Monitoring sites are divided into six different classes depending on their proximity to major sources of pollution:
|Kerbside||Sites with sample inlets within 1m of the kerb of a busy road. Sampling heights are within 2-3m.|
|Roadside||Sites with sample inlets between 1m and 5m of the kerbside. Sampling heights are within 2-3m of the ground.|
|Urban Background||Urban locations away from major sources and broadly representative of town/city-wide background concentrations, e.g. urban residential areas.|
|Suburban||Sites typical of residential areas on the outskirts of a town or city.|
|Rural||Rural locations distanced from major population centres, roads, industrial areas or other pollution sources.|
|Industrial||Sites where industrial emissions make an significant contribution to pollution levels.|
Each class of site will record pollution levels which can be considered broadly representative of similar locations nearby. For example, an 'Urban Background' site in an urban area will record levels similar to those found in many nearby residential areas, whereas a 'Roadside' site will record levels more representative of those close to roads of comparative size.
Care must be taken when comparing results from different classes of sites.