Compared with the pilot study, the results of this analysis provide a more accurate estimate of the numbers of buildings along the road links with the highest predicted NO₂ concentrations. A large amount of data has been used and although the method developed is partly automated, the results at each stage are checked carefully to ensure that the results are as accurate as possible based on the information available.

The method can be applied to assess exposure at any distance from the road for any pollutant for which concentration data is available. The NO₂ exposure results provided here can also be updated as the NO₂ modelling is improved, which may result in a different set of road links requiring analysis.

In this study, the households exposed to NO₂ have been assumed to be those within 10m of the kerbside. This distance is chosen somewhat arbitrarily, based on the knowledge that NO₂ concentrations decrease from the kerbside towards the buildings, although the rate of this decline is not known. Also the predictive model of air concentrations is only valid for roads where there is a canyon effect caused by buildings close to the roads. It is thought that this model is not reliable where the buildings are greater than 10m from the kerb.

The actual number of residential houses exposed to exceedances of the NO₂ objectives are likely to be considerably higher than stated here, as only 102 links have been considered compared with 761 which were predicted to exceed the NO₂ objective in 2005. The links considered were those with the highest predicted NO₂ concentrations. However, those with lower traffic flows, and hence lower predicted concentrations, are more likely to be residential roads, and therefore may represent higher risks in terms of exposure.

The results of this report may be of use to Local Authorities as a check against locally derived estimates of exposure or as a starting point in the analysis of likely areas requiring action.

Chapter 3         Chapter 5