44 Assessing the impact of particular policies on air quality is a complex science. Sophisticated modelling tools exist to forecast emissions from different sources and estimate the impact on ambient concentration levels of different pollutants at different locations.23 Government departments and agencies may need to consider air quality impacts in the design of their policies. For example, the Highways Agency's Design Manual for Roads and Bridges can be used to forecast the impact of new or existing road schemes on emissions of key pollutants from road transport.
45 Impacts on air quality are generally expressed in terms of either the total volume change in emissions of a particular pollutant from a particular source; the likely impact of this change on levels of ambient air quality in the affected area; or the total number of households likely to be affected by these changes.
46 In cases where detailed modelling is not possible, a reasoned statement of whether or not a particular policy is likely to result in greater or lesser emissions of particular pollutants should be included in the appraisal.
47 Research has been funded to develop a methodology for quantifying and monetising, where appropriate, the health and environmental impacts of air quality changes.24
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23 For a technical reference on the approach to air quality mapping and modelling, see "The Air Quality Strategy for England, Scotland, Wales and Northern Ireland" Defra, January 2000
24 Guidance can be found on the Defra website (http://www.defra.gov.uk) Defra has also sponsored research to generate empirical estimates of UK WTP for reductions in health risks associated with air pollution