US EPA report (EPA/600/R-06/063) July 2006
In the proposed rule on the National Ambient Air Quality Standards for particulate matter (PM), EPA committed to conduct a review and assessment of the numerous studies relevant to assessing the health effects of PM that were published too recently to be included in the 2004 PM Air Quality Criteria Document (AQCD). This report presents the findings of EPA’s survey and provisional assessment of such studies. EPA has screened and surveyed the recent literature and developed a provisional assessment that places those studies of potentially greatest relevance in the context of the findings of the 2004 PM AQCD. The focus is on: (a) epidemiologic studies that used PM2.5 or PM10-2.5 and were conducted in the U.S. or Canada, and (b) toxicology or epidemiologic studies that compared effects of PM from different sources, PM components, or size fractions. Given the limited time available, the provisional assessment presented here does not attempt to critically review individual studies or to provide the kind of full integration found in a typical AQCD.
This survey and assessment finds that that the new studies expand the scientific information and provide important insights on the relationships between PM exposure and health effects of PM. Taken in context, however, the new information and findings do not materially change any of the broad scientific conclusions regarding the health effects of PM exposure made in the 2004 PM AQCD. In brief, this report finds the following:
• Recent epidemiologic studies, most of which are follow-ups or extensions of earlier work, continue to find that long-term exposure to fine particles is associated with both mortality and morbidity, as was stated in the 2004 PM AQCD. Notably, a follow-up to the Six Cities study shows that an overall reduction in PM2.5 levels results in reduced long-term mortality risk. Both this study and the analysis of the ACS cohort data in Los Angeles suggest that previous studies may have underestimated the magnitude of mortality risks. Some studies provide more mixed results, including the suggestion that higher traffic density may be an important factor. In addition, the California Children’s Health Study reported that measures of PM2.5 exposure and PM components and gases were associated with reduction in lung function growth in children, increasing the evidence for increased susceptibility early in life, as was suggested in the 2004 PM AQCD. The results of recent epidemiologic and toxicology studies have also reported new evidence linking long-term exposure to fine particles with a measure of atherosclerosis development and, in a cohort of individuals with cystic fibrosis, respiratory exacerbations.
• Recent epidemiologic studies have also continued to report associations between acute exposure to fine particles and mortality and morbidity health endpoints. These include three multi-city analyses, the largest of which (in 204 counties) shows a significant association between acute fine PM exposures and hospitalization for cardiovascular and respiratory diseases, and suggestions of differential cardiovascular effects in eastern U.S. as opposed to western U.S. locations. The new studies support previous conclusions that short-term exposure to fine PM is associated with both mortality and morbidity, including a substantial number of studies reporting associations with cardiovascular and respiratory health outcomes such as changes in heart rhythm or increases in exhaled NO.
• New toxicology and epidemiologic studies have continued to link health outcomes with a range of fine particle sources and components. Several new epidemiologic analyses and toxicology studies have included source apportionment techniques, and the results indicated that fine PM from numerous sources, including traffic-related pollution, regional sulfate pollution, combustion sources, resuspended soil or road dust, are associated with various health outcomes. Toxicology studies continue to indicate that various components, including metals, sulfates, and elemental and organic carbon, are linked with health outcomes, albeit at generally high concentrations. Recent epidemiologic studies have also linked different fine PM components with a range of health outcomes; new studies indicate effects of the organic and elemental carbon fractions of fine PM that were generally not evaluated in earlier analyses.
• The recent epidemiologic studies greatly expand the more limited literature on health effects of acute exposure to thoracic coarse particles (PM10-2.5). The 2004 PM AQCD conclusion that PM10-2.5 exposure was associated with respiratory morbidity is substantially strengthened with these new studies; several epidemiologic studies, in fact, report stronger evidence of associations with PM10-2.5 than for PM2.5. In two new casecrossover studies, associations with thoracic coarse particles are robust to the inclusion of gaseous copollutants. For mortality, many studies do not report statistically significant associations, though one new analysis reports a significant association with cardiovascular mortality in Vancouver, Canada.
• Evidence of associations between long-term exposure to thoracic coarse particles and either mortality or morbidity remains limited.
• New toxicology studies have demonstrated that exposure to thoracic coarse particles, or PM sources generally representative of this size fraction (e.g., road dust), can result in inflammation and other health responses. Clinical exposure of healthy and asthmatic humans to concentrated ambient air particles comprised mostly of PM10-2.5 showed changes in heart rate and heart rate variability measures. The results are still too limited to draw conclusions about specific thoracic coarse particle components and health outcomes, but it appears that endotoxin and metals may play a role in the observed responses. Two studies comparing toxicity of dust from soils and road surfaces found variable toxic responses from both urban and rural locations.
• Significant associations between improvements in health and reductions in PM and other air pollutants have been reported in intervention studies or “found experiments.” One new study reported reduced mortality risk with reduced PM2.5 concentrations. In addition, several studies, largely outside the U.S., reported reduced respiratory morbidity with lowered air pollutant concentrations, providing further support for the epidemiological evidence that links PM exposure to adverse health effects.