Air pollution
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 or 8 million people each year. It is a significant risk factor for a number of diseases, including stroke, heart disease, chronic obstructive pulmonary disease, asthma, 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 and productivity losses caused by air pollution cost the world economy over $8trillion 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 ; electric vehicles produce no tailpipe emissions, but still produce the other emissions. Diesel trains, ships, and planes also cause air pollution.Agriculture and waste
, 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, 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.
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
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, 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
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
is mainly emitted by the burning of fossil fuels. is sometimes called an air pollutant, because it is the main greenhouse gas responsible for climate change. Although the World Health Organization recognizes 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 is deemed to regulate emissions. The Inflation Reduction Act of 2022 amended the Clean Air Act to define from fossil fuel burning explicitly as an air pollutant.Carbon monoxide
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 and.Ground-level ozone
is mostly created when 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.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. 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.