The average release rate of a given pollutant for a given source, relative to units of production or activity. EFs are usually expressed as the weight of contaminant divided by a unit weight, volume, distance or duration of associated activity that emits the pollutant (e.g., kg of sulfur dioxide emitted per ton of coal burned). EFs are usually obtained from data of varying degrees of accuracy and may be presented for either uncontrolled sources or facilities having air pollution control devices in place. In Canada, EFs are categorized as either published emission factors or as site-specific emission factors.