Air pollution is a chemical, physical (e.g. 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. Enforced air quality standards, like the Clean Air Act in the United States, have reduced the presence of some pollutants. While major stationary sources are often identified with air pollution, the greatest source of emissions is actually mobile sources, principally the automobile. Gases such as carbon dioxide, which contribute to global warming, have recently gained recognition as pollutants by some scientists. Others recognise the gas as being essential to life, and therefore incapable of being classed as a pollutant.
The World Health Organization estimates 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. Research 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. 
Air pollutants are classified as either directly released or formed by subsequent chemical reactions. A direct release air pollutant is one that is emitted directly from a given source, such as the carbon monoxide or sulfur dioxide, all of which are byproducts of combustion; whereas, a subsequent air pollutant is formed in the atmosphere through chemical reactions involving direct release pollutants. The formation of ozone in photochemical smog is the most important example of a subsequent air pollutant.
Anthropogenic sources (human activity) related to burning different kinds of fuel
Combustion-fired power plants (coal-fired power plants contribute 40% of world's sulphur and mercury)
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 emissions
Burning fossil fuels
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, 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.
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.
Pine trees, which emit volatile organic compounds (VOCs).
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.
Air quality standards
In Canada, air quality standards are typically determined by a federal-provincial process known as the Canadian Council of Ministers for the Environment (CCME). CCME created the Canada-Wide Standards for ozone and fine particulate matter (PM 2.5). National Ambient Air Quality Objectives also exist for criteria air contaminants, although these are used less frequently than the CCME standards.
Managing air quality in the United Kingdom is the responsibility of DEFRA. DEFRA core purpose is to improve the current and future quality of life a broad range of responsibilities of which environmental protection and air quality are key. DEFRA has recently (16th August 2006) launched a wiki to help create an environmental contract in collaboration with members of the UK community.
The present secretary for state the Rt Honorable David Milliband said in a recent speech:
If citizens, businesses and nations are to change their behaviour, they must be confident that their actions will be reciprocated. Citizens need to know their neighbours are committed and that together their actions will have critical mass. Business need to know that the bar is being raised, but it is being raised for all businesses. Nations need to know that others will follow suit.
This succinctly summarises the reason why the UK is harmonising its air quality strategy with the European Union air pollution strategy; so that communities that commit to improving air quality will not be penalised economically by becoming less competitive.
Air quality targets set by DEFRA are mostly aimed at local government representatives responsible for the management of air quality in cities, where air quality management is the most urgent. The UK has established an air quality network where levels of the key air pollutants are published by monitoring centers. Air quality in Oxford, Bath and London is particuarly poor. One controversial study created by Calor Gas and published in the Guardian newspaper compared walking in Oxford on an average day to smoking over sixty light cigarettes.
'Cigarette equivalents' is obviously a headline capturing measure and more reliable and accepted comparison data can be collected from UK Air Quality Archive which allows the user to compare a cities management of pollutants against objectives set by DEFRA in 2000.
It is, however, important to evaluate several aspects of air pollution, and especially to take into consideration overall average values, rather than localized peak values sometimes cited. The UK National Air Quality Information Archive offers almost real-time monitoring "current maximum" air pollution measurements for many UK towns and cities. This source offers a wide range of constantly updated data, including:
Hourly Mean Ozone (µg/m³);
Hourly Mean Nitrogen dioxide (µg/m³);
max 15 min mean Sulphur dioxide (µg/m³);
8 Hourly Mean Carbon monoxide (mg/m³), and
24 Hour mean PM10 Particles (µg/m³ Grav Equiv)
DEFRA acknowledges that air pollution has a significant effect on health and has produced a simple banding system that is used to create a daily warning system that is issued by the BBC Weather service to indicate air pollution levels. DEFRA has published guidelines for people suffering from respiratory and heart diseases.
In the 1960s, 70s, and 90s, the United States Congress enacted a series of Clean Air Acts which significantly strengthened regulation of air pollution. Individual U.S. states, some European nations and eventually the European Union followed these initiatives. The Clean Air Act sets numerical limits on the concentrations of a basic group of air pollutants and provides reporting and enforcement mechanisms.
In 1999, the United States EPA replaced the Pollution Standards Index (PSI) with the Air Quality Index (AQI) to incorporate new PM2.5 and Ozone standards.
Atmospheric dispersion modeling
main article: Atmospheric dispersion modeling
The basic technology for analyzing air pollution is through the use of a variety of mathematical models for predicting the transport of air pollutants in the lower atmosphere. The principal methodologies are:
Point source dispersion, used for simple industrial sources.
Line source dispersion, used for airport and roadway air dispersion modeling
Area source dispersion, used for forest fires or duststorms
Photochemical models, used to analyze reactive pollutants such as form smog
The point source problem is the best understood, since it involves simpler mathematics and has been studied for a long period of time, dating back to about the year 1900. It uses a Gaussian dispersion model to forecast the air pollution isopleths, with consideration given to wind velocity, stack height, emission rate, stability class (a measure of atmospheric turbulence). This model has been extensively validated and calibrated with experimental data for all sorts of atmospheric conditions.
The roadway air dispersion model was developed starting in the late 1950s and early 1960s in response to requirements of the National Environmental Policy Act and the U.S. Department of Transportation (then known as the Federal Highway Administration) to understand impacts of proposed new highways upon air quality, especially in urban areas. Several research groups were active in this model development, among which were: the Environmental Research and Technology (ERT) group in Lexington, Massachusetts, the ESL Inc. group in Sunnyvale, California and the California Air Resources Board group in Sacramento, California. The research of the ESL group received a boost with a contract award from the U.S. Environmental Protection Agency to validate a line source model using sulfur hexafluoride as a tracer gas. This program was successful in validating the line source model developed by ESL inc. Some of the earliest uses of the model were in court cases involving highway air pollution, the Arlington, Virginia portion of Interstate 66 and the New Jersey Turnpike widening project through East Brunswick, New Jersey.
Area source models were developed in 1971 through 1974 by the mid 1970s by the ERT and ESL groups, but addressed a smaller fraction of total air pollution emissions, so that their use and need was not as widespread as the line source model, which enjoyed hundreds of different applications as early as the 1970s. Similarly photochemical models were developed primarily in the 1960s and 1970s, but their use was more specialized and for regional needs, such as understanding smog formation in the Livermore Valley, California.
Indoor air pollution, or Indoor air quality (IAQ)
The lack of ventilation indoors concentrates air pollution where people have greatest exposure times. Radon (Rn) gas, a carcinogen, is exuded from the Earth in certain locations and trapped inside houses. Researchers have found that radon gas is responsible for over 1,800 deaths annually in the United Kingdom. Building materials including carpeting and plywood emit formaldehyde (H2CO) gas. Paint and solvents give off volatile organic compounds (VOCs) as they dry. Lead paint can degenerate into dust and be inhaled. Intentional air pollution is introduced with the use of air fresheners, incense, and other scented items. Controlled wood fires in stoves and fireplaces can add significant amounts of smoke particulates into the air, inside and out. Indoor air pollution may arise from such mundane sources as shower water mist containing arsenic or manganese, both of which are damaging to inhale. The arsenic (As3+) can be trapped with a shower nozzle filter.
Indoor pollution fatalities may be caused by using pesticides and other chemical sprays indoors without proper ventilation, and many homes have been destroyed by accidental pesticide explosions.
Carbon monoxide (CO) poisoning is a quick and silent killer, often caused by faulty vents and chimneys, or by the burning of charcoal indoors. 56,000 Americans died from CO in the period 1979-1988. Chronic carbon monoxide poisoning can result even from poorly adjusted pilot lights. Smoke inhalation is a common cause of death in victims of house fires. Traps are built into all domestic plumbing to keep deadly sewer gas, hydrogen sulfide, out of interiors. Clothing emits tetrachloroethylene, or other dry cleaning fluids, for days after dry cleaning.
Though its use has now been banned in many countries, the extensive use of asbestos in industrial and domestic environments in the past has left a potentially very dangerous material in many localities. Asbestosis is a chronic inflammatory medical condition affecting the tissue of the lungs. It occurs after long-term, heavy exposure to asbestos, e.g. in mining or in the installation or removal of asbestos-containing materials from structures. Sufferers have severe dyspnea (shortness of breath) and are at an increased risk regarding several different types of lung cancer. As clear explanations are not always stressed in non-technical literature, care should be taken to distinguish between several forms of relevant diseases. According to the World Health Organisation (WHO), these may defined as; asbestosis, lung cancer, and mesothelioma (generally a very rare form of cancer, when more widespread it is almost always associated with prolonged exposure to asbestos).
Biological sources of air pollution are also found indoors, as gases and airborne particulates. Pets produce dander, people produce dust from minute skin flakes, dust mites in bedding, carpeting and furniture produce enzymes and micron-sized fecal droppings, inhabitants emit methane, mold forms in walls and generates mycotoxins and spores, air conditioning systems can incubate Legionnaires' disease and mold, toilets can emit feces-tainted mists , and houseplants, soil and surrounding gardens can produce pollen, dust, and mold. Indoors, the lack of air circulation allows these airborne pollutants to accumulate more than they would otherwise occur in nature.
There are many available air pollution control technologies and urban planning strategies available to reduce air pollution; however, worldwide costs of addressing the issue are high. Enforced air quality standards, like the Clean Air Act in the United States, have reduced the presence of some pollutants.
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).
The following items are commonly used as pollution control devices by industry or transportation devices. They can either destroy contaminants or remove them from an exhaust stream before it is emitted into the atmosphere.
Mechanical collectors (dust cyclones, multicyclones)
Fabric filters (baghouses)
Low NOx burners
Selective catalytic reduction (SCR)
Selective non-catalytic reduction (SNCR)
Exhaust gas recirculation
Catalytic converter (also for VOC control)
Adsorption systems, such as activated carbon
Acid Gas/SO2 control
Flue gas desulfurization
Sorbent Injection Technology
Electro-Catalytic Oxidation (ECO)
Dioxin and furan control
Ambient cleaning systems
Source capturing systems
Continuous emissions monitoring systems (CEMS)
 Greenhouse effect and ocean acidification
The greenhouse effect is a phenomenon whereby carbon dioxide levels are thought to create a condition in the upper atmosphere, causing a trapping of excess heat and leading to increased surface temperatures. It shares this property with many other gases, and water vapour produces a larger effect than carbon dioxide. Other greenhouse gases include methane and NOx. This effect has been understood by scientists for about a century, and technological advancements during this period have helped increase the breadth and depth of data relating to the phenomenon. A number of studies have investigated the potential for long-term rising levels of atmospheric carbon dioxide to cause slight increases in the acidity of ocean waters and the possible effects of this on marine ecosystems. However, carbonic acid is a very weak acid, and is utilised by marine organisms during photosynthesis.
 See also
Air pollution dispersion modeling books
Air pollution in British Columbia
Air Quality Index
AP 42 Compilation of Air Pollutant Emission Factors
Asian brown cloud
Atmospheric dispersion modeling
Compilation of atmospheric dispersion models
Flue gas desulfurization
Flue gas emissions from fossil fuel combustion
Global Atmosphere Watch
International Agency for Research on Cancer (IARC)
National Ambient Air Quality Standards
Smog and Haze
Spare the Air program