In July 1997, EPA issued National Ambient Air Quality Standards for Fine Particles (PM 2.5). The standards include an annual standard set at 15 micrograms per cubic meter, based on the 3-year average of annual mean PM 2.5 concentrations and a 24-hour standard of 65 micrograms per cubic meter, based on the 3-year average of the 98th percentile of 24-hour concentrations.

Fine particle pollution is a mixture of microscopic solids and liquid droplets suspended in air. Fine particles can be emitted directly (such as smoke from a fire) or formed in the atmosphere from power plant, industrial and mobile source emissions of gases such as sulfur dioxide and nitrogen oxides. Fine particles less than or equal to 2.5 micrometers in diameter (called PM2.5 and measuring about one-thirtieth the diameter of an average human hair), pose the greatest risk. These particles can get deep into the lungs, and some may even get into the bloodstream.