Particulate matter (PM) is a term for particles and liquid droplets suspended in the air. Particulate matter originates from a variety of sources including diesel trucks, power plants, wood stoves and industrial processes. The chemical and physical compositions of these various particles vary widely. Health risks include premature death from heart and lung disease, aggravation of heart and lung diseases and respiratory and cardiovascular effects.

The current 24-hour fine particle standard is 35 µg/m3 and the annual fine particle standard is 15 µg/m3.

Unlike the ozone standard, there is no classification system for fine particulate matter; an area either meets the standard (attainment) or exceeds the standard (nonattainment). The EPA has designated the 20-county metro Atlanta as a nonattainment area for fine particular matter effective April 5, 2005. For more information on particulate matter reference "An Introduction to Particulate Matter for Transportation Officials." (PDF).