Portable Air Cleaners Are Associated with Low Indoor PM2.5 during the 2025 Los Angeles Wildland-Urban Interface Fires

Abstract Portable air cleaners (PACs) are recommended during wildfire smoke events, yet large-scale evidence of their protective benefit in residential settings remains limited. This study analyzed data from 736 PACs with integrated sensors (placed in 492 households) and 153 outdoor monitors throughout Los Angeles during the January 2025 wildland-urban interface fires. During peak fire conditions (January 7–11), the mean PM2.5 indoor/outdoor (I/O) ratio was 0.28, with indoor concentrations averaging below 15 μg/m3 across 95% of the study sample. An analysis of proximity to active fire perimeters showed that I/O ratios remained stable over the full range of distances despite a pronounced outdoor concentration gradient, and wind direction and speed data indicated that outdoor smoke was substantially attenuated indoors. A mass-balance model estimated that PAC use reduced potential indoor PM2.5 exposure by a median of 65.9% (interquartile range = 54.4–74.3%). Findings from this large-scale field evaluation suggest that PACs can serve as a practical, scalable option for managing degraded indoor air quality during smoke events.