PM Panel Studies
Scientific Problem and Policy Issues
In 1998, the U.S. Environmental Protection Agency (EPA) Administrator issued new National Ambient Air Quality Standards (NAAQS) for PM2.5 (particulate matter less than 2.5 micrometers in diameter) based on findings from epidemiological studies that demonstrated a link between increased levels of ambient PM2.5 and excess mortality. Following this issuance, Congress directed the National Research Council (NRC) to determine the most important research priorities for PM to be addressed by EPA. Some of the epidemiological studies that formed the basis for the new NAAQS PM2.5 standard suggested that ambient levels of gaseous co-pollutants (e.g., CO, O3, and NO2) may have confounded the relationship between PM2.5 and mortality. This is reflected in NRC Research topic #1: “What are the quantitative relationships between concentrations of particulate-matter and gaseous copollutants measured at stationary outdoor air-monitoring sites, and the contributions of these concentrations to actual personal exposures, especially for potentially susceptible subpopulations and individuals?”
The National Exposure Research Laboratory (NERL) conducted exposure studies in Baltimore, Maryland and Research Triangle Park (RTP), North Carolina to determine the relationships between actual personal exposures and ambient concentrations of PM and gaseous co-pollutants (e.g., O3, CO, and NO2).
The objective of the NERL Longitudinal PM Panel Studies was to characterize the relationships between personal exposures, residential indoor, and ambient concentrations of PM2.5 and associated gaseous co-pollutants (O3, CO, and NO2). A primary focus of the studies was to determine the factors that affect the variability and, specifically, the contribution of ambient concentrations to personal exposures of PM2.5 and gaseous co-pollutants. The Baltimore study was conducted over a 28-day period during the summer of 1998 in a high-rise retirement facility involving an elderly (ages > 65) cohort of 20 subjects. The study included measurements of PM2.5 and NO2 personal exposures and measurements of PM2.5, O3, CO, and NO2 at a central indoor location and at a central ambient site. The RTP study was conducted over four seasons from June 2000 to June 2001 involving two cohorts: 27 African-Americans with controlled hypertension living in a low/moderate socioeconomic status area in southeast Raleigh, North Carolina and 8 mixed-race individuals with implanted cardiac defibrillators living in and near Chapel Hill, North Carolina. Measurements were made of personal exposure (PM2.5 and O3), indoors at each participant’s residence (PM2.5, CO, and NO2) and at a central ambient monitoring site (PM2.5, O3, CO, and NO2) for seven days over each of the four seasons. The results from the exposure studies were summarized using general univariate statistics, and, in the RTP study, time-series regression analyses and mixed-model analyses were used to quantify the relationships between ambient, indoor, and personal exposures to PM2.5 and the gaseous co-pollutants
Results and Implications
Results from the studies indicated ambient levels of PM2.5 and O3 were similar in RTP and Baltimore (comparison of summer season only). Personal PM2.5 exposures, however, were about two-fold higher in Baltimore than in RTP. Indoor levels of O3 in both studies represented only about 10% of the corresponding outdoor O3 levels. Ambient levels of CO were about two-fold higher in Baltimore while ambient NO2 was about two-fold higher in RTP. In both studies, similar correlations existed between ambient levels of PM2.5 and the ambient gaseous co-pollutants. Relationships between personal exposures, indoor concentrations, and ambient concentrations also were comparable between Baltimore and RTP. In RTP, concentrations of ambient gaseous pollutants (O3, CO, and NO2) were poor predictors of personal exposures to the gases themselves. In fact, ambient O3 served as a better predictor of personal PM2.5 exposures than it was for personal O3 exposures. Of all the pollutants, PM2.5 was the only one for which there were significant relationships between ambient concentrations, indoor concentrations, and personal exposures. These results suggest that ambient O3 acted as a surrogate of personal PM2.5 exposures. Ambient CO and NO2 acted as neither confounders nor surrogates of personal exposures. This is important because it suggests that confounding of ambient gaseous co-pollutants with ambient PM2.5 may exist, but personal exposures are not affected by such relationships. These findings are very important for epidemiologists because they show that observed health effects are not confounded by ambient concentrations of the gaseous co-pollutants because levels of ambient gases are poorly associated with personal exposures.
Research Collaboration and Publications
The NERL Longitudinal PM Panel Studies were designed and conducted by research teams at the U.S. EPA’s National Exposure Research Laboratory and the U.S. EPA’s National Health and Environmental Effects Research Laboratory.
Wallace, L., Williams, R., Suggs, J. Use of Indoor-Outdoor Sulfur Concentrations to Estimate the Infiltration Factor, Outdoor Exposure Factor, Penetration Coefficient, and Deposition Rate for Individual Homes (APM-214). US EPA, National Exposure Research Laboratory, Research Triangle Park, NC. EPA/600/R-06/023.
Wallace, L., Williams, R., Suggs, J. et al., Exposure of High-Risk Subpopulations to Particles: Final Report—APM-21. US EPA, National Exposure Research Laboratory, Research Triangle Park, NC. EPA/600/R-03/145. February, 2004.
Williams, R. et al., Preliminary Particulate Matter Mass Concentrations Associated with Longitudinal Panel Studies: Assessing Human Exposures of High Risk Subpopulations to Particulate Matter. US EPA, National Exposure Research Laboratory, Research Triangle Park, NC. EPA/600/R-01/086.