PM 2.5 refers to airborne particles less than 2.5 micrometers in diameter. These microscopic particles can consist of several chemical compounds and are created by combustion sources, such as coal-fired power plants, steel and coke works and motor vehicles. PM 2.5 can travel deep into the lungs and studies have reported long– and short-term exposure to be associated with serious health conditions, including respiratory ailments and cardiovascular disease. Concentrations of PM 2.5 typically are not uniform across a region and can be affected by weather, topography, local and upwind emissions sources and other factors. PM 2.5 is measured in micrograms per cubic meter. In the data, the higher the number, the higher the concentration of the pollutant. The Clean Air Act requires the EPA to set National Ambient Air Quality Standards for six major air pollutants, including PM 2.5. The current standard for PM 2.5 annual average is 12 micrograms per cubic meter based on a three-year average.