Estimating Error in Using Ambient PM2.5 Concentrations as Proxies for Personal Exposures: A Review

Background: Several methods have been used to account for measurement error inherent in using ambient concentration of particulate matter <2.5 mu m/m(3) (PM2.5) as a proxy for personal exposure. Such methods commonly rely on the estimated correlation between ambient and personal PM2.5 concentrations (r). However, studies of r have not been systematically and quantitatively assessed for publication bias or heterogeneity. Methods: We searched 7 electronic reference databases for studies of the within-participant correlation between ambient and personal PM2.5. Results: We identified 567 candidate studies, 18 (3%) of which met inclusion criteria and were abstracted. The studies were published between 1999 and 2008, representing 619 nonsmoking participants aged 6-93 years in 17 European and North American cities. Correlation coefficients (median 0.54; range 0.09-0.83) were based on a median of 8 ambient-personal PM2.5 pairs per participant (range 5-20) collected over 27-547 days. Overall, there was little evidence for publication bias (funnel plot symmetry tests: Begg's log-rank test, P = 0.9; Egger's regression asymmetry test, P = 0.2). However, strong evidence for heterogeneity was noted (Cochran's Q test for heterogeneity, P < 0.001). European locales, eastern longitudes in North America, higher ambient PM2.5 concentrations, higher relative humidity, and lower between-participant variation in r were associated with increased r. Conclusions: Characteristics of participants, studies, and the environments in which they are conducted may affect the accuracy of ambient PM2.5 as a proxy for personal exposure.