Air pollution is the introduction of harmful chemicals, biological wastes, and particulate matter into the atmosphere. Pollution has harmful effects on humans as well as on all other living beings.
Pollutants can be classified into two main categories:
Primary pollutants which are directly emitted into the atmosphere such as carbon monoxide from exhaust of a motor vehicle.
Secondary pollutants which are not emitted directly into atmosphere but are formed in air when primary pollutants interact.
Major primary pollutants include:
Carbon monoxide (CO) is produced by incomplete combustion of fuels like petrol, natural gas, coal or wood. It is a colourless and odourless gas but very poisonous in nature.
Carbon dioxide (CO2) is produced by complete combustion of fuels in motor vehicles and various industries. It is a colourless, odourless and non-toxic gas. (A person dies in atmosphere of carbon dioxide due to lack of oxygen and not due to its toxic nature).
Sulphur oxides (SOx) (mainly sulphur dioxide, SO2) are produced by combustion of coal and petroleum and also produced in volcanoes. It is also produced in various industrial processes. Oxidation of sulphur dioxide (SO2) to sulphur trioxide (SO3) results in formation of sulphuric acid (H2SO4) which causes acid rain.
Nitrogen oxides (NOx) especially nitrogen dioxide, NO2 is a reddish brown gas with pungent smell. It catalyses the oxidation of SO2 to SO3 and indirectly causes acid rain.
Volatile organic compounds (VOCs) include methane, benzene, toluene and xylene. While methane is a major green house gas, others are suspected to be carcinogens (cancer inducing).
Particulate matter consists of tiny particles of solids or liquids suspended in air. These are also called ‘suspended particulate matter (SPM)’. The major sources for these include volcanoes, dust storms and burning of fuels. These can cause heart and lung diseases and breathing disorders.
Chloro-fluorocarbons (CFCs) are used as refrigerants in air conditioners and refrigerators and are harmful to the ozone layer which protect us from harmful ultraviolet rays.
Major secondary pollutants include:
Photochemical smog (smoke + fog) formed by the action of ultraviolet light from the sun on particulate matter or formed due to burning of coal and petrol in an atmosphere containing SO2. It prevents dissipation of pollutants and causes breathing disorders.
Ground level ozone (O3) is formed from NOx and VOCs. It is a constituent of smog. Normally ozone occurs in stratosphere and prevents UV radiations from reaching earth’s surface. At ground level, when inhaled, it is harmful for health of humans and animals.