Ambient smoke exposure and indoor air quality in eastern Massachusetts during the 2023 wildfire season.

Widespread North American wildfires in 2023 led to exposure to ambient wildfire smoke outside of traditionally wildfire-prone regions. The objective was to evaluate levels of indoor air pollutants in relation to ambient wildfire smoke exposure in eastern Massachusetts. Using a real-time multipollutant sensor system in five Boston area households, this study assessed indoor fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), and total volatile organic compound concentrations (TVOC) two days before and during days of hazardous wildfire smoke exposure (smoke days). The relationship between ambient PM_2.5 from regulatory monitors and indoor PM_2.5 before and during smoke days was investigated by mixed effects linear regression. During smoke days and the preceding non-smoke days, median indoor PM_2.5 was 9.9 µg/m³ and 3.5 µg/m³ (p < 0.001), respectively; median NO₂ was 20.5 ppb and 18.4 ppb (p = 0.11); median TVOC was 6,715 µg/m³ and 5,361 µg/m³ (p = 0.35). A 1% increase in ambient PM_2.5 was associated with a 0.93% increase in indoor PM_2.5 on smoke days (95% CI, 0.54%-1.32%) and a 0.34% increase on non-smoke days (95% CI, 0.17%-0.66%), though interaction testing of smoke day status was not statistically significant (p = 0.14). In Northeastern US homes, indoor PM_2.5 increased significantly during ambient wildfire smoke exposure, which may reflect increased infiltration and increased indoor particle-generating activities during smoke days.Implications: This study reports on household exposure to wildfire smoke in eastern Massachusetts, finding that indoor PM_2.5 more than doubled compared to preceding non-smoke days, while indoor NO₂ and TVOC did not significantly rise. Though the generalizability of this study is limited by the small number of homes studied, the findings suggest that more investigation is needed to understand indoor air pollution during future wildfire smoke exposure in regions not traditionally wildfire-prone and to inform mitigation efforts.