There are two types of particulate matter: coarse and fine. Coarse particles, or PM 10, range in size from 2.5 to 10 micrometers in diameter; fine particles, or PM 2.5, are less than 2.5 micrometers in diameter.

Fine particles are tiny pieces of matter that are released into the air when fuels like coal, natural gas, gasoline and diesel motor vehicle fuel, oil and wood are burned.

Collectively, fine particles are found in smoke or haze.

Fine particles are about 30 times smaller than the diameter of a human hair.

Fine particle pollution is a year-round concern, but pollution levels tend to peak in the winter months.

Five central Indiana counties – Hamilton, Hendricks, Johnson, Marion and Morgan – were designated by the U.S. Environmental Protection Agency as non-attainment of the fine particle (PM 2.5) standard in April 2005. Click here for more information.