An emission factor is a tool used to estimate emissions to the environment, and 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 ton of coal combusted). Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average). Emission factor based calculations are a cost effective means to "screen" one's emissions prior to, and in some instances in place of, actual testing. The US EPA has published emission factors for numerous activities after decades of investigation.