The term "particulate matter" (PM) includes both solid particles and liquid droplets found in air. Many manmade and natural sources emit PM
directly or emit other pollutants that react in the atmosphere to form PM. These solid and liquid particles come in a wide range of sizes.
Particles less than 10 micrometers in diameter (PM10) pose a health concern because they can be inhaled into and accumulate in the
respiratory system Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine"particles and are believed to
post the largest health risks. Because of their small size, fine particles can lodge deeply into the lungs. Sources of fine particles include
all types of combustion (motor vehicles, power plants, wood burning, etc.) and some industrial processes. Particles with diameters between
2.5 and 10 micrometers are referred to as "coarse." Sources of coarse particles include crushing or grinding operations, and dust from paved
or unpaved roads.