Pollen can be as small as 10 micrometers (µm), tobacco smoke particles are about 0.01 µm, and smog (translucent cubes of smoke and fog primarily resulting from automobile combustion) as small as 0.001 µm.
The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants: particle pollution, photochemical oxidants and ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Examples of other toxic air pollutants include benzene, found in gasoline; perchlorethlyene, emitted from some dry cleaning facilities; methylene chloride, used as a solvent and paint stripper by a number of industries; asbestos released form some building materials, plastic and textiles; refrigerants released from vehicle air conditioners; airborne formaldehyde from smoke, cosmetics and disinfectants.