From Outdoor to Indoor: Amplified Inequities in Wildfire PM_2.5 Exposure across the Contiguous United States

Exposure to wildfire smoke is a major public health issue, yet most studies focus on outdoor air, overlooking that people spend most of their time indoors. We estimated 2020 indoor wildfire-related fine particulate matter (PM_2.5) concentrations across 72,537 census tracts in the contiguous United States by combining outdoor concentrations with infiltration factors. Annual-average indoor wildfire PM_2.5 concentrations ranged from 0.02 to 6.2 μg/m³ across census tracts, with a pronounced west-east gradient and a population-weighted national average of 0.241 μg/m³. Disparities were modest at the national level but became more pronounced across states. Populations living in disadvantaged communities (DACs) experienced higher outdoor exposures than those in non-DACs in 30 of 49 states and higher indoor exposures in 47 states. Racial-ethnic disparities followed a similar pattern: although Black populations experienced below-average exposure nationally, above-average exposure occurred in 18 states outdoors and 31 states indoors. Inequality analyses further showed that indoor infiltration amplified disparities across several social vulnerability dimensions, including income and disability. These findings demonstrate that indoor infiltration amplified wildfire PM_2.5 inequities and highlight the need for indoor-focused and location-specific studies.