Characteristics of personal exposure to metals in PM and their implications for epidemiological studies: New insights from a panel study of the elderly in Beijing.

Air pollution levels monitored at fixed sites, known as ambient concentrations, are often used as surrogate indicators of individual exposure levels. However, this could lead to exposure misclassification, a major challenge in environmental epidemiology research. Discrepancies between ambient concentrations and personal exposure characteristics, their underlying causes, and their potential impacts on health effect assessments remain unclear, particularly for the metals in fine particulate matter (PM). Based on a panel study conducted in Beijing, we investigated the discrepancies in daily average concentrations of nine metals in personal and ambient PM samples. Specific personal exposure sources and outdoor activity time can impact personal PM metal exposure levels. Indoor sources were found to be the primary contributors to these discrepancies when ambient concentrations were low, while infiltration factors became the dominant influence at higher ambient concentrations. Using inflammatory biomarkers as examples, we further explored the impacts of ambient concentration and personal exposure as indicators for health effect assessment. The confidence intervals for associations between single metals and inflammatory biomarkers were wider for ambient concentration, but narrower for personal exposure. An overall increase of one IQR in the concentrations of nine metals in personal exposure was significantly associated with the following increases in inflammatory biomarkers: 0.16 [95 % CI: 0.04-0.28] in MCP-1, 0.13 [95 % CI: 0.05-0.21] in MDC, 0.49 [95 % CI: 0.41-0.76] in MIP-1β, 0.32 [95 % CI: 0.07-0.57] in sCD40L, and 0.22 [95 % CI: 0.0-0.44] in TNF-α. In contrast, no such significant associations were found for ambient concentrations. Taken together, indoor sources and infiltration factors largely determine the discrepancies between ambient concentrations and personal exposure levels, which can further affect the accuracy of health effect assessments. Personal exposure is a better indicator for characterizing metal exposure levels.