Assessing Indoor Versus Outdoor PM2.5 Concentrations During the 2025 Los Angeles Fires Using the PurpleAir Sensor Network

In January 2025, a series of fast-moving wildland-urban-interface (WUI) fires swept through the Los Angeles (LA) metropolitan area, causing severe air pollution. While the impacts of WUI fires on outdoor air quality have been extensively studied, indoor exposure remains less understood, despite most people sheltering indoors during WUI fires. This study investigates the spatial and temporal patterns of indoor and outdoor PM₂.₅ concentrations across the South Coast Air Basin, with a focus on Los Angeles County during the LA fires. Using high-resolution data from co-located indoor and outdoor PurpleAir sensors, we analyze hourly PM₂.₅ levels and indoor/outdoor ratios. Outdoor PM₂.₅ concentrations spiked sharply during the fires, reaching unhealthy levels. Indoor concentrations increased concurrently but to a lesser extent, reflecting the partial shielding effect of indoor environments from outdoor air pollution. The mean daily indoor/outdoor PM₂.₅ ratio was 0.50 during LA fire days, lower than that ratio (0.81) during non-fire days. Indoor/outdoor PM₂.₅ ratios across sensors showed a wide distribution, reflecting differences in building characteristics and occupant behavior, such as the use of air purifiers. These findings emphasize the need for guidance and interventions to reduce indoor PM ₂.₅ exposure and protect public health during extreme WUI fire events.