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

During the 2025 Los Angeles Eaton urban firestorm, the airborne plume rapidly changed as fires shifted from structures to wildland, tracked hourly by the potassium-to-lead ratio.

• Why it matters: Urban fire smoke is chemically distinct from wildland smoke but remains poorly understood, leaving exposure and emergency response frameworks insufficient as fires increasingly burn into cities.
• What they did: Real-time airborne trace element measurements were taken during the Eaton urban firestorm, with plume evolution tracked hourly using the potassium-to-lead ratio.
• The result: Compared to wildfire smoke, the urban plume was enriched more than 50-fold in lead, 7-fold in halogens, and 3-fold in arsenic, and peak structural burning is estimated to have exposed 7 million residents (4.3–8.0 million) to 24-hour lead >50 nanograms per cubic meter.