The report noted that the zones most impacted by traffic-related pollution are up to 300 to 500 meters from highways and other major roads, and calculated that for large cities in North America that would include 30% to 45% of the population.

The panel was convened by the HEI Board of Directors to examine the rapidly growing body of studies purporting to show a relationship between exposure to traffic and disease, and systematically reviewed more than 700 worldwide studies of emissions from motor vehicles and exposure to and health effects of traffic-related air pollution. Overall, the Panel concluded:.

• Many countries have implemented more stringent emission controls and made steady progress in reducing the emissions from motor vehicles and improving air quality. However, the rapid growth of the world’s motor-vehicle fleet, the expansion of metropolitan areas, and the increasing dependence on motor vehicles have resulted in an increase in the fraction of the population living and working in close proximity to busy highways and roads — counteracting to some extent the benefits of pollution control regulations and technologies.

• Based on a synthesis of the best available evidence, the panel identified an exposure zone within a range of up to 300 to 500 meters from a highway or a major road as the area most highly affected by traffic emissions and estimated that 30% to 45% of people living in large North American cities live within such zones.

• Many aspects of the epidemiologic and toxicologic evidence relating adverse human health effects to exposure to primary traffic-generated air pollution remain incomplete. However, using rigorous criteria to assess cause and effect, the Panel concluded that the evidence is sufficient to support a causal relationship between exposure to traffic-related air pollution and exacerbation of asthma among children.

• The Panel also found suggestive evidence of a causal relationship with onset of childhood asthma, non-asthma respiratory symptoms, impaired lung function, total and cardiovascular mortality, and cardiovascular morbidity, although the data were not sufficient to support causality. For a number of other health outcomes, there was limited evidence of associations, although the data were either inadequate or insufficient to draw firmer conclusions.

• The Panel’s conclusions should be considered in the context of the continuing progress to reduce emissions from motor vehicles. Since the epidemiology studies are based on past estimates of exposure from older vehicles, they may not provide an accurate guide to estimating health associations in the future.

• In light of the large number of people residing within 300 to 500 meters of major roads, the Panel concluded that the sufficient and suggestive evidence for these health outcomes indicates that exposures to traffic-related pollution are likely to be of public health concern and deserve public attention.

The Panel also identified a number of top priority research needs to fill key gaps in the understanding of emissions, exposure, and health.

Resources

• Traffic-Related Air Pollution: A Critical Review of the Literature on Emissions, Exposure, and Health Effects