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Air pollution occurs when harmful substances including particulates and biological molecules are
introduced into Earth's atmosphere. It may cause diseases, allergies or death of humans; it may also
cause harm to other living organisms such as animals and food crops, and may damage
the natural or built environment. Human activity and natural processes can both generate air
pollution.
Indoor air pollution and poor urban air quality are listed as two of the world's worst toxic pollution
problems in the 2008 Blacksmith Institute World's Worst Polluted Places report.[1] According to the
2014 World Health Organization report, air pollution in 2012 caused the deaths of around 7 million
people worldwide,[2] an estimate roughly echoed by one from the International Energy Agency.[3][4]
Carbon dioxide (CO2) - Because of its role as a greenhouse gas it has been described as
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"the leading pollutant"[5] and "the worst climate pollution".[6] Carbon dioxide is a natural component
of the atmosphere, essential for plant life and given off by the human respiratory system.[7] This
question of terminology has practical effects, for example as determining whether the U.S. Clean Air Act
is deemed to regulate CO2 emissions.[8] CO2 currently forms about 405 parts per million (ppm) of
earth's atmosphere, compared to about 280 ppm in pre-industrial times,[9] and billions of metric tons of
CO2 are emitted annually by burning of fossil fuels.[10] CO2 increase in earth's atmosphere has been
accelerating.[11]
Sulfur oxides (SOx) - particularly sulfur dioxide, a chemical compound with the formula SO2. SO2 is
produced by volcanoes and in various industrial processes. Coal and petroleum often contain sulfur
compounds, and their combustion generates sulfur dioxide. Further oxidation of SO2, usually in the
presence of a catalyst such as NO2, forms H2SO4, and thus acid rain.[2] This is one of the causes for
concern over the environmental impact of the use of these fuels as power sources.
Nitrogen oxides (NOx) - Nitrogen oxides, particularly nitrogen dioxide, are expelled from high
temperature combustion, and are also produced during thunderstorms by electric discharge. They can
be seen as a brown haze dome above or a plume downwind of cities. Nitrogen dioxide is a chemical
compound with the formula NO2. It is one of several nitrogen oxides. One of the most prominent air
pollutants, this reddish-brown toxic gas has a characteristic sharp, biting odor.
Carbon monoxide (CO) - CO is a colorless, odorless, toxic yet non-irritating gas. It is a product of
incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of
carbon monoxide.
Volatile organic compounds (VOC) - VOCs are a well-known outdoor air pollutant. They are categorized
as either methane (CH4) or non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas
which contributes to enhanced global warming. Other hydrocarbon VOCs are also significant
greenhouse gases because of their role in creating ozone and prolonging the life of methane in the
atmosphere. This effect varies depending on local air quality. The aromatic NMVOCs benzene, toluene
and xylene are suspected carcinogens and may lead to leukemia with prolonged exposure. 1,3-
butadiene is another dangerous compound often associated with industrial use.
Particulates, alternatively referred to as particulate matter (PM), atmospheric particulate matter, or fine
particles, are tiny particles of solid or liquid suspended in a gas. In contrast, aerosol refers to combined
particles and gas. Some particulates occur naturally, originating from volcanoes, dust storms, forest and
grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in
vehicles, power plants and various industrial processes also generate significant amounts of aerosols.
Averaged worldwide, anthropogenic aerosols—those made by human activities—currently account for
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approximately 10 percent of our atmosphere. Increased levels of fine particles in the air are linked to
health hazards such as heart disease,[12] altered lung function and lung cancer.
Persistent free radicals connected to airborne fine particles are linked to cardiopulmonary
disease.[13][14]
Toxic metals, such as lead and mercury, especially their compounds.
Chlorofluorocarbons (CFCs) - harmful to the ozone layer; emitted from products are currently banned
from use. These are gases which are released from air conditioners, refrigerators, aerosol sprays, etc. On
release into the air, CFCs rise to the stratosphere. Here they come in contact with other gases and
damage the ozone layer. This allows harmful ultraviolet rays to reach the earth's surface. This can lead
to skin cancer, eye disease and can even cause damage to plants.
Ammonia (NH3) - emitted from agricultural processes. Ammonia is a compound with the formula NH3. It
is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly
to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers.
Ammonia, either directly or indirectly, is also a building block for the synthesis of many pharmaceuticals.
Although in wide use, ammonia is both caustic and hazardous. In the atmosphere, ammonia reacts with
oxides of nitrogen and sulfur to form secondary particles.[15]
Odours — such as from garbage, sewage, and industrial processes
Radioactive pollutants - produced by nuclear explosions, nuclear events, war explosives, and natural
processes such as the radioactive decay of radon.
Secondary pollutants include:
Particulates created from gaseous primary pollutants and compounds in photochemical smog. Smog is a
kind of air pollution. Classic smog results from large amounts of coal burning in an area caused by a
mixture of smoke and sulfur dioxide. Modern smog does not usually come from coal but from vehicular
and industrial emissions that are acted on in the atmosphere by ultraviolet light from the sun to form
secondary pollutants that also combine with the primary emissions to form photochemical smog.
Ground level ozone (O3) formed from NOx and VOCs. Ozone (O3) is a key constituent of the
troposphere. It is also an important constituent of certain regions of the stratosphere commonly known
as the Ozone layer. Photochemical and chemical reactions involving it drive many of the chemical
processes that occur in the atmosphere by day and by night. At abnormally high concentrations brought
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about by human activities (largely the combustion of fossil fuel), it is a pollutant, and a constituent of
smog.
Peroxyacetyl nitrate (C2H3NO5) - 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 particulates
Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental
degradation through chemical, biological, and photolytic processes. Because of this, they have been
observed to persist in the environment, to be capable of long-range transport, bioaccumulate in human
and animal tissue, biomagnify in food chains, and to have potentially significant impac
