Fire at the boundary: Airborne toxic metals from the 2025 Los Angeles urban firestorm

Urban fire smoke is chemically distinct from wildland fire smoke, yet remains poorly understood. Using real-time airborne trace element measurements during the 2025 Los Angeles Eaton urban firestorm, we show that the plume evolved rapidly as fire transitioned from structures to wildland, tracked hourly by the potassium-to-lead ratio. Compared to wildfire smoke, the urban plume was enriched more than 50-fold in lead, 7-fold in halogens, and 3-fold in arsenic, a toxic mixture linked to burning vehicles, electronics, and building materials, a fundamentally different fuel from conventional wildland vegetation. During peak structural burning, we estimate approximately 5 million residents experienced 24-hour lead concentrations exceeding 50 ng m −3 , over 80 times background levels. Unlike wildfire smoke that has been the focus of decades of air quality research, this plume represents an emerging class of urban fire air pollution that demands revised exposure frameworks and emergency response protocols as fires increasingly burn into cities.