A primary air pollutant is emitted directly into the air from its source. Primary air pollutants are known to cause many health problems, negatively impact the environment and cause property damage. When in the atmosphere, these pollutants can undergo chemical reactions, forming other pollutants, which are called secondary pollutants.
The six most common air pollutants are lead, particulate matter, ozone, carbon monoxide, nitrogen oxides and sulfur dioxide. Of these pollutants, the two responsible for the most widespread health threats are particulate matter and ozone. Ground-level ozone is created when oxides of nitrogen combine with volatile organic compounds in the sunlight.