"Bad air quality" and "Air quality" redirect here. For the obsolete medical theory, seeMiasma theory. For the measurement of air pollution, seeAir quality index.
Air pollution is the presence of substances in theair that are harmful to humans, other living beings or the environment. Pollutants can begases, likeozone ornitrogen oxides, or small particles likesoot and dust. Both outdoor and indoor air can be polluted.
Construction anddemolition produces dust, but also other pollutants. The direct particles from construction and demolition are relatively coarse.[8][9] Construction also has an indirect impact on air quality, ascement production is one of the main sources of particle pollution.[5] Though banned in many countries,asbestos persists in older buildings, where it poses a risk of lung disease when disturbed.[10] Building materials including carpeting andplywood emitformaldehyde, a gas which can cause difficulty breathing andnausea.[11]
Road vehicles produce a significant amount of all air pollution. For instance, they may be responsible for a third to half of allnitrogen dioxide emissions,[12][13] and are a major cause ofclimate change.[14] Vehicles with petrol and diesel engines produce about half of their emissions from theirexhaust gas, and the other half fromnon-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.[15]Diesel trains, ships andplanes also cause air pollution.[16]
Open dumps of waste are a common source of air pollution in low-income countries. They can promote the growth of microbes which pollute water and air, and be a source of toxins. Throughopen burning of waste—whetherself-ignited or burned on purpose—soot, methane, and other pollutants are released.[22] The waste in landfills itself also produces methane.[23] Globally, a quarter ofsolid waste is not collected and another quarter is not disposed of properly.[24]
As of 2023, more than 2.3 billion people indeveloping countries rely on burning polluting fuels such asfirewood, agricultural waste,dry dung,coal orcharcoal for cooking, which causes harmfulhousehold air pollution.[25]Kerosine, another polluting fuel, is used in many countries forlighting 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.[26]
Gas stoves for cooking contribute to indoor air pollution by emittingNO2,benzene and carbon monoxide.[27]Toasters can produce particulate pollution.[28] Similarly,heating systems such asfurnaces and other types of fuel-burning heating devices release pollutants into the air.[29] In some developed countries, including the UK and Sydney, Australia,wood stoves are the major source of particulate pollution in urban areas.[30][31] Wood stoves can also emit carbon monoxide andNOx.[11]
Other sources of indoor air pollution are building materials, biological material and tobacco smoke. Biological material, such asdander,house dust mite,mold andpollen, can come from humans, animals or plants. Some of this material can trigger allergies, such asallergic rhinitis.[11] Fumes frompesticides, paints,cleaning products andpersonal care products can be substantial, and make up an increasing share of outdoor air pollution as transportation is getting cleaner.[32]
Dust from desert can cause poor air quality far from its source. For instance, dust from theGobi Desert in China and Mongolia can reachHawaii, and dust from theSahara reaches theAmazon rainforest in South America.[33]
Radon is a radioactive gas that can build up in buildings from thesoil. It can causelung cancer, especially insmokers. Levels are generally low, but can be elevated in buildings with "leaky" foundations or areas with soils rich inuranium.[34] Volcanic eruptions can be a large source of sulfur dioxide and also produces particle pollution.[35]
Vegetation can emit gases that contribute to ozone formation and particle pollution. This is especially true in warmer climates and during the growth season.[36] These gases react with human pollution sources to produce a seasonal haze.[37]Black gum, poplar, oak andwillow emit gases that can raise ozone levels up to eight times more than low-impact tree species.[38]Wildfires, which have become more severe and more common due to climate change, release fine particles. They are a major source of air pollution.[39]
Air pollutants can be tiny solid or liquid particles dispersed in the air (calledaerosols), or gases.[40] 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 includecarbon 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: they are both emitted directly and formed from other primary pollutants.[41]
Ammonia (NH3) is emitted mainly by overuse of synthetic nitrogenfertilisers on farmland, and frommanure and urine fromlivestock.[42] At typical concentrations in the air, it is not harmful to health directly. However, ammonia can react with other pollutants in the air to formammonium sulfate ornitrate salts, contributing to particulate matter pollution. Furthermore, when ammonia isdeposited onto the soil, it can harm ecosystems viaeutrophication.[43]
Carbon dioxide (CO2) is mainly emitted by the burning of fossil fuels.[44] CO2 is sometimes called an air pollutant, because it is the maingreenhouse gas responsible forclimate change.[45][46] Although theWorld Health Organization recognizes CO2 as a climate pollutant, it does not include the gas in itsAir Quality Guidelines or set recommended targets for it.[47] 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. TheInflation Reduction Act of 2022 amended the Clean Air Act to define CO2 from fossil fuel burning explicitly as an air pollutant.[48]
Carbon monoxide (CO) is a colorless, odorless, toxic gas.[49] It is a product ofcombustion 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 andbonfires are the main source of outdoors CO.[50] Indoors, CO is a larger problem and mainly comes from cooking and heating.[51] In poorly ventilated spaces, CO canaccumulate 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 andCH4.[52]
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.[53] Due to the influence of temperature and sunlight on this reaction, high ozone levels are most common on hot summer afternoons.[54] It is the main gas inphotochemical smog.[55]
It can be harmful to human health, but also to some materials, forests, plants and crops.[56] Smog is a particular problem in big cities where it cannot easily be transported away by wind, for instance cities built in valleys surrounded by mountains.[57] 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.[53]
A NASA video on satellite measurement of nitrogen oxides, showing declining levels due to regulation. However, some regions did see rising levels of NO2 pollution, for instance those withfracking orcivil unrest.
Nitrogen oxides (NOx), particularly nitrous oxide (NO), are mostly created by the burning of fossil fuels, and in lesser amounts bylightning.Nitrogen dioxide (NO2) is formed from NO in a reaction with other atmospheric gases.[58][59] NO and NO2 can form acid rain, can form into ahaze, and can causenutrient pollution in water.[60] NO2 is a reddish-brown toxic gas with a strong odor, whereas NO is odorless and colorless.[61]
Particulate matter (PM), also known as particle pollution, includes all airborne substances that are not gases.[8][62] It is a mix of microscopic solid particles or droplets suspended in a gas.[63]
It consists of a large variety of materials and chemical compounds including toxic substances, and they vary strongly in size.[8] Coarse PM (PM10) is 10 micrometer (μm) or smaller in diameter, fine PM (PM2.5) is smaller than 2.5 μm, andultrafine particles are 0.1 μm or smaller.[64] Smaller particles pose more risk to health, as they can reach the bloodstream.[62][64] A definitive link between fine particulate pollution and higher death rates in urban areas was established by theHarvard Six Cities study, published in 1993.[65]
Sea spray, wildfires, volcanoes anddust 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.[66] Most particulate matter is formed in the atmosphere from precursor gases. For instance,sulfate comes from SO2,nitrate from NO2, andammonium is formed from ammonia.Soot on the other hand is directly emitted from combustion, and consists of black carbon and organic compounds.[67]Particulate matter can have a cooling effect locally on the climate, as it reflects sunlight away from Earth's surface.[68]
Sulfur dioxide (SO2), an acidic and corrosive gas, is produced mostly by burningcrude oil and coal. These fossil fuels often contain sulfur compounds, and their combustion generates sulfur dioxide.[69] In Europe and North America, SO2 is mostly found in areas with significant shipping and industry, as road traffic fuels are regulated.[70] Smaller amounts of SO2 are released fromsmelting and volcanoes.[71]
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 damagingleafs and decreasing growth.[71] Further oxidation of SO2, mostly taking place in cloud droplets, formssulfuric acid (H2SO4), which is one of the components ofacid rain.[72]
Volatile organic compounds (VOC) are a class of carbon-based chemicals that exist as gases at room temperature, found both indoors and outdoors.[73] They can causephotochemical smog and form aerosols impacting climate. The group includes methane,acetone andtoluene. Some cancause cancer, such asbutadiene andbenzene,[74] with benzene being released fromcigarette smoking. Methane is a greenhouse gas and the second-largest driver ofglobal warming. Other VOCs contribute to climate warming because they help form ground-level ozone, a greenhouse gas.[75]
Chlorofluorocarbons (CFCs) are a group of compounds which harm theozone layer. They were widely used inaerosol sprays, in refrigerants and infire suppression. Due to their chemical stability, CFCs persist in the atmosphere and eventually reach thestratosphere (the upper atmosphere).[80] There, they break down under the impact ofUV light, which releaseschlorine. 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 asskin ageing andskin cancer.[81]
PM2.5 Levels Across the World's 5 Most Populated Nations in 2019
Exposure to air pollution varies widely across the world and across groups.[82] 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.[83] Similarly, people engaging in strenuous exercise inhale more pollutants than those at rest.[84] On the other hand, people can reduce their exposure by wearinghigh-quality face masks or by usingair purifiers.[85]
Share of the population exposed to air pollution levels above WHO guidelines, 2017
For some pollutants, low exposure can be seen as safe, whereas other pollutants have negative health effects even at low levels.[86] 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 PM2.5.[87] The new limit fornitrogen dioxide (NO2) became 75% lower.[88] For all pollutants together, the WHO concluded that 99% of the world population is exposed to harmful air pollution.[89]
For some pollutants such asblack 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.[90] A large portion of total daily exposure occurs as short peaks of high concentrations.[91]
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 highinequalities in incomes and healthcare, like 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.[92] Residents in public housing, who are generally low-income and cannot easily move to healthier neighborhoods, are highly affected by nearbyrefineries and chemical plants.[93] Additionally, lower-income communities more often perform polluting activities, such as using solid biofuels for cooking.[94][95] In the United States, Blacks and Latinos generally face more pollution than Whites and Asians.[96]
Exposure to outdoor air pollution is worst in lower-middle income countries in line with theenvironmental Kuznets curve, which postulates that pollution is worst in economies that rely on manufacturing but have not yet been able to prioritize environmental regulation.[92] Indoor air pollution is worst in low-income countries, in particularlysouth-east Asia, the western Pacific and Africa.[98]
Outdoor air pollution is usually concentrated in densely populatedmetropolitan areas. Urbanization leads to a rapid rise in prematuremortality due to air pollution in fast-growing tropical cities.[99] Indoor air pollution on the other hand is most common inrural areas, which may lack access to clean cooking fuels.[98]
A map published in 2025 byClimate 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.[100]
The share of total deaths from indoor air pollution, 2017
Air pollution is an important risk factor for various diseases, such asCOPD (a common lung disease),stroke, heart disease,lung cancer andpneumonia. Indoor air pollution is also associated withcataract.[86] Even at very low levels (under theWorld Health Organization recommended levels), fine particulates can continue to cause harm.[101] However, according to the WHO, 99% of the world's population lives in areas with air pollution that exceeds WHO recommended levels.[102]
Pollutants strongly linked to ill health include particulate matter,[103] carbon monoxide,[104] nitrogen dioxide (NO2), ozone (O3),[105] and sulphur dioxide (SO2). Fine particulates are especially damaging, as they can enter thebloodstream via the lungs and reach other organs.[106][107] Air pollution causes disease by drivinginflammation andoxidative stress, suppressing theimmune system and by damagingDNA.[86]
People living inpoverty, babies andolder people are also disproportionately affected by air pollution; pregnancy is also more risky when exposed to air pollution.[108] Communities with a lowsocioeconomic status andminority groups are also more vulnerable to pollution than more privileged communities.[109] Lower-income groups might for instance have lessaccess to healthcare.[92]
Estimates of yearly deaths from air pollution range from 6.7 million to 8.8 million. In comparison, war caused 50,000 yearly deaths and terrorism 25,000.Deaths in 2021 from air pollution per 100,000 inhabitants (IHME)
Estimates of deaths due to air pollution vary.[110] The 2024Global 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 byindoor air pollution (3.1 million) and ozone (0.5 million).[111]
The WHO estimates that 6.7 million people die from air pollution each year, 4.2 million due to outdoor air pollution.[112] Roughly 68% of outdoor air pollution-related premature deaths were due tocoronary heart disease and stroke, 14% due to COPD and 14% due to lung infections (lower respiratory tract infections).[112]
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.[113][114] The global meanloss 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.[115]
Regional deaths due to air pollution depend not only on the§ regional exposure, but also on how large and how old the population is, and how healthy people are overall.[116]
In some countries, more than 20% of deaths are attributed to air pollution, for instance in China, Nepal, Bangladesh, Laos and North Korea. In South America, around 4% of deaths are from air pollution, while in countries such as Australia, Canada and the US, this number is under 3%.[117]
In absolute number, 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.[118] Annual premature European deaths from air pollution are estimated at 416,000[119] to 800,000.[114] The UK saw some 17,000 deaths in 2021 due to air pollution and the US saw 64,000. Nigeria, Indonesia and Pakistan each saw over 200,000 deaths resulting from air pollution.[120]
Deaths caused by accidents and air pollution from fossil fuel use in power plants exceed those caused by production ofrenewable energy.[2]
The burning offossil fuels is the largest source of air pollution deaths.[121] There are estimated 4.5 million annual premature deaths worldwide due to pollutants released by high-emission power stations and vehicle exhausts.[122] PM2.5 formed from emissions fromcoal-fired power plants could be more harmful than other types of fine particulate matter.[123]
TheWorld Health Organization (WHO) estimates that cooking-related pollution causes 3.8 million annual deaths.[124] The Global Burden of Disease study estimated the number of deaths in 2021 at 3.1 million.[125]
There is strong evidence that air pollution increases the risk ofcardiovascular disease, includingstroke,high blood pressure, andcoronary heart disease.[126] According to the Global Burden of Disease Study, air pollution is responsible for 27% of deaths from strokes worldwide and 28% of coronary heart disease.[127] The risks are highest in regions with higher air pollution (i.e. Asia), for elderly and for people who are overweight.[126]
Air pollution is a leading risk factor for stroke, particularly in developing countries where pollutant levels are highest.[127] 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).[128][129] The mechanisms linking air pollution to increased cardiovascular mortality are not fully understood, but likely includesystemic inflammation andoxidative stress.[130]
Air pollution is associated with increased development, hospitalization and mortality andCOPD (chronic obstructive pulmonary disease).[131] COPD is a common disease which causes restricted airflow and breathing difficulties, and is the fourth-largest cause of death globally.[132] Nearly half of global COPD deaths are due to air pollution.[127] Fine particles (PM2.5) and NO2 are associated with increased risk of developing COPD.[133] In children, air pollution can hinder lung development, which may increase their susceptibility to COPD later in life.[131]
Air pollution is further associated with increased risk of asthma and worsening of symptoms, and this effect seems stronger in children.[134] For adults, fine particles (PM2.5) or NO2 seem linked toasthma onset too.[135] Short-term exposure to ozone makes asthma worse in children.[136] There is limited evidence on (almost) fatal asthma attacks in children: ground-level ozone and PM2.5 seem to increase its risk.[137]
Dark factory-emitted clouds obscuring the Clark Avenue Bridge in Cleveland, Ohio in July 1973
Around 265,000 lung cancer deaths were attributed globally in 2019 to exposure tofine particulate matter, PM2.5, suspended in the air.[138] Exposure to indoor air pollution, including radon, caused another 170,000 lung cancer deaths.[138] Lung cancer was also more common among people exposed to NO2 and black carbon.[139]
Outdoor air pollution may increase risk of other types of cancer too, but the evidence is not as clear as for lung cancer.[140] For example, there may be a relationship betweenkidney cancer and PM2.5 and NO2 levels.[141] Household air pollution – from cooking with solid fuels, but also from radon in building material – has been associated withcervical,oral, andesophageal cancer.[140]
Stillbirths,miscarriages andbirth defects are all more likely when the mother is exposed to air pollution during pregnancy.[83] Exposure to air pollution also raises the chance a baby has alow birth weight. The impacts might be due to pollutants directly impacting theplacenta orfetus, or indirectly via the mother's health (as air pollution can cause systemic inflammation and oxidative stress).[83]
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.[83] The source of PM2.5 differs greatly by region. In South and East Asia, pregnant women are frequently exposed to indoor air pollution because of wood and otherbiomass 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 asdust storms.[142]
Including older children, polluted air 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).[83] Children in low- or middle-income countries are exposed to higher levels of fine particulate matter than those in high income countries.[143] Further health effects of air pollution on children include asthma, pneumonia and lower respiratory tract infections.[144] There is possibly a link between exposure to air pollution during pregnancy and after birth andautism in children.[145][146]
Air pollution is linked to various diseases of thebrain. It increases the risk ofdementia.[147] Indoor air pollution exposure during childhood may negatively affect cognitive function and neurodevelopment.[148][149]Prenatal exposure may also affect neurodevelopment.[150][151] It may also contribute toneurodegenerative diseases such as Parkinson's disease.[149]
Exposure to air pollution may also drivemental health issues, such asdepression andanxiety.[152] In particular, air pollution from the use of solid fuels was associated with a higher depression risk.[153] Depression risk andsuicide 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.[154]
Problems with thinking (cognitive issues) are also associated with air pollution. In people over the age of 40, both NOx and PM2.5 have been linked to general cognitive problems. PM2.5 was also associated with reducedverbal fluency (for instance, number of animals one can list in a minute) and worseexecutive functions (likeattention andworking memory). Similarly, children tended to fare worse in tests involving working memory when there was NOx, PM2.5, or PM10 pollution.[155]
Monument damaged by acid rainEffects of acid rain in theJizera Mountains, Czech Republic
Naturally, water in the atmosphere is slightly acidic. Some pollutants can form strong acids, making rainwater much more acidic. Key acids that causeacid rain arenitric acid (HNO3),sulfuric acid (H2SO4) andhydrochloric acid (HCl). HCl comes fromcoal combustion. H2SO4 forms from SO2, which comes from the burning of coal and oil and from some industrial processes like smelting. HNO3 forms from NO2, which is formed during high-temperature combustion. The term acid rain not only refers to rain, but also to pollution from hail, fog and snow.[156]
Acid rain caused substantial damage in the 1970, includinglake acidification andforest diebacks in Northern Europe. Due to the changed acidity in water bodies and soils, essential nutrients such asmagnesium andcalcium became soluble and could be washed away. Other elements, such asaluminium, which were toxic to vegetation, became available for the roots to absorb. Acid rain also impacts buildings and statues made of specific stones (e.g.marble,calcite orfreestone), as the stone reacts chemically with the acid in the water and erodes.[156]
Air pollution can settle (deposit) on the soil or in water, causing various problems. For instance, ammonia and nitric acid in the air can contribute to nutrientpollution in the water, a process calledeutrophication. At first, the extra nutrients help plants grow, but dense plant growth blocks sunlight from reaching the bottom. Plants in the lower layers then die, and with fewer plants producing oxygen, the oxygen level drops. This harms organisms that need oxygen to live,[157] and can lead to the loss of sensitive species.[158]
Various studies have estimated the impacts of air pollution onagriculture, especially ozone. Ozone acts as anoxidant and reducesphotosynthesis. One study estimated that for a 1% increase in ozone concentrations, there would be a global economic loss of $10 billion each year. For PM2.5, a 1% increase in pollution levels would lead to around $5 billion in losses, especially in colder climates.[159] After air pollutants enter the agricultural environment, they not only directly affect agricultural production and quality, but also enter agricultural waters and soil.[160] Air pollution further decreases theproductivity of labourers via health impacts.[159]
TheCOVID-19 lockdowns created a natural experiment to examine the links between air quality and agricultural output. In India, the lockdown improved air quality, which enhanced surface greenness and photosynthetic activity. Both forests and crops saw positive effects; the improvement was most pronounced for crops.[161]
Air pollution has a strong impact on the economy via its health effects – such as reducedproductivity at work and the costs ofhealthcare – and its effects on crop yield. It also affectstourism, biodiversity,forestry andwater quality. Tourism may be negatively affected due to decreasedvisibility and damage tocultural heritage.[162] People may be more prone to accidents due to air pollution. Increased NO2 levels are for instance linked to construction site accidents.[163]
In terms of the welfare cost on human health (non-market costs), aWorld Bank study found that PM2.5 pollution in 2019 cost theworld economy over $8 trillion, over 6% of globalGDP. In India and China, the loss of GDP was over 10%. Around 85% of this loss globally came from the loss of life, the rest from increased ill health.[164]: 23–24 The costs of lives lost are calculated using theValue of Statistical Life, a number that tries to estimate how much people would bewilling to pay to reduce their risk of dying.[164]: xiii This number differs by country and is difficult to estimate forlow- and middle-income countries.[165]
The direct market impacts on productivity loss, healthcare use and crop losses were estimated to rise to 1% of GDP by 2060, according to the OECD. TheCaspian region andChina would see the largest impact.[166] Air pollution also has an impact on energy production, as it reduces the amount of sunlight that reachessolar panels. It also causes the panels to become dirty, further reducing their energy output.[167]
Mummified remains of people inPeru,Egypt and Britain show that ancient people in these regions suffered fromblackening of the lungs caused by open fires in poorly ventilated homes. Recorded complaints of air pollution go back to theGreek andRoman period. Outdoor air pollution became a problem with the rise of cities, caused by household smoke and by early industrial activities (such as smelting andmining). In particular, lead levels, found in Arctic ice cores, were about ten times higher in the Roman period than in the period before.[168]
During theIndustrial Revolution, outdoor air pollution started to rise strongly, mostly due to the large-scale burning of coal. This occurred first in Britain, then in the rest of Northern Europe and the United States. By the 19th century, buildings around industrial plants started to blacken, while plants and trees in public parks started to wither. Smoke-induced fogs reduced the amount of sunlight city-dwellers got, contributing to cases ofrickets, a childhood disease caused by lack of sunlight and poor diet.[169]
However, the business and political leadership of the industrial cities were enthusiastic backers of the industry: heavy black smoke meant prosperity, high profits and high wages.[170]
Poster in the Soviet Union praising "The smoke of chimneys is the breath of Soviet Russia"
Themiasma theory was a prominent idea in the 18th and 19th centuries that gave a false explanation of how deadly epidemics likecholera,yellow fever andmalaria ("bad air") originated and spread. It said that illnesses were caused by breathing in a mysterious "miasma" --a harmful vapor that arose from decaying organic matter. Epidemics often came in the summer because that is when people spent more time outside. The theory motivated an enormous emphasis on public sanitation in major cities to remove smelly pollution, especially human and animal excrement, from streets and back alleys. The theory collapsed when physicians accepted the newgerm theory of disease in the late 19th century. Germs coughed up by an infected person or spread by certain types ofmosquitos orhookworms were the real reason people caught an infectious disease.[171]
In the 1830s, anti-smoke groups emerged in Britain, followed by groups in the United States in the 1880s. Legislation against pollution was weak however, as it was seen to conflict with industrial interests. During theinterwar period of 1920s and 1930s, a move from coal to gas and oil meant there was less air pollution, but this trend reversed whenWorld War II broke out.[169] The United Kingdom suffered its worst air pollution during the 1952Great Smog of London, with some 12,000 deaths, which led to theClean Air Act 1956.[172] The1948 Donora Smog in the US, killing 20 people, prompted the US to start regulating air pollution.[169][173] Japan followed in the 1960, but other heavily polluted regions, such as theSoviet Union and China, did not implement effective regulation.[169]
Technological disasters have caused severe problems with air pollution. The world's worst pollution disaster was the 1984Bhopal Disaster in India. Leaked industrial vapours from theUnion Carbide factory (later bought byDow Chemical Company), killed at least 20,000 people and affected around 600,000.[174]
From the 1950s, smog in developed countries was regulated, but other pollutants were not. Acid rain, caused by sulphur dioxide, became a major issue as it spread across borders. In the 1990s, for instance, Japan experienced acid rain from Chinese and Korean industry. International cooperation was needed to curb acid rain, and various coalitions were started. In 1975, it was discovered that certain chemicals caused ahole in the ozone layer; thanks to successful international negotiations, these chemicals were banned worldwide. There has been far less success in combatting climate change, andgreenhouse gas emissions, mostly from fossil fuels, continue to rise.[175]
Air pollution can be monitored using different techniques. For instance,satellites andremote sensing are used to track PM, NO2 and ozone.[176] 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.[177] Low-cost monitors can also be used for indoor air quality monitoring.[178] Finally, air quality sensors can be incorporated intodrones to measure air pollution higher up in the air.[179] Some websites attempt to map air pollution levels using available data.[180][181]
Air quality indexes (AQIs) offer a simple way to communicate changes in air quality and associated health risks to a wide audience. 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. These indexes can indicate when air quality is good, when it is dangerous for sensitive groups (e.g. children with asthma) and when it is a general hazard.[182]
Map of simulated particle pollution. Wildfire smoke is in green, desert dust is in orange, light blue issea salt and white shows sulphate particle pollution.
When direct data is unavailable or when projecting future air pollutant levels, estimates can be derived using models or emission factors.[183] Air pollutant emission factors are typical values that link the amount of a pollutant released into the air to a related activity. This could for instance be the typical amount of particulate matter released from a coal-power station.[184] TheUnited States Environmental Protection Agency has published a compilation of air pollutant emission factors for a wide range of industrial sources,[185] as did theEuropean Environment Agency.[186]
Air quality models usemeteorological and emissions data tosimulate how pollutants disperse and react in the atmosphere. Regulatory agencies use them to assess whether a new source of air pollution would exceed acceptable pollution levels, for permitting purposes. They can also be used to predict future pollution levels under different policy scenarios.[187] There are models for local pollution, but also for cross-boundary pollution.[188]
A growing number of countries regulates waste, which include national or city-widewaste management systems, opening managedlandfills,landfill gas capture (for electricity production), andwaste separation.[22] In agriculture, air pollution can be minimised by not overusing fertilisers and by not feeding excess protein to livestock.[192]
Theavoid-shift-improve framework groups efforts to cut pollution from vehicles into reducing travel, shifting to sustainable transport, and improving vehicle technology.[194] Reducing motor vehicle travel can curb pollution. One strategy is to buildcompact cities, so that amenities are close by and cars are not needed.[195] Motor traffic can be reduced by creating morewalkable cities and by investing incycling infrastructure.[196]Working from home is another way of avoiding motorised traffic.[194]
Various technologies are available for clean cooking, to replace traditional biomass stoves orthree-stone fires. For example, a switch to cooking withbiogas,bioethanol, electricity, natural gas, or LPG (liquified petroleum gas) significantly reduces air pollution.Improved cookstoves, which use biomass more efficiently, improve air quality less, but can be an intermediate solution if clean cookstoves or their fuels are not available. These clean cooking devices, including those run on fossil fuels, usually have a smaller climate impact than traditionalbiomass stoves.[200]
Kerosine for lighting can be replaced with efficientLED lights, for instance, solar-powered LED lights.[201] Combustion of fossil fuels for space heating can be replaced by the use of electricity inheat pumps.[202]Ventilation improves indoor air quality, but leads to outdoor air pollution, which can in turn reduce localindoor air quality.[203]
Air pollution in Shanghai, China, in 2013. China launched a "war against pollution" in 2014, which drastically lowered particle pollution levels across the country.[204]
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 Organization's recommendations.[205]
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,[206] which specify maximum atmospheric concentrations for specific pollutants. Other examples of air quality laws around the world include theClean Air Act in Britain, theUS Clean Air Act, and theTA Luft in Germany.[207] Air pollution laws might also put limits on theemissions of air pollutants, e.g. from vehicles.[208]
The World Health Organization'sGlobal 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.[209]
Some air pollution action has been successful at the international level, such as theMontreal Protocol,[210] which phased out harmful ozone-depleting chemicals. It was ratified worldwide. On the other hand,international action on climate change, has been less successful.[211] The 1997Kyoto Protocol introduced modest reduction targets for some countries but lacked strong enforcement,[212] while the 2015Paris Agreement set no binding limits, instead encouraging all countries to raise their ambition over time.[175]
In 2022, theUN General Assembly passed a resolution recognizing the right to a clean, healthy, and sustainable environment as a human right. The resolution is not legally binding. This resolution followed the declaration from theUN Human Rights Council published earlier that year.[213]
While many countries have air pollution laws, they differ in how they can be enforced vialitigation. In the European Union, individual countries, including France, have been fined by the EU for not complying with air quality rules. The revisedAmbient Air Quality Directive also makes it possible for individuals in the EU to seek compensation.[214] While China allows litigation on environmental grounds, it is rare as it is seen as risky.[215] InChile, the right to a healthy environment is part of theconstitution, and the Supreme Court found that the government has to act to provide clear air because of this.[216]
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