This study compares an indoor-outdoor air-exchange mass balance model (IO model) with a chemical mass balance (CMB) model. The models were used to determine the contribution of outdoor sources and indoor resuspension activities to indoor particulate matter (PM) concentrations. Simultaneous indoor and outdoor measurements of PM concentration, chemical composition, and air-exchange rate were made for five consecutive days at a single-family residence using particle counters, nephelometers, and filter samples of integrated PM with an aerodynamic diameter of less than or equal to 2.5 m (PM 2.5 ) and PM with an aerodynamic diameter of less than or equal to 5 m (PM 5 ). Chemical compositions were determined by inductively coupled plasma massspectrometry. During three high-activity days, prescribed activities, such as cleaning and walking, were conducted over a period of 4 -6 hr. For the remaining two days, indoor activities were minimal. Indoor sources accounted for 60 -89% of the PM 2.5 and more than 90% of the PM 5 for the high-activity days. For the minimal-activity days, indoor sources accounted for 27-47% of PM 2.5 and 44 -60% of the PM 5 . Good agreement was found between the two mass balance methods. Indoor PM 2.5 originating outdoors averaged 53% of outdoor concentrations.
INTRODUCTIONMany epidemiological studies associate outdoor particulate matter (PM) with human health effects. 1 Much of human exposure to PM, however, occurs indoors. 2 Connecting this exposure to outdoor concentrations of PM, therefore, requires identifying the fraction of indoor PM originating outdoors. The connection between outdoor measurements and indoor exposure is the infiltration of these particles into buildings coupled with their fate in the indoor air. Quantifying this component of exposure by separating PM of indoor and outdoor origin has become a recent research priority. 3,4 Fully developing this connection is critical to explaining the epidemiology of PM and human health. 5 Determining the contribution of outdoor and indoor PM sources to indoor PM concentrations is especially important when measuring PM concentrations in occupied residences. Many human activities generate or resuspend significant amounts of PM, potentially obscuring the impact of outdoor PM. This indoor-generated PM is correlated to human activities and indoor processes and is not correlated to outdoor concentrations. 5 Furthermore, indoor and outdoor sources have different composition and potentially have different toxicological effects. 4 Finally, appropriate mitigation measures may differ between sources.Recent studies recognize the necessity of estimating the effect of outdoor PM on indoor concentrations and separating that effect from indoor-generated PM. 6 -9 These studies generally rely on indoor-outdoor air-exchange mass balance models (IO models) and measurements. Unique tracers with limited indoor sources also may provide a measure of the impact of outdoor PM on indoor air. 10 In this study, an IO mass balance is performed on a residential...