There are many types of air pollutants. The exact composition and concentration of pollutants depend on the source activity or process, the type of fuel and/or chemicals involved, and in some cases the meteorological conditions under which the pollutant is emitted. Air pollutants are pervasive, and are responsible for a range of adverse health and environmental effects. These pollutants include hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), sulfur dioxide (SO2), ozone, volatile organic compounds (VOCs), hydrogen sulfide (H2S), and toxic air contaminants such as lead (Pb).

Greenhouse gases such as carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and high-global warming potential gases (e.g., perfluorocarbons, sulfur hexafluoride, hydrofluorocarbons, nitrogen trifluoride, hydrofluoroethers and ozone depleting substances) have been implicated in global warming effects.

Sources of air pollution may also emit quantities of other substances which are often referred to collectively as toxic or “hazardous” air pollutants (HAPs). These pollutants can have more serious health impacts than some of the general pollutants, depending on the level of exposure. In many cases, toxic pollutants constitute a small fraction of the total HC or PM emissions.