Brief Airborne Particle Excursions Reveal Dose-Rate Dependent Health Risks, Supporting a Middle Ground in the Airborne Particulate Matter (PM) Debate. Robert A. Michaels, RAM TRAC Corporation, 3100 Rosendale Road, Schenectady, New York 12309; and Michael T. Kleinman, Department of Community and Environmental Medicine, University of California College of Medicine, Irvine, California
Twenty-four-hour average airborne PM levels permissible under the National Ambient Air Quality Standard (NAAQS) are associated with mortality and morbidity, motivating EPA to review the NAAQS. NAAQS revision has been controversial, in part because the effects, though statistically significant, are weak and remain unexplained. This study inquired whether 24-hour PM levels might encompass undetected brief, but health-significant, excursions. PM excursions were found to be health significant based upon four a priori criteria: 1. real-time measurements of PM concentrations included brief excursions well above the 24-hour PM average, 2. exposing animals and humans briefly (up to a few hours) to the higher PM concentrations attained in such excursions caused adverse health effects, 3. the severity of effects was greater when a given time-weighted average PM concentration included excursions than when exposure to the same time-weighted PM average was constant over the full exposure duration, and 4. adverse effects, though more severe with longer exposure, were more strongly associated with short-term exposures when exposure duration was normalized, that is, quantified as the degree of effect per hour of exposure. Evidence for a causation model involving higher concentrations of PM acting over shorter durations was obtained in a recent incident in Queen Creek, Arizona (outside Phoenix). Approximately 500 people attending a four-day outdoor music festival were overcome, apparently by airborne PM, over a period of hours, after an increase in wind activity had lofted dry vegetation dust, probably including fragments of pollen and alfalfa, into the air. We will report on the status of our investigation of this incident as of the December conference date. Given the recent association of asthma in urban children with cockroach allergen, we will also report on our critical examination of the possible importance of biological and non-biological immunogenic aerosols in explaining epidemiological associations of asthma exacerbation and other effects, including mortality, in communities. Our data suggest that greater public health protection can be achieved via a middle ground in the PM risk management debate. Specifically, controlling brief PM excursions via a relatively high, one-hour PM mass limit may offer greater public health protection than controlling full-day PM averages via a lower, 24-hour PM limit.
Supported in part by Rupprecht and Patashnick, Inc., Albany, New York.