Particulate matter or PM pollution refers mostly to dust and tiny particles varying from ~ 0.01 microns - 1000 microns. The composition of particulate matter can be anything from inorganic particles (dust, soot, ash, salt spray over the oceans, etc.) to organic particles (bacteria, mold, and pet dander allergens, etc.). The larger particles are heavy enough to settle out, while the smaller and lighter particles reamin suspended in the atmosphere. Particle pollution of most concern and regulation includes "inhalable coarse particles," with diameters larger than 2.5 micrometers and smaller than 10 micrometers and "fine particles," with diameters that are 2.5 micrometers and smaller. These are referred to as PM2.5 and PM10.