We conducted a multi-pollutant exposure study in Baltimore, MD, in which 15 non-smoking older adult subjects (>64 years old) wore a multi-pollutant sampler for 12 days during the summer of 1998 and the winter of 1999. The sampler measured simultaneous 24-hr integrated personal exposures to PM 2.5 , PM 10 , SO 4 2- IMPLICATIONS Particulate epidemiologic studies typically use stationary, ambient concentrations as a surrogate for personal exposure. Questions have been raised concerning the

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... of ambient PM 2.5 measures as a surrogate of exposure due to variability in personal-ambient associations and confounding by PM 2.5 co-pollutants. This paper examines the personal particulate and gaseous exposures for a cohort of older adults and investigates their relationship to corresponding ambient concentrations and to one another. Findings from these analyses will help interpret results from epidemiological studies. exposures was lowest when individuals spent the majority of their time in well-ventilated indoor environments. Results also indicate that the potential for confounding by PM 2.5 co-pollutants is limited, despite significant correlations among ambient pollutant concentrations. In contrast to ambient concentrations, PM 2.5 exposures were not significantly correlated with personal exposures to PM 2.5-10 , PM 2.5 of non-ambient origin, O 3 , NO 2 , and SO 2 . Since a confounder must be associated with the exposure of interest, these results provide evidence that the effects observed in the PM 2.5 epidemiologic studies are unlikely to be due to confounding by the PM 2.5 co-pollutants measured in this study.