Particulate matter is the general term for a mixture of solid particles, including pieces of dust, soot, dirt, ash, smoke, and liquid droplets or vapor directly emitted into the air, where they are suspended for long periods of time. Particulates can affect breathing, damage paints, and reduce visibility. These particles derive from stationary, mobile, and natural sources. Such sources include forest fires and volcanic ash; emissions from power plants, motor vehicles, wood stoves, and waste incineration; and dust from mining, paved and unpaved roads, and wind erosion.
In 1987, the EPA changed its regulatory focus from total suspended particulates (TSPs) to PM-10, suspended particulates that are 10 micrometers or smaller. The change was due to the recognition that smaller particles are more likely to be inhaled deeper into the lungs. Therefore, PM-10 is a better indicator of health impact than TSP. In 1997, the EPA changed the standards again, and it now regulates particles 2.5 micrometers and smaller (PM-2.5). Ambient air quality data (which begins in 1957 for about 60 areas) indicate that urban air quality for PM, as measured by TSP, has been improving since 1957. From 1988-1997, air quality concentrations of PM-10 measured at monitoring sites across the country decreased 25 percent.