Air quality standards are prescribed by Central Pollution Control Board. There are three types of standards:
i) Emission standards, which limit the emission of pollutants from a source; stock emission standard are the maximum tolerable level of a pollutant that are permitted to escape the chimney of an industry or the stack of a thermal power plant.
ii) Immersion standards, which specify the ambient air quality, i.e. the amounts of various pollutants, which are tolerable in space surrounding the source of generation. Immersion standards are the maximum levels of a pollutant, which can be tolerated in the air mass of the surrounding areas of the industry/factory or a thermal power plant. These are also called as ambient air standards.
iii) A third category of standards can be defined separately, for the sake of convenience; the vehicular emission standards. These standards have been prescribed only recently. Vehicular emission standards are the maximum tolerable limits of the levels of various pollutants that are allowed to escape the exhaust duct of an automobile.
a) Emission Standards
Various emission standards have been prescribed for different industries. The maximum permissible level of dust in the form of suspended particular matter (SPM) is 150 mg/Nm3 (read as milligram per normal cubic meter). Dust comes from stacks of thermal power plants, iron and steel sintering plants, fertilizer manufacturing units, small industry boilers and stone quarrying activities. Other industrial units such as metal refineries, brick kilns and synthetic fiber plants belch out other pollutants in the form of acid mist consisting of droplets of dissolved gases suspended in air. These droplets consist mainly of aqueous solution of varying proportions of gases like SO2, oxides of nitrogen (NOx), CS2, CO2, and H2S. The total permissible level of acid mist in the stack exhaust should not exceed 50 mg/NM3.
b) The Immission Standards
The gaseous pollutants released from the stacks of various industries are ultimately deposited in the air surrounding these units, that is, in the ambient air. Obviously, regardless of how much of pollutants are released from the stack, there is a limit to the amount of individual pollutants that can be allowed to accumulate in the zone surrounding a factory, kiln or unit. The ambient air quality or immission air standards define the maximum permissible level/concentration of individual pollutants, which can be allowed to prevail in the surrounding air.
c) Vehicular Emission Standards
Automobile emissions of all types are thought to contribute 60 percent of total air pollutants in western countries. The gases and solid particles coming from automobile exhaust pipes consist mainly of carbon monoxide, lead compounds and oxides of nitrogen (NOx), as well as unburnt hydrocarbons.