Using time- and size-resolved particulate data to quantify indoor penetration and deposition behavior

Because people spend approximately 85-90% of their time indoors, it is wide ly recognized that a significant portion of total personal exposures to amb ient particles occurs in indoor environments. Although penetration efficien cies and deposition rates regulate indoor exposures to ambient particles, f ew data exist on the levels or variability of these infiltration parameters , in particular for time- and size-resolved data. To investigate ambient pa rticle infiltration, a comprehensive particle characterization study was co nducted in nine nonsmoking homes in the metropolitan Boston area. Continuou s indoor and outdoor PM2.5 and size distribution measurements were made in each of the study homes over weeklong periods. Data for nighttime, nonsourc e periods were used to quantify infiltration factors for PM2.5 as well as f or 17 discrete particle size intervals between 0.02 and 10 mum. Infiltratio n factors for PM2.5 exhibited large intra- and interhome variability, which was attributed to seasonal effects and home dynamics. As expected, minimum infiltration factors were observed for ultrafine and coarse particles. A p hysical-statistical model was-used. to estimate size-specific penetration e fficiencies and deposition rates for these study homes. Our data show that the penetration efficiency depends on particle size, as well as home charac teristics. These results provide new insight on the protective role of the building shell in reducing indoor exposures to ambient particles, especiall y for tighter (e.g., winterized) homes and for particles with diameters gre ater than 1 mum.