The air pollutant substances include gases (oxides of nitrogen, oxides of sulphur, carbon monoxide, hydrocarbons, etc), particulate matter (smoke, dust, fumes, aerosols), addictive material and many other. Most of these substances are naturally present in low concentrations and are usually considered to be harmless.
Air pollutants are divided into two categories :
(i) Primary pollutants (ii) Secondary pollutants.
The Primary pollutants are those which are emitted directly from the sources. These include particulate matter such as ash, smoke, dust fumes, etc.; inorganic gases such as sulphur dioxide, carbon monoxide, hydrogen sulphides, nitric oxide, ammonia, hydrogen fluoride and carbon dioxide; hydrocarbons those which are formed in the atmosphere by chemical interactions among primary pollutants and normal atmospheric constituents. Pollutants such as sulphur trioxide, nitrogen dioxide, ozone, aldehydes, ketons, various sulphate and nitrate salts, etc, are the secondary pollutants. The major processes that convert the pollutants fro the primary forms to their secondary forms are oxidation, dissociation and dissolution. Like :
| Reaction | Primary Pollutant | Secondary Pollutant |
| Oxidation | Sulphur dioxide Carbon monoxide Hydrogen sulphide |
Sulphur trioxide Carbon dioxide Sulphur dioxide |
| Dissociation | Nitrogen dioxide Oxygen (molecular) Hydrocarbons and aldehydes |
Nitric Oxide + Oxygen Oxygen (atomic) Organic radicals |
| Dissolution | Carbon dioxide Sulphur trioxide Nitrogen dioxide |
Carbonic acid Carbonic acid Nitric / Nitrous acid |
More than 90% of the air pollution is created by the following five primary pollutants :
(i) Sulphur oxides (ii) Nitrogen oxides (iii) Carbon monoxide (iv) Hydrocarbons (v) Particular
| Air pollutants | Major sources |
| Sulphur oxides | Combustion of fossil fuels |
| Nitrogen oxides | Transportation (mostly automobiles) Combustion of fuels (mostly natural gas and coal) |
| Carbon monoxide | Transportation (mostly automobiles) Agricultural burning |
| Hydrocarbons | Transportation (mostly automobiles) Industrial process Evaportion of organic solvents. |
| Particulates | Industrial processes Combustion of fuels Agricultural burning |