The spatial distribution of sources of stressors is one of the most important factors influencing environmental impacts on health.  Although pollutants (and many other hazards) are mobile, their intensity typically declines with distance away from their source, so that patterns of risk to human health broadly reflect the source distribution.  Partly for this reason, place of residence relative to the source has often been used as a proxy for exposure (or risk) in epidemiological studies - for example, of landfill sites, road traffic, nuclear power stations - with the consequence that the exposure-response functions available for use in the assessment relate to source distribution.  Information on source distribution is therefore vital in most assessments, either as direct input into the exposure metric or (more generally) as a basis for modelling the distribution and environmental fate of the hazard. 

Sources clearly vary in the character of their distribution, and in the structure of each specific source.  Some are localised, others more widely dispersed; some exist as clearly definable entities (e.g. a building); others are more diffuse and relate to a broad, and possibly poorly defined area.  The spatial distribution of sources can thus be represented in different ways:

• as points - e.g. a chimney stack
• as lines - e.g. a road
• as areas - e.g. an industrial zone 

Which is most relevant for the assessment depends on the spatial scale of the analysis, the specific hazards and environmental pathways, and target populations of interest. 

Detailed information on source location and distribution can be obtained from a number of different data providers, and in different forms including:

• land cover maps (and the satellite data on which they are based) - e.g. the Corine Land Cover 2000
• Infrastructure maps - e.g. road maps
• national or regional topographic maps
• emission inventories - e.g. the European Pollutant Release and Transfer Register (E-PRTR)

These and other data sources are listed and described in the Data section of the Toolkit.  A range of data, and links to other data providers, are also available via the European Environment Agency.