Pollutants are released into our atmosphere by many methods. Smokestacks from factories have been increasing due to the rise of industry in the last century. These are known as stationary sources. Mobile sources, on the other hand, include exhaust from motor vehicles, trains, airplanes, etc. The pollution from these sources is measured by the amount emitted as well as the amount of pollution in the ambient air. Both of these sources contribute significant amounts of contaminants into the air that we breathe. Air pollution standards are a way of limiting emissions and controlling the damage.
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