhopal Disaster in India. Leaked industrial vapours from the Union Carbide factory, belonging to Union Carbide, Inc., U.S.A. (later bought by Dow Chemical Company), killed at least 3787 people and injured from 150,000 to 600,000.
The United Kingdom suffered its worst air pollution event when the 4 December Great Smog of 1952 formed over London. In six days more than 4,000 died and more recent estimates put the figure at nearer 12,000. The worst single incident of air pollution to occur in the US occurred in Donora, Pennsylvania, in late October 1948, when 20 people died and over 7,000 were injured.
An accidental leak of anthrax spores from a biological warfare laboratory in the former USSR in 1979 near Yekaterinburg (formerly Sverdlovsk) is believed to have caused at least 64 deaths.

## Measurement and monitoring
### Monitoring

Air pollution can be monitored using different techniques. For instance, satellites and remote sensing is used to track PM, NO$_2$ and ozone. Many regions have a network of monitoring stations, with good coverage in India, China, Europe and the US. Poor coverage exist however for a number of highly-polluted countries, such as Chad and Iran. The density of measurements is improving as there are more low-cost techniques to measure air pollution. Low-cost monitors can also be used for indoor air quality monitoring. Finally, air quality sensors can be incorporated into drones to measure air pollution higher up in the air. Some websites attempt to map air pollution levels using available data.

### Air quality indexes

Air quality indexes (AQIs) offer a simple way for governments to communicate changes in air quality and associated health risks, especially during short-term pollution episodes, such as wildfires. An AQI is essentially a health protection tool people can use to help reduce their short-term exposure to air pollution by adjusting activity levels during increased levels of air pollution. Examples include Canada's Air Quality Health Index (AQHI), Malaysia's Air Pollution Index, and Singapore's Pollutant Standards Index.

### Emission factors

Air pollutant emission factors are reported representative values that aim to link the quantity of a pollutant released into the ambient air to an activity connected with that pollutant's release. The weight of the pollutant divided by a unit weight, volume, distance, or time of the activity generating the pollutant is how these factors are commonly stated (e.g., kilograms of particulate emitted per tonne of coal burned). These criteria make estimating emissions from diverse sources of pollution easier. Most of the time, these components are just averages of all available data of acceptable quality, and they are thought to be typical of long-term averages.
The United States Environmental Protection Agency has published a compilation of air pollutant emission factors for a wide range of industrial sources. The United Kingdom, Australia, Canada, and many other countries have published similar compilations, as well as the European Environment Agency.

## Reduction and regulation

Pollution prevention seeks to prevent pollution such as air pollution and could include adjustments to industrial and business activities such as designing sustainable manufacturing processes (and the products' designs) and related legal regulations as well as efforts towards renewable energy transitions.

### Industry

Various pollution control technologies and strategies are available to reduce air pollution. For instance, industrial plants can install scrubbers, such as flue gas desulfurization or catalysts to remove NO$_x$. Stringent environmental regulations, effective control technologies and shift towards the renewable source of energy also helping countries like China and India to reduce their sulfur dioxide pollution.
In the power sector, a very effective means to reduce air pollution is the transition to renewable energy or nuclear power. According to 2015 study 2015 the switch to 100% renewable energy in the United States would eliminate about 62,000 premature deaths per year and about 42,000 in 2050, if no biomass were used. This would save about $600 billion in health costs a year due to reduced air pollution in 2050, or about 3.6% of the 2014 U.S. gross domestic product.

### Transport

The avoid-shift-improve framework groups efforts to cut pollution from vehicles into reducing travel, shifting to sustainable transport, and improving vehicle technology. Reducing motor vehicle travel can curb pollution. One strategy is to build compact cities, so that amenities are close by and cars are not needed. Motor traffic can be reduced by creating more walkable cities and by investing in cycling infrastructure. Working from home is another way of avoiding motorised traffic. After Stockholm reduced motor vehicle traffic in the central city with a congestion tax, nitrogen dioxide and PM$_{10}$ pollution declined, as did acute asthma attacks in children.
Traffic can be shifted to cleaner modes of transport, for instance by increased use of public transport. The 9-Euro-Ticket scheme in Germany which allowed people to buy a monthly pass allowing use on all local and regional transport (trains, trams and busses) for 9 euro (€) for one month of unlimited travel saved 1.8 million tons of CO$_2$ emissions during its three-month implementation from June to August 2022.
Finally, road vehicles can be improved from increased fuel efficiency, conversion to cleaner fuels, and conversion to electric vehicles. For example, buses in New Delhi, India, have run on compressed natural gas since 2000, to help eliminate the city's "pea-soup" smog. Phase-out of fossil fuel vehicles is a critical component of a shift to sustainable transport. However, even in electric vehicles, rubber tires produce significant amounts of air pollution, ranking as 13th worst pollutant in Los Angeles.
Areas downwind (over 20 miles) of major airports have more than double total particulate emissions in air than other areas, even when factoring in areas with frequent ship calls, and heavy freeway and city traffic like Los Angeles. Aviation biofuel mixed in with jetfuel at a 50/50 ratio can reduce jet derived cruise altitude particulate emissions by 50–70%, according to a NASA led 2017 study (however, this should imply ground level benefits to urban air pollution as well).

### Cooking and heating

Heating and home stoves, which contribute significantly to regional air pollution, can be replaced with a much cleaner fossil fuel, such as natural gas, or, preferably, renewables, in poor countries. Biodigesters can be used in poor nations where slash and burn is prevalent, turning a useless commodity into a source of income. The plants can be gathered and sold to a central authority that will break them down in a large modern biodigester, producing much needed energy to use.
Combustion of fossil fuels for space heating can be replaced by using heat pumps and seasonal thermal energy storage.
Induced humidity and ventilation both can greatly dampen air pollution in enclosed spaces, which was found to be relatively high inside subway lines due to braking and friction and relatively less ironically inside transit buses than lower sitting passenger automobiles or subways.

### Waste and agriculture

Open dumps of waste are a common source of air pollution in low-income countries. The waste either self-ignites or is burned on purpose, releasing soot, methane and other pollutants. A growing number of countries regulates waste, which include national or city-wide waste management systems, opening managed landfills, landfill gas capture (for electricity production), and waste separation.

### Air quality modeling

Numerical models either on a global scale using tools such as GCMs (general circulation models coupled with a pollution module) or CTMs (Chemical transport model) can be used to simulate the levels of different pollutants in the atmosphere. These tools can have several types (Atmospheric model) and different uses. These models can be used in forecast mode which can help policy makers to decide on appropriate actions when an air pollution episode is detected. They can also be used for climate modeling including evolution of air quality in the future, for example the IPCC (Intergovernmental Panel on Climate Change) provides climate simulations including air quality assessments in their reports (latest report accessible through their site).

### Campaigning, public awareness and litigation

In 2022, the UN General Assembly passed a resolution recognizing the right to a clean, healthy, and sustainable environment as a human right.
In the United Kingdom, air pollution campaigning currently involves a mixture of grassroots activism (by groups such as Mums for Lungs and individual campaigners such as Rosamund Kissi-Debrah), public health awareness (through events such as Clean Air Day), legal work (advanced by activist lawyers such as ClientEarth), and more traditional campaigning (by environmental groups such as Greenpeace and Friends of the Earth, public health advocacy groups such as British Lung Foundation and Asthma UK, and organizations that raise health and safety issues, such as the British Safety Council). Citizen science projects combine scientific research with public health awareness raising and grassroots environmental campaigning.
In 2019, the Court of Justice of the EU, found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO$_2$) in 12 air quality zones.

### Laws and regulations

Although a majority of countries have air pollution laws, 43% of countries lack a legal definition of air pollution, 34% lack outdoor air quality standards, and just 31% have laws for tackling pollution originating from outside their borders. Few countries have limits that are as strict as the World Health Organisation's recommendations.
Some air pollution laws include specific air quality standards, such as the U.S. National Ambient Air Quality Standards and E.U. Air Quality Directive, which specify maximum atmospheric concentrations for specific pollutants. Other examples of air quality laws around the world include the Clean Air Act in Britain, the US Clean Air Act, and TA Luft in Germany. Some air pollution laws put limits on emissions (e.g. from vehicles), as well as air standards.
The World Health Organization's Global Air Quality Guidelines encourage improvements in a similar way to national standards, but are "recommendations" and "good practice" rather than mandatory targets that countries must achieve.

### Best practices

Cities and towns often work together, both nationally and internationally, to share best practices for improving air quality. Initiatives of this kind include the BreatheLife Network of 79 cities (a campaign launched in 2016 by the Climate and Clean Air Coalition, World Health Organization, United Nations Environment Programme, and World Bank), World Cities Day and the International Day of Clean Air for Blue Skies (two UN initiatives), and the Partnership for Healthy Cities (sponsored by the World Health Organization and health organization Vital Strategies).
Other networks include the C40 Cities Climate Leadership Group, a public 'non-state' network of the world's leading cities that aims to curb their greenhouse emissions. The C40 has been identified as 'governance from the middle' and is an alternative to intergovernmental policy. It has the potential to improve urban air quality as participating cities "exchange information, learn from best practices and consequently mitigate carbon dioxide emissions independently from national government decisions". A criticism of the C40 network is that its exclusive nature limits influence to participating cities and risks drawing resources away from less powerful city and regional actors.

### Indigenous people

Because Indigenous people frequently experience a disproportionate share of the effects of environmental degradation and climate change, even while they have made very little contribution to the processes causing these changes, environmental justice is especially important to them. Indigenous peoples have been marginalized and their lands and resources have been exploited as a result of historical and continuing colonization, institutional injustices, and inequality.
Indigenous groups frequently lack the political and financial clout to influence policy decisions that impact their lands and means of subsistence or to lessen the effects of climate change. This makes the already-existing inequalities in these communities' social, economic, and health conditions worse. Furthermore, traditional ecological knowledge and Indigenous knowledge systems provide insightful information about sustainable resource management and climate change adaptation techniques. To promote persistence and environmental justice, Indigenous viewpoints must be acknowledged and integrated into efforts to mitigate the effects of climate change and adapt to them.
Combating climate change necessitates an all-encompassing strategy that recognizes the interdependence of social, economic, and environmental elements. This entails defending treaty rights, advancing Indigenous sovereignty and self-determination, and aiding Indigenous-led projects for sustainable development and environmental preservation.

## Hotspots

Air pollution hotspots are areas where air pollution emissions expose individuals to increased negative health effects. They are particularly common in highly populated, urban areas, where there may be a combination of stationary sources (e.g. industrial facilities) and mobile sources (e.g. cars and trucks) of pollution. Fine particulate matter such as diesel soot, which contributes to more than 3.2 million premature deaths around the world each year, is a significant problem. It is very small and can lodge itself within the lungs and enter the bloodstream. Diesel soot is concentrated in densely populated areas, and one in six people in the U.S. live near a diesel pollution hot spot, such as Cancer Alley along the Mississippi River.
While air pollution hotspots affect a variety of populations, some groups are more likely to be located in hotspots. Previous studies have shown disparities in exposure to pollution by race and/or income. Hazardous land uses (toxic storage and disposal facilities, manufacturing facilities, major roadways) tend to be located where property values and income levels are low. Low socioeconomic status can be a proxy for other kinds of social vulnerability, including race, a lack of ability to influence regulation and a lack of ability to move to neighborhoods with less environmental pollution. These communities bear a disproportionate burden of environmental pollution and are more likely to face health risks such as cancer or asthma.
Studies show that patterns in race and income disparities not only indicate a higher exposure to pollution but also higher risk of adverse health outcomes. Communities characterized by low socioeconomic status and racial minorities can be more vulnerable to cumulative adverse health impacts resulting from elevated exposure to pollutants than more privileged communities. Blacks and Latinos generally face more pollution than Whites and Asians, and low-income communities bear a higher burden of risk than affluent ones. Racial discrepancies are particularly distinct in suburban areas of the Southern United States and metropolitan areas of the Midwestern and Western United States. Residents in public housing, who are generally low-income and cannot move to healthier neighborhoods, are highly affected by nearby refineries and chemical plants.

### Cities

Air pollution is usually concentrated in densely populated metropolitan areas, especially in developing countries where cities are experiencing rapid growth and environmental regulations are relatively lax or nonexistent. Urbanization leads to a rapid rise in premature mortality due to air pollution in fast-growing tropical cities. However, even populated areas in developed countries have unhealthy levels of pollution, with Los Angeles and Rome being two examples. Between 2002 and 2011 the incidence of lung cancer in Beijing near doubled. While smoking remains the leading cause of lung cancer in China, the number of smokers is falling while lung cancer rates are rising .
| World's Most Polluted Cities 2020 | 2020 Average | 2019 Average |
| Hotan, China | 110.2 | 110.1 |
| Ghaziabad, India | 106.6 | 110.2 |
| Bulandshahr, India | 98.4 | 89.4 |
| Bisrakh Jalalpur, India | 96.0 | - |
| Bhiwadi, India | 95.5 | 83.4 |
Tehran was declared the most polluted city in the world on May 24, 2022.