Emission factors provide estimates of the typical or average rate of emission (per unit of activity) of a specified substance from a specified source or process.  'Activity' in this context can be characterised in different ways, according to the type of source and purpose of the emission factor: e.g. per unit (mass, volume) of material or energy consumed, per unit of output produced, per individual (employee, consumer, animal, livestock unit) or per distance travelled (for transport sources).  Together with matching data on source intensity, the factors thus provide a means of estimating contaminant releases.

Emission factors are derived in several different ways, depending to a large degree on the type of source activity and the contaminant concerned.  For major controlled or confined sources, such as large combustion plants, direct measurement of releases is possible during routine operation (e.g. by in-stack monitoring); measurements across a sample of these facilities can then be used to estimate emission factors per unit of activity.  For smaller or more diffuse sources, such as road vehicles or farming activities, estimates are usually based on observations under experimental conditions, designed to simulate normal activities.  Alternatively, factors may be approximated by comparing inputs of the substance of interest contained in the raw materials with that in the usable products; losses can then be estimated by differencing.  Where other methods are not possible, estimates may also be made by establishing a relationship between concentrations in the environment and local source activity levels (e.g. between measured concentrations of pollen in the amosphere and the density of source species in the surrounding area). 

In all cases, the uncertainties involved in reported emission factors also need to be recognised.  These derive primarily from the limited representativeness of the situations from which the data are gathered; inevitably these do not necessarily reflect the range of local conditions and individual behaviours (e.g. in terms of equiment maintenance and use) which may exist.  As a consequence, emission estimates based on the use of standard emission factors are highly approximate, and need to be treated with caution.

Databases of emission factors have been extensively developed for emissions into the atmosphere. Effort to develop and use these factors has been motivated, especially, by international initiatives to control climate change and long-range air pollution, but national factors have also been devised for more local uses.   Equivalent factors for other media tend to be scarce, and indeed the very concept of emission factors is much less highly evolved in relation to noise, electro-magnetic radiation, water or soil pollution.  In these cases, modelling has to rely on other ways of estimating emissions - for example, by reference to industry standards and government regulations on emission levels (such as EU regulations on noise from outdoor equipment and household appliances).   

Links to a selection of emission factor databases are given below, and factsheets and data for some key European sources are included in the Data section of the Toolkit.  Links to a range of other, ad hoc, data sets are provided in the panel to the left.