Pollutants may be primary or secondary or both: Primary being gases that are emitted directly into the air and secondary being pollutants that are formed through reactions with primary pollutants.
Major primary pollutants produced by human activity include:
Sulfur dioxide (SO2)- produced by volcanoes and various industrial processes such as the burning of fuels, such as coal and petroleum that contain sulfur compounds.
Sulfur dioxide is released into the air when gas from the burning of fuels is not treated. In addition to contributing to photochemical smog, it also causes acid rain when it dissolves into the rain water, forming sulfur trioxide, then oxidised further into sulfuric acid. When rain falls, it corrodes concrete and metallic structures. Its also the lowers pH of the soil and water bodies, killing aquatic life forests and other vegetation, indirectly affecting us humans.
Nitrogen oxides (NOx) - especially nitrogen dioxide are emitted from high temperature combustion where there are high amounts of energy. 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. Toxic NO2 is one of the most prominent air pollutants.
Carbon monoxide (CO)- is a 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. It can cause problems to the lungs when breathed in.
Carbon dioxide (CO2) - a colorless, odorless, non-toxic greenhouse gas associated with ocean acidification, emitted from sources such as combustion, cement production, and respiration.
Volatile organic compounds - One example, methane (CH3) is an extremely efficient greenhouse gas which contributes to enhanced global warming.
Particulate matter - fine particles of solid or liquid suspended in a gas. Contributes to photochemical smog. Increased levels of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer.
Chlorofluorocarbons(CFCs) - harmful to the ozone layer emitted from products currently banned from use.
Radioactive pollutants - produced by nuclear explosions, war explosives, and natural processes such as the radioactive decay of radon.
and Secondary pollutants include:
Ground level ozone (O3) - Ozone is a key constituent of the troposphere and the Ozone layer. It is formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone. At abnormally high concentrations brought about by human activities (largely the combustion of fossil fuel), it is a pollutant, and a constituent of smog. Exposure to ozone and the pollutants that produce it is linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems. Long-term exposure to ozone increases risk of death from respiratory illness.