Wildfire Smoke Is Associated with Larger Outdoor–Indoor PM2.5 Difference in U.S. Homes: A Multi-Region Paired-Sensor Analysis, 2019–2024

Wildfire smoke contributes substantially to episodic PM2.5 exposure, yet outdoor measurements may not represent indoor conditions. We analyzed indoor PurpleAir sensors and nearby outdoor monitors from U.S. residences (2019–2024) to estimate smoke-day changes in the outdoor–indoor PM2.5 difference and characterize heterogeneity across regions. After data quality control and the application of completeness criteria, 509 monitor pairs contributed 250,873 monitor-days. Smoke days were assigned using the NOAA Hazard Mapping System smoke-plume polygons. Pair-specific time-series models estimated smoke-day changes in the outdoor–indoor PM2.5 difference, which were pooled using random-effects meta-analysis; heterogeneity was summarized by clustering indoor and outdoor smoke–non-smoke contrasts. In the unadjusted summary, the mean outdoor PM2.5 was 8.61 vs. 5.63 µg/m3 on smoke vs. non-smoke days and the mean indoor PM2.5 was 6.33 vs. 5.09 µg/m3, reflecting an increase in the mean outdoor–indoor difference from 0.54 to 2.27 µg/m3 (p < 0.001). The pooled smoke-day effect on the outdoor–indoor difference was 0.88 µg/m3 (95% CI: 0.80, 0.96). Clustering identified four distinct response patterns, most commonly outdoor increases exceeding indoor increases, with smaller subsets showing extreme outdoor amplification or net indoor reductions under modest outdoor increases. These results indicate that indoor protection during smoke episodes is common but variable and support exposure characterization beyond outdoor concentrations alone.