According to the US EPA, Emission Factors (EFs) are representative values that attempt 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 a pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant. Typically for greenhouse gasses (GHGs), EFs are written as the mass of a gas per unit of the emissions-producing activity.
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.