PDS 65: Exposure assessment: implications for epidemiology, Exhibition Hall (PDS), Ground floor, August 27, 2019, 1:30 PM - 3:00 PM Background: Though individuals spend ~90% of their time indoors, ambient air pollution data are frequently used in exposure assessment for epidemiologic studies. We sought to understand the relation between indoor and outdoor concentrations of black carbon (BC) and additional housing characteristics associated with indoor BC concentrations among cohort study participants in Denver, CO. Methods: Households from the Healthy Start cohort were selected for monitoring based on diversity in housing type and built environmental characteristics. For one week in spring and summer 2018, households were provided two low-cost air samplers, one for outdoor and one for indoor measurement. Participants completed questionnaires addressing housing specifics such as building type, flooring, and use of heating and cooling systems. BC was measured using transmissometry and weekly mean concentrations were log transformed prior to model fitting. A linear model was fit using all available predictors obtained from surveys as well as outdoor BC concentrations. Model selection was performed using stepwise backwards elimination; model fit was evaluated using AIC. Results: A total of 25 households participated in the study: 11 filters had BC measurements below LOD, and data were available for 39 filters. The average (SD) indoor and outdoor concentrations were 1.1 (0.8) and 1.2 (0.6) μg/m3, respectively. Outdoor BC concentrations were significant predictors of log(indoor BC) in a single-predictor model (β = 0.49, p = 0.02, R2=0.12). The final model included outdoor BC, single family home, tile flooring, and use of electric heater and had an adjusted R2 of 0.5. Conclusion: Outdoor BC and housing characteristics were able to account for ~50% of the variability in indoor BC concentrations measured in Denver, CO homes. In the absence of personal monitoring, household characteristics and time-activity patterns may be used in calibrating ambient air pollution concentrations for personal exposure estimation.