PM_2.5 of Ambient Origin:  Estimates and Exposure Errors Relevant to PM Epidemiology

Abstract

Epidemiological studies routinely use central-site particulate matter (PM) as a surrogate for exposure to PM of ambient (outdoor) origin. Below we quantify exposure errors that arise from variations in particle infiltration to aid evaluation of the use of this surrogate, rather than actual exposure, in PM epidemiology. Measurements from 114 homes in 3 cities from the Relationship of Indoor, Outdoor and Personal Air (RIOPA) study were used. Indoor PM 2.5 of outdoor origin was calculated: 1) assuming a constant infiltration factor, as would be the case if central-site PM were a "perfect surrogate" for exposure to outdoor particles; 2) including variations in measured air exchange rates across homes; 3) also incorporating home-to-home variations in particle composition, and 4) calculating sample-specific infiltration factors. The final estimates of PM 2.5 of outdoor origin take into account variations in building construction, ventilation practices, and particle properties that result in home-to-home and day-to-day variations in particle infiltration. As assumptions became more realistic (from the first, most constrained model to the fourth, least constrained model), the mean concentration of PM 2.5 of outdoor origin increased. Perhaps more importantly, the bandwidth of the distribution increased. These results quantify several ways in which the use of central site PM results in underestimates of the ambient PM 2.5 exposure distribution bandwidth. The result is larger uncertainties in relative risk factors for PM 2.5 than would occur if epidemiological studies used more accurate exposure measures. In certain situations this can lead to bias.

Fine organic particulate matter dominates indoor-generated PM2.5 in RIOPA homes

“…Constant values of k and P were calculated by fitting measured C in , C out , and a to Eq. (1) using non-linear regression analysis (NLIN, SAS Version 8.02; SAS Institute, Cary, NC, USA; Meng et al, 2005b). C ig values were then calculated as the home-by-home difference between C in and C og .…”

“…Air exchange rate, temperature and relative humidity were also measured in each home. PM 2.5 samples or subsets of samples were analyzed for mass (Meng et al, 2005a), functional groups , elements (Meng et al, 2005b), OC and EC, gas and particle phase PAHs (Naumova et al, 2002(Naumova et al, , 2003, and Chlordanes (Offenberg et al, 2004). Gas phase aldehydes (Liu et al, 2006) and volatile organic compounds were also measured.…”

Variability in the fraction of ambient fine particulate matter found indoors and observed heterogeneity in health effect estimates

“…13 As a result, exposure to ambient PM 2.5 mostly occurs in the indoor environment and, specifically, within the residence. Importantly, the fraction of ambient PM 2.5 that penetrates and persists indoors (F) varies temporally 14 and spatially, 15 and is different for different components of the PM 2.5 mixture. 16 Exposure metrics that rely on central site concentrations do not account for this variability, nor do they account for changes in PM 2.5 properties (i.e., composition and size distribution) that result from outdoor-toindoor transport.…”

Modeling residential fine particulate matter infiltration for exposure assessment

“…Studies examining residential PM 2.5 infiltration (F inf , defined as the fraction of ambient PM 2.5 that penetrates indoors and remains suspended) have found substantial variation both between residences and within residences over time (Allen et al, 2003;Wallace et al, 2003;Meng et al, 2005;Wallace and Williams, 2005;Barn et al, 2007). As individuals spend, on average, nearly 70% of their time inside their homes (Klepeis et al, 2001), failure to consider F inf differences may contribute to exposure misclassification and impact the precision and magnitude of health effect estimates in epidemiological studies.…”

“…Meng et al (2005) examined F inf and the contributions of ambient PM 2.5 to residential indoor PM 2.5 concentrations. They found that the use of central site PM 2.5 as a surrogate for exposure to PM 2.5 of ambient origin significantly underestimates the distribution of exposures, resulting in larger uncertainties in reported relative risks.…”