Analysis of the effects of ultrafine particulate matter while accounting for human exposure

Particulate matter (PM) has been associated with mortality in several epidemiological studies. The US EPA currently regulates PM10 and PM2.5 (mass concentration of particles with diameter less than 10 and 2.5 mu m, respectively), but it is not clear which size of particles are most responsible for adverse heath outcomes. A current hypothesis is that ultrafine particles with diameter less than 0.1 mu m are particularly harmful because their small size allows them to deeply penetrate the lungs. This paper investigates the association between exposure to particles of varying diameter and daily mortality. We propose a new dynamic Factor analysis model to relate the ambient concentrations of several sizes of particles with diameters ranging from 0.01 to 0.40 mu m with mortality. We introduce a Bayesian model that converts ambient concentrations into simulated personal exposure using the EPA's Stochastic Human Exposure and Dose Simulator, and relates simulated exposure with mortality. Using new data from Fresno, CA, we find that the 4-day lag of particles with diameter between 0.02 and 0.08 mu m is associated with mortality. This is consistent with the small particles hypothesis. Copyright (C) 2008 John Wiley & Sons, Ltd.