Of all of the pollutants, that are contnuously dispersed into the atmosphere by various sources fine particulate matter has the greatest effect on human health. PM is defined as a mixture of solid particles and liquid droplets found in the air. It includes “inhalable coarse particles,” with aed larger than 2.5 microns and smaller than 10 microns and “fine particles,” with diameters that are 2.5 micrometers and smaller. The particles comprising PM may come in many sizes and shapes and can be made up of hundreds of different chemicals. Some particles, known as primary particles are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires.