Air pollution

• 1. Introduction

  ›

  Air pollution kills 7 to 8 million people every year. That is more than one in eight deaths, scattered across every continent and nearly every household. The substances doing the killing are often invisible. Some are gases like ozone and nitrogen oxides. Others are particles so small they slip from the lungs into the bloodstream and travel to distant organs. They come from burning fossil fuels, from cooking fires, from wildfires, from dust storms, and from volcanoes. The World Bank has put a price on the damage: more than 8 trillion dollars a year, over 6 percent of global GDP. How did a problem this large grow so quietly? Why does the air outside a power plant and the air over a kitchen stove harm different people in different ways? And why have some efforts to clean the air succeeded spectacularly while others have barely moved? The answers begin with where these substances come from, what they do inside the body, and the long struggle to name them as a danger at all.

• 2. Sources Outside And In

  ›

  Lignite and coal produce the most air pollution of any fuel burned for electricity, followed by oil, then fossil gas and biomass. Methane leaks are common in oil and gas production, and oil refineries emit a wide range of pollutants. Construction and demolition add their own load, with cement production standing as one of the main sources of particle pollution. Asbestos, banned in many countries, still lurks in older buildings, where disturbing it risks lung disease.

  Road vehicles produce a large share of all air pollution, perhaps a third to half of all nitrogen dioxide emissions. Petrol and diesel engines release about half their emissions from the exhaust pipe. The other half comes from tire and brake wear and the disturbance of the road surface. Electric vehicles produce no tailpipe emissions but still create those non-exhaust particles.

  Agriculture contributes heavily, both from crops and from animals. Cattle digesting food emit methane, which helps create ground-level ozone, and farming is a major source of ammonia. Open dumps of waste are common in low-income countries, releasing soot and methane when burned. Globally, a quarter of solid waste is never collected and another quarter is not disposed of properly.

  As of 2023, more than 2.3 billion people in developing countries burn polluting fuels for cooking, such as firewood, agricultural waste, dry dung, coal, or charcoal. The health effects fall most heavily on women, who usually do the cooking, and on young children. Globally, 12 percent of outdoor fine particle pollution comes from household cooking. Even the gas stove emits benzene and carbon monoxide, and in the UK and in Sydney, Australia, wood stoves are the major source of urban particulate pollution.

  Nature supplies its own pollution across vast distances. 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, a radioactive gas, seeps up from uranium-rich soils and can cause lung cancer, especially in smokers. Black gum, poplar, oak and willow emit gases that can raise ozone levels up to eight times more than low-impact tree species, a hint that even the forest plays a part in the chemistry overhead.

• 3. The Chemistry Of Harm

  ›

  Carbon monoxide is colorless, odorless, and toxic, a product of burning natural gas, coal, or wood. In poorly ventilated spaces it can build to levels that make people lose consciousness and die. Pollutants like it are called primary, produced directly by a source and unchanged once released. Secondary pollutants form later, when primary ones react with each other or with the atmosphere.

  Ground-level ozone is the classic secondary pollutant, created when nitrogen oxides and volatile organic compounds mix in sunlight. High ozone is most common on hot summer afternoons, and it is the main gas in photochemical smog. Once formed, it can linger for days or weeks and travel far from where it began, a particular curse for cities built in valleys ringed by mountains.

  Particulate matter is the deadliest category, both indoors and out. Coarse particles, PM10, measure 10 micrometers or smaller; fine particles, PM2.5, are smaller than 2.5 micrometers; ultrafine particles are 0.1 micrometers or less. Smaller is more dangerous, because the tiniest particles reach the bloodstream. The Harvard Six Cities study, published in 1993, established a definitive link between fine particulate pollution and higher death rates in urban areas.

  Ammonia at typical air concentrations does not harm health directly. Yet it reacts with other pollutants to form ammonium sulfate or nitrate salts, feeding particulate matter, and when it settles on soil it can damage ecosystems through eutrophication. Sulfur dioxide, acidic and corrosive, comes mostly from burning crude oil and coal, and further oxidation inside cloud droplets turns it into the sulfuric acid that helps make acid rain.

  Some pollutants are stubborn travelers. Persistent organic pollutants resist degradation, move across long distances, build up in bodies, and concentrate up the food chain. The harm of the pesticide DDT, one such compound, was popularized by Rachel Carson's 1962 book Silent Spring. Chlorofluorocarbons, once common in aerosol sprays and refrigerants, drift up to the stratosphere, where UV light frees their chlorine to destroy ozone in the layer that shields the Earth's surface.

• 4. Who Breathes The Worst Air

  ›

  Children breathe more rapidly than adults and closer to the ground, where vehicle exhaust and dust concentrate. People exercising hard inhale more than those at rest. The risk is not spread evenly, and as evidence grew that even very low levels of pollutants harm health, the World Health Organization halved its recommended safe limit for particulate matter, from 10 micrograms per cubic meter to 5, in 2021. Under that new guideline, 97 percent of the global population is exposed to unsafe levels of fine particles. Across all pollutants, the WHO concluded that 99 percent of the world is exposed to harmful air.

  Income and race shape exposure sharply, especially in countries with high inequality such as the United States. Polluting industries and roads are more often placed in poorer communities, where residents are also more likely to work outdoors. People in public housing, generally low-income and unable to move, live near refineries and chemical plants. In the United States, Blacks and Latinos generally face more pollution than Whites and Asians.

  Geography draws its own map. In 2024 the most polluted city was Byrnihat, India, with a PM2.5 concentration of 128, followed by Delhi at 108, Karaganda, Kazakhstan at 105, Mullanpur, India at 102, and Lahore, Pakistan at 102. Outdoor pollution tends to be worst in lower-middle income countries, in line with the environmental Kuznets curve, which holds that pollution peaks in economies that rely on manufacturing but have not yet prioritized regulation. Indoor pollution is worst in low-income countries, particularly in south-east Asia, the western Pacific, and Africa.

  A map published in 2025 by Climate TRACE found that fine particles and other toxins are released near the homes of about 1.6 billion people. About 900 million of them lie in the path of super-emitting facilities such as power plants, refineries, ports, and mines.

• 5. The Toll On The Body

  ›

  The 2024 Global Burden of Disease Study estimated that air pollution contributed to 8.1 million deaths in 2021, more than one in eight deaths worldwide. Outdoor particulate pollution was the largest single cause at 4.7 million, followed by indoor particulate pollution at 3.1 million and ozone at 0.5 million. The global mean loss of life expectancy from air pollution in 2015 was 2.9 years, far more than the 0.3 years lost to all forms of direct violence.

  The heart and brain bear much of the burden. Air pollution is responsible for 27 percent of stroke deaths worldwide and 28 percent of coronary heart disease deaths. A systematic analysis of 17 risk factors across 188 countries tied air pollution to nearly one in three strokes, 34 percent in developing countries against 10 percent in developed ones. The disease extends into the mind: pollution raises the risk of dementia, and exposure to solid-fuel pollution was linked to a higher risk of depression, strongest in people over 65.

  The lungs suffer in proportion. Nearly half of global COPD deaths are due to air pollution, a disease that is itself the fourth-largest cause of death globally. In children, pollution can hinder lung development and worsen asthma, and short-term ozone exposure makes childhood asthma worse. About 265,000 lung cancer deaths were attributed globally in 2019 to fine particulate matter, with another 170,000 from indoor pollution including radon.

  The damage reaches the unborn. Over a third of preterm births were associated with air pollution in 2021, contributing to more than half a million newborn deaths. Polluted air killed over 700,000 children in 2021, including 709,000 under five years of age. For most of these links, only correlation could be shown, since the randomized trials that prove causation are nearly impossible in environmental medicine. Scientists at BIPS in Bremen managed to demonstrate a causal relationship for at least some problems, including diabetes and high blood pressure, using a special study design.

• 6. Damage Beyond Breathing

  ›

  Acid rain caused substantial damage in the 1970s, including lake acidification and forest diebacks in Northern Europe. The key culprits are nitric acid, sulfuric acid, and hydrochloric acid, the last drawn from coal combustion. As waters and soils turned acidic, nutrients like magnesium and calcium washed away, while aluminium, toxic to plants, became available to their roots. The acid also eats at buildings and statues made of marble, calcite, or freestone.

  Farming pays a measurable price, especially from ozone, which acts as an oxidant and reduces photosynthesis. One study estimated that a 1 percent rise in ozone would bring a global economic loss of 10 billion dollars each year, with a 1 percent rise in PM2.5 costing about 5 billion, especially in colder climates. The COVID-19 lockdowns offered a natural experiment: in India, cleaner air boosted surface greenness and photosynthetic activity, with crops benefiting most.

  The economic blow runs deeper still. A World Bank study found that PM2.5 pollution in 2019 cost the world economy over 8 trillion dollars, more than 6 percent of global GDP, with losses in India and China exceeding 10 percent of GDP. About 85 percent of that loss came from the value of lives lost, calculated using the Value of Statistical Life. Pollution even hampers clean energy, reducing the sunlight reaching solar panels and dirtying them so they generate less power.

• 7. Naming The Danger

  ›

  Mummified remains from Peru, Egypt, and Britain show that ancient people suffered blackened lungs from open fires in poorly ventilated homes. Complaints about air pollution reach back to the Greek and Roman period, and lead levels in Arctic ice cores ran about ten times higher in Roman times than before. With the Industrial Revolution, outdoor pollution rose sharply from the large-scale burning of coal, first in Britain, then across Northern Europe and the United States. By the 19th century, buildings near industrial plants blackened and park trees withered, and smoke-induced fogs cut the sunlight enough to contribute to rickets.

  For a long time the danger was misread. The miasma theory, prominent in the 18th and 19th centuries, blamed epidemics like cholera and yellow fever on a harmful vapor rising from decaying matter. It collapsed only when physicians accepted the germ theory of disease in the late 19th century. Even then, legislation lagged: anti-smoke groups emerged in Britain in the 1830s and in the United States in the 1880s, but laws stayed weak against industrial interests. The business and political leaders of industrial cities welcomed heavy black smoke as a sign of prosperity, high profits, and high wages.

  Disaster forced the issue. The 1952 Great Smog of London killed some 12,000 people and led to the Clean Air Act 1956. The 1948 Donora smog in the United States killed 20 people and prompted American regulation, with Japan following in 1960. The worst pollution disaster of all came in 1984, when leaked industrial vapors from the Union Carbide factory in Bhopal, India, later bought by the Dow Chemical Company, killed at least 20,000 people and affected about 600,000.

  Carbon dioxide poses a question that history has not settled. It is the main greenhouse gas behind climate change, and although the World Health Organization recognizes it as a climate pollutant, it sets no air-quality target for it. The Inflation Reduction Act of 2022 amended the U.S. Clean Air Act to define carbon dioxide from fossil fuel burning explicitly as an air pollutant, a decision with real consequences for what the law may regulate.

• 8. Cleaning The Air

  ›

  Buses in New Delhi, India, switched to compressed natural gas after 2000 to cut the city's thick pea soup smog, a single concrete move within a larger toolkit. Industrial plants can fit scrubbers like flue-gas desulfurization or catalysts to remove nitrogen oxides. In the power sector, the most effective step is the shift to renewable energy or nuclear power, since switching coal to fossil gas reduces pollution without eliminating it. For transport, the avoid-shift-improve framework groups efforts into reducing travel, shifting to cleaner modes, and improving vehicle technology.

  The evidence for design choices is concrete. In 2006, Lawrence D. Frank and his co-authors published a study, Many Pathways from Land Use to Health, later cited over 1,930 times. It found that a 5 percent increase in walkability was associated with a 6.5 percent reduction in vehicle miles driven, a 5.6 percent reduction in nitrogen oxides, and a 5.5 percent reduction in volatile organic compounds. For households, a switch to biogas, bioethanol, electricity, natural gas, or LPG significantly reduces pollution, while kerosene lamps can give way to solar-powered LED lights.

  Laws have done much of the heavy lifting, though unevenly. The 1956 Clean Air Act in Britain and the 1963 US Clean Air Act stand out as effective, yet 43 percent of countries still lack a legal definition of air pollution, and few set limits as strict as the World Health Organization recommends. International results split sharply. The Montreal Protocol, ratified worldwide, almost eliminated ozone-depleting chemicals, while the 1997 Kyoto Protocol and the 2015 Paris Agreement set weak or non-binding climate targets.

  The air is slowly becoming a matter of rights. In 2022 the UN General Assembly recognized the right to a clean, healthy, and sustainable environment as a human right, though the resolution is not legally binding. In Chile, where that right is written into the constitution, the Supreme Court found that the government must act to provide clean air, a sign of how the fight may move next from the laboratory and the legislature into the courtroom.