It is widely suspected that the toxicity of ambient particulate matter is most strongly associated with the fine and ultrafine fractions, and that particle number concentrations may be a better metric for the assessment of public health burden than PM_(10). However, our ability to test this hypothesis is limited by the lack of data available for environmental exposure to particle number concentrations, which are less influenced by non-local emissions than PM_(10) and which exhibit greater spatial gradients close to sources, which in urban areas, is road traffic. In the UK, road transport is the primary source of ultrafine particles (UFP), contributing more than 80% of particulate matter in terms of number within the urban environment. Consequently, peak concentrations tend to occur near to roads. As such, journey-time exposures may contribute a disproportionately large fraction of total personal exposures to UFP. In addition, there is some evidence that it is these short-term peak exposures which are responsible for adverse health effects, with peak exposures of less than 1hour being the most damaging to health.Therefore even a short commute can be responsible for a person’s daily peak exposure.
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