Estimating Error in Using Residential Outdoor PM(2.5) Concentrations as Proxies for Personal Exposures: A Meta-analysis

BACKGROUND: Studies examining the health effects of particulate matter ≤ 2.5 μm in aerodynamic diameter (PM(2.5)) commonly use ambient PM(2.5) concentrations measured at distal monitoring sites as proxies for personal exposure and assume spatial homogeneity of ambient PM(2.5). An alternative proxy—the residential outdoor PM(2.5) concentration measured adjacent to participant homes—has few advantages under this assumption. OBJECTIVES: We systematically reviewed the correlation between residential outdoor PM(2.5) and personal PM(2.5) (r̄(j)) as a means of comparing the magnitude and sources of measurement error associated with their use as exposure surrogates. METHODS: We searched seven electronic reference databases for studies of the within-participant residential outdoor-personal PM(2.5) correlation. RESULTS: The search identified 567 candidate studies, nine of which were abstracted in duplicate, that were published between 1996 and 2008. They represented 329 nonsmoking participants 6–93 years of age in eight U.S. cities, among whom r̄(j) was estimated (median, 0.53; range, 0.25–0.79) based on a median of seven residential outdoor-personal PM(2.5) pairs per participant. We found modest evidence of publication bias (symmetric funnel plot; p(Begg) = 0.4; p(Egger) = 0.2); however, we identified evidence of heterogeneity (Cochran’s Q-test p = 0.05). Of the 20 characteristics examined, earlier study midpoints, eastern longitudes, older mean age, higher outdoor temperatures, and lower personal-residential outdoor PM(2.5) differences were associated with increased within-participant residential outdoor-personal PM(2.5) correlations. CONCLUSIONS: These findings were similar to those from a contemporaneous meta-analysis that examined ambient-personal PM(2.5) correlations (r̄(j) = median, 0.54; range, 0.09–0.83). Collectively, the meta-analyses suggest that residential outdoor-personal and ambient-personal PM(2.5) correlations merit greater consideration when evaluating the potential for bias in studies of PM(2.5)-mediated health effects.