This report identifies those air pollutants which, on the basis of current knowledge, appear to warrant closest attention when assessing the impact of road traffic upon human health, the natural environment and the built environment within a short distance of a highway. A multi-stage screening process has been adopted to deal with many hundreds of pollutants associated with road vehicles. Evidence suggests that the following pollutants are potentially of greatest importance: nitrogen dioxide, carbon monoxide, lead, benzene and polyaromatic hydrocarbons in terms of risks to health; and smoke (particulates) and possibly acid precursors in terms of amenity, public perception of nuisance and/or effects on the built environment. The technical literature on these pollutants is assessed with a view to identifying dose-response relationships, thresholds for no adverse effects, or, in the case of pollutants for which there is no evidence of a threshold, the degree of associated risk. On the basis of this, the various air quality criteria - standards, guidelines and limit values - are examined for their suitability as a means of gauging the impact of new or modified roads upon public health and the environment. Appendices are included which list: (a) the chemicals present in vehicle exhausts; (b) properties of selected compounds; (c) IARC (International Agency for Research on Cancer) categories; and (d) air quality criteria.

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