Types and Sources of Air Pollution

Air Pollutant:  Any airborne substance (solids, liquid or gas) which threatens the health of people, animals
           or vegetation, and/or which harms structures

                Natural Sources:  volcanic emissions, forest and range fires, wind-blown dust, pollen grains

                Human-Related Sources:  burning of fuels, by-products of industrial processes or chemical
                            processes

Primary Pollutant:  Any pollutant which enters the air directly

Secondary Pollutant:  A pollutant which is formed in a chemical reaction involving a primary pollutant and
           either air or another pollutant