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Nitrogen oxides (NOx) – especially nitrogen dioxide are emitted from
high temperature combustion. Nitrogen dioxide is the chemical compound
with the formula NO2. It is one of the several nitrogen oxides. This reddish-
brown toxic gas has a characteristic sharp, biting odor. NO2 is one of the
most prominent air pollutants.
Carbon monoxide – is a colorless, odorless, non-irritating but very poi-
sonous 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
but is also a gas vital to living organisms. It is a natural gas in the atmos-
phere.
Volatile organic compounds – VOCs are an important outdoor air pol-
lutant. In this field they are often divided into the separate categories of me-
thane (CH4) and non-methane (NMVOCs). Methane is an extremely efficient
greenhouse gas which contributes to enhanced global warming. Other hydro-
carbon VOCs are also significant greenhouse gases via their role in creating
ozone and in prolonging the life of methane in the atmosphere, although the
effect varies depending on local air quality.
Particulate matter – Particulates, alternatively referred to as particulate
matter (PM) or fine particles, are tiny particles of solid or liquid suspended in
a gas. In contrast, aerosol refers to particles and the gas together. Sources of
particulate matter can be man-made or natural. Some particulates occur natu-
rally, originating from volcanoes, dust storms, forest and grassland fires, liv-
ing 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.
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. 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 nutri-
tional 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.
Odors – such as from garbage, sewage, and industrial processes
Radioactive pollutants – produced by nuclear explosions, war explo-
sives, and natural processes such as the radioactive decay of radon.
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