Use of Personal-Indoor-Outdoor Sulfur Concentrations to Estimate the Infiltration Factor and Outdoor Exposure Factor for Individual Homes and Persons

Abstract

A study of personal, indoor, and outdoor exposure to PM_2.5 and associated elements has been carried out for 37 residents of the Research Triangle Park area in North Carolina. Participants were selected from persons expected to be at elevated risk from exposure to particles, and included 29 persons with hypertension and 8 cardiac patients with implanted defibrillators. Participants were monitored for 7 consecutive days in each of four seasons. One goal of the study was to estimate the contribution of outdoor PM_2.5 to indoor concentrations. This depends on the infiltration factor F_inf, the fraction of outdoor PM_2.5 remaining airborne after penetrating indoors. After confirming with our measurements the findings of previous studies that sulfur has few indoor sources, we estimated an average F_inf for each house based on indoor/outdoor sulfur ratios. These estimates ranged from 0.26 to 0.87, with a median value of 0.55. Since these estimates apply only to particles of size similar to that of sulfur particles (0.06−0.5 μm diameter), and since larger particles (0.5−2.5 μm) have lower penetration rates and higher deposition rates, these estimates are likely to be higher than the true infiltration factors for PM_2.5 as a whole. In summer when air conditioners were in use, the sulfur-based infiltration factor was at its lowest (averaging 0.50) for most homes, whereas the average F_inf for the other three seasons was 0.62−0.63. Using the daily estimated infiltration factor for each house, we calculated the contribution of outdoor PM_2.5 to indoor air concentrations. The indoor-generated contributions to indoor PM_2.5 had a wider range (0−33 μg/m³) than the outdoor contributions (5−22 μg/m³). However, outdoor contributions exceeded the indoor-generated contributions in 27 of 36 homes. A second goal of the study was to determine the contribution of outdoor particles to personal exposure. This is determined by the “outdoor exposure factor” F_pex, the fraction of outdoor PM_2.5 contributing to personal exposure. As with F_inf, we estimated F_pex by the personal/outdoor sulfur ratios. The estimates ranged from 0.33 to 0.77 with a median value of 0.53. Outdoor air particles were less important for personal exposures than for indoor concentrations, with the median outdoor contribution to personal exposure just 49%. We regressed the outdoor contributions to personal exposures on measured outdoor PM_2.5 at the central site. The regressions had R² values ranging from 0.19 to 0.88 (median = 0.73). These values provide an indication of the extent of misclassification error in epidemiological estimates of the effect of outdoor particles on health.