For air pollution produced during an industrial process, the ratio of output (air pollutants or other by-product such as waste heat) to some measure of productivity, such as input quantity of fuel or raw materials or amount of final product. As written by the U. S. Environmental Protection Agency, an emission factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the weight of pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., kilograms of particulate emitted per megagram of coal burned).