Air pollution is a chemical, particulate matter, or biological agent that modifies the natural characteristics of the atmosphere. The atmosphere is a complex, dynamic natural gaseous system that is essential to support life on planet Earth. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health as well as to the Earth's ecosystems.

Worldwide air pollution is responsible for large numbers of deaths and cases of respiratory disease. While major stationary sources are often identified with air pollution, the greatest source of emissions is actually mobile sources, mainly automobiles. Gases such as carbon dioxide, which contribute to global warming, have recently gained recognition as pollutants by some scientists.

There are many substances in the air which may impair the health of plants and animals (including humans), or reduce visibility. These arise both from natural processes and human activity. Substances not naturally found in the air or at greater concentrations or in different locations from usual are referred to as 'pollutants'.

Pollutants can be classified as either primary or secondary. Primary pollutants are substances directly emitted from a process, such as ash from a volcanic eruption or the carbon monoxide gas from a motor vehicle exhaust.

Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants react or interact. An important example of a secondary pollutant is ground level ozone - one of the many secondary pollutants that make up photochemical smog.

Note that some pollutants may be both primary and secondary: that is, they are both emitted directly and formed from other primary pollutants.

Major primary pollutants produced by human activity include:

• Sulfur oxides (SO_x) especially sulfur dioxide are emitted from burning of coal and oil.
• Nitrogen oxides (NO_x) especially nitrogen dioxide are emitted from high temperature combustion. Can be seen as the brown haze dome above or plume downwind of cities.
• Carbon monoxide is colorless, odorless, non-irritating but very poisonous gas. It is a product by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of carbon monoxide.
• Carbon dioxide (CO₂), a greenhouse gas emitted from combustion and respiration.
• Volatile organic compounds (VOC), such as hydrocarbon fuel vapors and solvents.
• Particulate matter (PM), measured as smoke and dust. PM₁₀ is the fraction of suspended particles 10 micrometers in diameter and smaller that will enter the nasal cavity. PM_2.5 has a maximum particle size of 2.5 µm and will enter the bronchies and lungs.
• Toxic metals, such as lead, cadmium and copper.
• Chlorofluorocarbons (CFCs), harmful to the ozone layer emitted from products currently banned from use.
• Ammonia (NH₃) emitted from agricultural processes.
• Odors, such as from garbage, sewage, and industrial processes
• Radioactive pollutants produced by nuclear explosions and war explosives, and natural processes such as radon.

Secondary pollutants include:

• Particulate matter formed from gaseous primary pollutants and compounds in photochemical smog, such as nitrogen dioxide.
• Ground level ozone (O₃) formed from NOx and VOCs.
• Peroxyacetyl nitrate (PAN) similarly formed from NOx and VOCs.

Minor air pollutants include:

• A large number of minor hazardous air pollutants. Some of these are regulated in USA under the Clean Air Act and in Europe under the Air Framework Directive.
• A variety of persistent organic pollutants, which can attach to particulate matter.

Anthropogenic sources (human activity) related to burning different kinds of fuel

• "Stationary Sources" as smoke stacks of power plants, manufacturing facilities, municipal waste incinerators
• "Mobile Sources" as motor vehicles, aircraft etc.
• Combustion-fired power plants
• Controlled burn practices used in agriculture and forestry management
• Motor vehicles generating air pollution emissions.
• Marine vessels, such as container ships or cruise ships, and related port air pollution.
• Burning wood, fireplaces, stoves, furnaces and incinerators

Other anthropogenic sources

• Oil refining, power plant operation and industrial activity in general.
• Chemicals, dust and crop waste burning in farming, (see Dust Bowl).
• Fumes from paint, hair spray, varnish, aerosol sprays and other solvents.
• Waste deposition in landfills, which generate methane.
• Military uses, such as nuclear weapons, toxic gases, germ warfare and rocketry.

Natural sources

• Dust from natural sources, usually large areas of land with little or no vegetation.
• Methane, emitted by the digestion of food by animals, for example cattle.
• Radon gas from radioactive decay within the Earth's crust.
• Smoke and carbon monoxide from wildfires.
• Volcanic activity, which produce sulfur, chlorine, and ash particulates.

Health effects

The World Health Organization states that 4.6 million people die each year from causes directly attributable to air pollution. Many of these mortalities are attributable to indoor air pollution. Worldwide more deaths per year are linked to air pollution than to automobile accidents. Published in 2005 suggests that 310,000 Europeans die from air pollution annually. Direct causes of air pollution related deaths include aggravated asthma, bronchitis, emphysema, lung and heart diseases, and respiratory allergies. The US EPA estimates that a proposed set of changes in diesel engine technology (Tier 2) could result in 12,000 fewer premature mortalities, 15,000 fewer heart attacks, 6,000 fewer emergency room visits by children with asthma, and 8,900 fewer respiratory-related hospital admissions each year in the United States.

The worst short term civilian pollution crisis in India was the 1984 Bhopal Disaster. Leaked industrial vapors from the Union Carbide factory, belonging to Union Carbide, Inc., U.S.A., killed more than 2,000 people outright and injured anywhere from 150,000 to 600,000 others, some 6,000 of whom would later die from their injuries. The United Kingdom suffered its worst air pollution event when the December 4th Great Smog of 1952 formed over London. In six days more than 4,000 died, and 8,000 more died within the following months. An accidental leak of anthrax spores from a biological warfare laboratory in the former USSR in 1979 near Sverdlovsk is believed to have been the cause of hundreds of civilian deaths. The worst single incident of air pollution to occur in the United States of America occurred in Donora, Pennsylvania in late October, 1948, when 20 people died and over 7,000 were injured.

The health effects caused by air pollutants may range from subtle biochemical and physiological changes to difficulty in breathing, wheezing, coughing and aggravation of existing respiratory and cardiac conditions. These effects can result in increased medication use, increased doctor or emergency room visits, more hospital admissions and premature death. The human health effects of poor air quality are far reaching, but principally affect the body's respiratory system and the cardiovascular system. Individual reactions to air pollutants depend on the type of pollutant a person is exposed to, the degree of exposure, the individual's health status and genetics. People who exercise outdoors, for example, on hot, smoggy days increase their exposure to pollutants in the air.

Reduction efforts

There are many air pollution control technologies and urban planning strategies available to reduce air pollution; however, worldwide costs of addressing the issue are high.  Of course, these costs are a small fraction of the economic damage that air pollution will inflict on every nation of earth. Within the last decade the cost of air pollution annually in most of Europe is between 1-3 percent GDP and is at least 5 percent GDP of China.

Many countries have programs to or are debating how to reduce dependence on fossil fuels for energy production and shift toward renewable energy technologies or nuclear power plants.

Efforts to reduce pollution from mobile sources includes primary regulation (many developing countries have permissive regulations), expanding regulation to new sources (such as cruise and transport ships, farm equipment, and small gas-powered equipment such as lawn trimmers, chainsaws, and snowmobiles), increased fuel efficiency (such as through the use of hybrid vehicles), conversion to cleaner fuels (such as bioethanol, biodiesel), or conversion to electric vehicles with renewable energy sources (batteries or clean fuel such as hydrogen being used for transport and storage).