Uncertainties relating to the health effects of particulate air pollution: The US EPA’s particle standard

Although the epidemiologic associations between urban particulate air pollution and human mortality and morbidity have been accumulating for several years, the causal agents (a specific chemical component, a specific particle size range, one or more pollutant combinations, etc.), and the physiological mechanisms behind the associations have yet to be identified. Significant questions regarding confounding effects due to weather, indoor air pollutant exposures and co-pollutants (that accompany particulate matter) stubbornly remain. The events in the United States began with recent epidemiological associations, followed by a lawsuit forcing the US EPA to accelerate the standard-setting process, and finally controversy over the scientific basis of the new standard. In contrast to the potential risks posed by particulate air pollution, many of the sources of such particles are positive contributors to human health; control measures to meet the proposed standard may therefore produce offsetting enhanced mortality and morbidity. In order to establish the information required for well-informed public health policies, a substantial research program is needed because of uncertainties relating to, the affected individuals, the potential causal agents, and the consequences of particle-control activities. Not only are the remaining scientific questions significant, but the particle exposure/health effects associations also call into question some of the current scientific assumptions relating to the nature of effects of population exposures to low concentrations of pollutants.