From the Inside Out – Application of the Mass Balance Model for PM Exposure Assessment in Residential Settings Under the Influences of Indoor and Outdoor Factors

The application of the widely used mass balance model in determining portable air purifier (PAP) effectiveness in particulate matter (PM) removal was not validated in occupied residential settings. The corresponding size-resolved information and measurements for the model parameters and PAP effectiveness were also limited to better characterize human exposure to indoor PM. Additionally, effects of ambient factors, such as meteorology, and their long-term impacts on occupant indoor exposure to outdoor PM was unclear. We achieved well-mixed environment and steady state of PM concentrations that met the mass balance model assumptions. Size-resolved particle deposition rate was determined using non-linear mixed effects model, whereas linear mixed effects model was used to estimate the slope between the measured and modeled effectiveness for validation purpose. To evaluate the impact of ambient factors on PM exposure, we assembled data from two cohorts in the greater Boston area, assessing the monthly and long-term effect of temperature and other meteorology on Sr. Long-term meteorology was projected using 15 weather models for the past and future 20 years to estimate Sr for the corresponding periods with mixed effects models. Both particle deposition rate and portable air purifier effectiveness were highly particle size-dependent. Filtration was found to be the dominant removal mechanism for submicrometer particles, whereas deposition could play a more important role in ultrafine particle removal. There was reasonable agreement between measured and modeled effectiveness with size-resolved slopes ranging from 1.11±0.06 to 1.25±0.07 (mean±SE), except for particles <35 nm. Sr was found to be a robust measure of indoor exposure to outdoor PM, and temperature was its significant predictor. Seasonal effect of temperature was much more dominant when compared to long-term effect on Sr, which differed in the whole population and the subpopulation of naturally ventilated homes. However, long-term temperature effect was small, with maximum of <10% for summer Sr compared to the past. Findings from the studies improved characterization of indoor PM exposure. The study design and methods can be used in the future to better understand exposure scenarios and their correlation to health effects in other homes or populations.