Air pollution is the presence of substances in the air that are harmful to humans, other living beings or the environment. Pollutants can be gases, like ozone or nitrogen oxides, or small particles like soot and dust. Both outdoor and indoor air can be polluted.
Outdoor air pollution comes from burning fossil fuels for electricity and transport, wildfires, some industrial processes, waste management, demolition and agriculture. Indoor air pollution is often from burning firewood or agricultural waste for cooking and heating. Other sources of air pollution include dust storms and volcanic eruptions. Many sources of local air pollution, especially burning fossil fuels, also release greenhouse gases that cause global warming. However, air pollution may limit warming locally.
Air pollution kills 7 to 8 million people each year. It is a significant risk factor for a number of diseases, including stroke, heart disease, chronic obstructive pulmonary disease (COPD), asthma, coronavirus and lung cancer. Particulate matter is the most deadly, both for indoor and outdoor air pollution. Ozone affects crops, and forests are damaged by the pollution that causes acid rain. Overall, the World Bank has estimated that welfare losses (premature deaths) and productivity losses (lost labor) caused by air pollution cost the world economy over $8 trillion per year.
Various technologies and strategies reduce air pollution. Key approaches include clean cookers, fire protection, improved waste management, dust control, industrial scrubbers, electric vehicles and renewable energy. National air quality laws have often been effective, notably the 1956 Clean Air Act in Britain and the 1963 US Clean Air Act. International efforts have had mixed results: the Montreal Protocol almost eliminated harmful ozone-depleting chemicals, while international action on climate change has been less successful.
Human sources
Industry and construction
Burning fuel to generate electricity causes air pollution; lignite and coal produce the most air pollution, followed by oil, and then by fossil gas and biomass. Methane leaks are common in oil and gas production, and oil refineries emit a wide range of pollutants. Some hazardous air pollutants are produced in plastic and rubber production, whereas chloroform can be produced during water chlorination, and arsenic is found in the mining industry. Many polluting industries have been pushed out of richer nations, and China too has started to push its most polluting industries out of the country.
Construction and demolition produces dust, but also other pollutants. The direct particles from construction and demolition are relatively coarse. Construction also has an indirect impact on air quality, as cement production is one of the main sources of particle pollution. Though banned in many countries, asbestos persists in older buildings, where it poses a risk of lung disease when disturbed. Building materials including carpeting and plywood emit formaldehyde, a gas which can cause difficulty breathing and nausea.
Transportation
Road vehicles produce a significant amount of all air pollution. For instance, they may be responsible for a third to half of all nitrogen dioxide emissions, and are a major cause of climate change. Vehicles with petrol and diesel engines produce about half of their emissions from their exhaust gas, and the other half from non-exhaust emissions (tire and brake wear and erosion or disturbance of the road surface); electric vehicles produce no tailpipe emissions, but still produce the other emissions. Diesel trains, ships, and planes also cause air pollution.
Agriculture and waste
Agricultural emissions, both from crops and from animal agriculture, contribute substantially to air pollution. For instance, methane is emitted by the digestion of food by cattle, causing ground-level ozone. Agriculture is also a major source of ammonia, which can form fine particulate matter. Practices like slash-and-burn in forests like the Amazon cause large air pollution alongside deforestation.
Open dumps of waste are a common source of air pollution in low-income countries. They can be a source of toxins and can promote the growth of microbes that pollute water and air. Through open burning of waste—whether self-ignited or burned on purpose—soot, methane, and other pollutants are released. Organic waste in landfills itself also produces methane as it decomposes. Globally, a quarter of solid waste is not collected and another quarter is not disposed of properly.
Household sources
As of 2023, more than 2.3 billion people in developing countries rely on burning polluting fuels such as firewood, agricultural waste, dry dung, coal, or charcoal for cooking, which causes harmful household air pollution. Kerosene, another polluting fuel, is used in many countries for lighting and sometimes for space heating or cooking. Globally, 12% of outdoor fine particle pollution comes from household cooking. Health effects are concentrated among women, who are likely to be responsible for cooking, and young children.
Gas stoves for cooking contribute to indoor air pollution by emitting NO2, benzene, and carbon monoxide. Toasters can produce particulate pollution. Similarly, heating systems such as furnaces and other types of fuel-burning heating devices release pollutants into the air. In some developed countries, including the UK and Sydney, Australia, wood stoves are the major source of particulate pollution in urban areas. Wood stoves can also emit carbon monoxide and NOx.
Other sources of indoor air pollution are building materials, biological material and tobacco smoke. Biological material, such as dander, house dust mite, mold and pollen, can come from humans, animals or plants. Some of this material can trigger allergies, such as allergic rhinitis. Fumes from pesticides, paints, cleaning products and personal care products can be substantial, and make up an increasing share of outdoor and indoor air pollution as transportation is getting cleaner.
Natural sources
Dust from desert can cause poor air quality far from its source. For instance, dust from the Gobi Desert in China and Mongolia can reach Hawaii, and dust from the Sahara reaches the Amazon rainforest in South America.
Radon is a radioactive gas that can build up in buildings from the soil. It can cause lung cancer, especially in smokers. Levels are generally low, but can be elevated in buildings with "leaky" foundations or areas with soils rich in uranium. Volcanic eruptions can be a large source of sulfur dioxide and also produce particle pollution.
Vegetation can emit gases that contribute to ozone formation and particle pollution. This is especially true in warmer climates and during the growth season. These gases react with human pollution sources to produce a seasonal haze. Black gum, poplar, oak and willow emit gases that can raise ozone levels up to eight times more than low-impact tree species. Wildfires, which have become more severe and more common due to climate change, release fine particles. They are a major source of air pollution.
Major pollutants
Air pollutants can be tiny solid or liquid particles dispersed in the air (called aerosols), or gases. 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 carbon monoxide gas from car exhausts and sulfur dioxide from factories. Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants react with each other or with other parts of the atmosphere. Ground-level ozone is one example of a secondary pollutant. Some pollutants may be both primary and secondary — both are emitted directly and formed from other primary pollutants.
Ammonia
Ammonia (NH3) is emitted mainly by overuse of synthetic nitrogen fertilizers on farmland, and from manure and urine from livestock. At typical concentrations in the air, it is not harmful to health directly. However, ammonia can react with other pollutants in the air to form ammonium sulfate or nitrate salts, contributing to particulate matter pollution. Furthermore, when ammonia is deposited onto the soil, it can harm ecosystems via eutrophication.
Carbon dioxide
Carbon dioxide (CO2) is mainly emitted by the burning of fossil fuels. CO2 is sometimes called an air pollutant, because it is the main greenhouse gas responsible for climate change. Although the World Health Organization recognizes CO2 as a climate pollutant, it does not include the gas in its Air Quality Guidelines or set recommended targets for it. 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 CO2 emissions. The Inflation Reduction Act of 2022 amended the Clean Air Act to define CO2 from fossil fuel burning explicitly as an air pollutant.
Carbon monoxide
Carbon monoxide (CO) is a colorless, odorless, and 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 of it. Now, wildfires and bonfires are the main source of outdoors CO. Indoors, CO is a larger problem and mainly comes from cooking and heating. In poorly ventilated spaces, CO can accumulate to dangerous levels, and exposure may cause people to lose consciousness and die. When CO is destroyed in the atmosphere, it can raise levels of CO2 and CH4.
Ground-level ozone
Ground-level ozone (O3) is mostly created when NOx and volatile organic compounds mix in the presence of sunlight. It can also form from carbon monoxide or methane. Due to the influence of temperature and sunlight on this reaction, high ozone levels are most common on hot summer afternoons. It is the main gas in photochemical smog.
O3 can be harmful to human health, but also to some materials, forests, plants, and crops. Smog is a particular problem in big cities where it cannot easily be transported away by wind (e.g. cities built in valleys surrounded by mountains). When ground-level ozone is produced, it can linger in the air for days or weeks, and therefore be transported far from where it was first formed.
Nitrogen oxides
Nitrogen oxides (NOx), particularly nitric oxide (NO), are mostly created by the burning of fossil fuels, and in lesser amounts by lightning. Nitrogen dioxide (NO2) is formed from NO in a reaction with other atmospheric gases. NO and NO2 can form acid rain, can form into a haze, and can cause nutrient pollution in water. NO2 is a reddish-brown toxic gas with a strong odor, whereas NO is odorless and colorless.
Particulate matter
Particulate matter (PM), also known as particle pollution, includes all airborne substances that are not gases. It is a mix of microscopic solid particles or droplets suspended in a gas.
Particulate matter can contain a large variety of materials and chemical compounds including toxic substances, which can vary strongly in size. Coarse PM (PM10) is 10 micrometer (μm) or smaller in diameter, fine PM (PM2.5) is smaller than 2.5 μm, and ultrafine particles are 0.1 μm or smaller. Smaller particles pose more risk to health, as they can reach the bloodstream. 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.
Sea spray, wildfires, volcanoes and dust storms are the main natural sources of PM. Meanwhile, human sources include the burning of biomass and fossil fuels, as well as road emissions and dust resuspension. Human PM is usually finer than natural PM. Most particulate matter is formed in the atmosphere from precursor gases. For instance, sulfate comes from SO2, nitrate from NO2, and ammonium is formed from ammonia. Soot on the other hand is directly emitted from combustion, and consists of black carbon and organic compounds. Particulate matter can have a cooling effect locally on the climate, as it reflects sunlight away from Earth's surface.
Sulfur dioxide
Sulfur dioxide (SO2), an acidic and corrosive gas, is produced mostly by burning crude oil and coal. These fossil fuels often contain sulfur compounds, and their combustion generates sulfur dioxide. In Europe and North America, SO2 is mostly found in areas with significant shipping and industry, as road traffic fuels are regulated. Smaller amounts of SO2 are released from smelting and volcanoes.
High concentrations of SO2 in the air 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 pollution. At high concentrations, gaseous SOx can harm plants by damaging leafs and decreasing growth. Further oxidation of SO2, mostly taking place in cloud droplets, forms sulfuric acid (H2SO4), which is one of the components of acid rain.
Volatile organic compounds
Volatile organic compounds (VOCs) are a class of carbon-based chemicals that exist as gases at room temperature, found both indoors and outdoors. They can cause photochemical smog and form aerosols impacting climate. The group includes methane, acetone, and toluene. Some can cause cancer, such as butadiene and benzene, with benzene being released from cigarette smoking. Methane is a greenhouse gas and the second-largest driver of global warming. Other VOCs contribute to climate warming because they help form ground-level ozone, a greenhouse gas.
Other pollutants
Some heavy metals can be bad for health. For instance, lead exposure can lead to learning disabilities in children. In the atmosphere, heavy metals can exist in different states, such as particles or gases. One of the forms of chromium can cause cancer. Mercury is harmful both as an element and in an organic compound. In the atmosphere, it comes mostly from cement production, coal burning, and incinerators.
Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental degradation. They persist in the environment, are capable of long-range transmission, bioaccumulate in humans and animals, and biomagnify in food chains. The Stockholm Convention on Persistent Organic Pollutants identified pesticides and other POPs of concern. These include dioxins and furans which are created by waste combustion. POPs are usually either semi-volatile (gaseous only at higher temperatures) or non-volatile (emitted as particles). The harmful effects of the pesticide DDT, a POP, were popularized by Rachel Carson's 1962 book Silent Spring. PFASs and polycyclic aromatic hydrocarbons (PAHs) are other examples of POPs.
Chlorofluorocarbons (CFCs) are a group of compounds which harm the ozone layer. They were widely used in aerosol sprays, refrigerants, and fire suppression. Due to their chemical stability, CFCs persist in the atmosphere and eventually reach the stratosphere (the upper atmosphere). There, they break down under the impact of UV light, which releases chlorine. This in turn reacts with ozone, destroying it. As the ozone layer blocks harmful UV radiation from reaching the Earth's surface, its depletion leads to health risks such as skin ageing and skin cancer.
Exposure
Exposure to air pollution varies widely across the world and across groups. Children, for example, are more exposed because they breathe more rapidly than adults and closer to the ground, where pollution from vehicle exhaust and dust is more concentrated. Similarly, people engaging in strenuous exercise inhale more pollutants than those at rest. People can reduce their exposure by wearing high-quality face masks or by using air purifiers.
For some pollutants, low exposure can be seen as safe, whereas other pollutants have negative health effects even at low levels. As evidence has grown that even very low levels of air pollutants hurt human health, the WHO halved its recommended safe limit for particulate matter from 10 μg/m3 to 5 μg/m3 in 2021. Under the new guideline, nearly the entire global population—97%—is classified as exposed to unsafe levels of fine particles (PM2.5). The new limit for nitrogen dioxide (NO2) became 75% lower. For all pollutants together, the World Health Organization concluded that 99% of the world population is exposed to harmful air pollution.
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.
By socioeconomic group
While air pollution affects a variety of populations, some groups are more exposed. In many regions, there are disparities in exposure to pollution by race and income. This is especially true in countries with high inequalities in incomes and healthcare, such as the United States. Polluting industries and roads are more likely to be placed in poorer communities, and people in these communities are more likely to work outdoors, leading to additional exposure. Residents in public housing, who are generally low-income and cannot easily move to healthier neighborhoods, are highly affected by nearby refineries and chemical plants. Additionally, lower-income communities more often perform polluting activities, such as using solid biofuels for cooking. In the United States, Blacks and Latinos generally face more pollution than Whites and Asians.
By geographic area
Exposure to outdoor air pollution is worst in lower-middle income countries in line with the environmental Kuznets curve, which postulates that pollution is worst in economies that rely on manufacturing but have not yet been able to prioritize environmental regulation. Indoor air pollution is worst in low-income countries, in particularly south-east Asia, the western Pacific, and Africa.
Outdoor air pollution is usually concentrated in densely populated metropolitan areas. Urbanization leads to a rapid rise in premature mortality due to air pollution in fast-growing tropical cities. Indoor air pollution on the other hand is most common in rural areas, which may lack access to clean cooking fuels.
A map published in 2025 by Climate TRACE indicates that PM2.5 (fine particles) and other toxins are released near the homes of about 1.6 billion people, about 900 million of whom are in the path of "super-emitting" facilities such as power plants, refineries, ports, and mines.
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. According to the WHO, 99% of the world's population lives in areas with air pollution that exceeds WHO recommended levels. Even at very low levels (under the World Health Organization recommended levels), fine particulates can continue to cause harm.
Pollutants strongly linked to ill health include particulate matter, carbon monoxide, nitrogen dioxide (NO2), ozone (O3), and sulfur dioxide (SO2). 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.