Sources of pollution vary not only in their location and type but also in their intensity: the scale of the operations being carried out, the amounts of materials being consumed or produced, and thus the total quantities of waste materials being generated.  Data on source intensity are therefore valuable in impact assessments, both as inputs into emission models and, where these are not available, as proxies for emissions in their own right.

Measures of source intensity

Source intensity can be described in many different ways, and which is most appropriate or informative is likely to depend on circumstances, including the type of source activity being considered, the specific pollutants or other agents of interest, and the environmental pathways and health impacts of concern.  Typically, sources can be characterised in terms of four main attributes:

• the geographic volume or extent of the operation (e.g. total operational area; road length);
• operational numbers (e.g. numbers of workers employed; numbers of road users; livestock numbers; numbers of households or residents);     
• quantities of inputs consumed (e.g. energy consumption; total volume or mass of raw materials);
• quantities of throughputs or outputs produced (e.g. volume or mass of materials produced; value of total sales; traffic volume).

No one of these tells the complete picture about the magnitude of source activities, and clearly each is likely to give only an approximate measure of the potential impacts on the environment.  In terms of emissions, measures from further down the causal chain (i.e. more closely related to output) are likely to provide better indicators.  Nevertheless, this is is not always true, for rates of release are influenced also by where within the process waste materials are produced and the effectiveness of emission controls.  In the case of animal farming, for example, livestock numbers may be a better predictor of emission potential than meat or milk production because even non-productive animals generate wastes; in the case of road transport, emissions are highly dependent on traffic speed, so may vary with road type (e.g. between motorways and urban streets), independently of traffic flow.     

Data types and sources

A wide range of data on source activities is generally collected, as part of business surveys, taxation returns and reporting under environmental regulations.  For some major industrial sources, these may be available at a site-specific level (e.g. as part of industrial registers); likewise, detailed data on traffic flows and composition may be available for individual network links where monitoring or modelling is carried out for the purpose of management.  In many cases, however, detailed data such as these are not readily available for reasons of commercial confidentiality, public privacy or security.  In these cases, aggregated data have to be used (e.g. statistical data on employment, consumption or production; population census data).  For some widely distributed emission sources, this aggregation may not greatly degrade the quality of the data as a basis for analysis; for localised sources, or where the releases are likely to be locally confined (e.g. due to release into soils or confined water bodies), aggregation can greatly distort the apparent distribution of source activities.  In these cases, therefore, some form of modelling may be necessary to try to enhance the spatial resolution of the data (see link to Modelling source activity, in panel to left). 

Links to data sources, and factsheets on a selection of sources, are given in the Data section of the Toolkit.