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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 (NO 2 ), 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/m 3 and 3.5 µg/m 3 ( p  < 0.001), respectively; median NO 2 was 20.5 ppb and 18.4 ppb ( p  = 0.11); median TVOC was 6,715 µg/m 3 and 5,361 µg/m 3 ( 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 NO 2 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.