Metal Toxin Signatures in the Smoke of Wildland–Urban Interface Fires

Alireza Namayandeh , Frida Daniela Garcia Ledezma, Scott Fendorf

Wildland–urban interface (WUI) fires burn natural and built environments simultaneously, generating smoke from vegetation, soil, buildings, vehicles, and infrastructure. Airborne metals can provide source signatures and pose inhalation hazards, yet the sources and physicochemical properties of these metals in WUI fire smoke remain poorly resolved. This uncertainty limits identification of burned urban material contributions and toxic-metal exposure assessment. Here, we compared smoke from the 2025 Los Angeles fires (Eaton Fire specifically) with smoke from nine wildland fires and showed that Eaton Fire smoke carried distinct urban metal signatures alongside geologic inputs. Lead (Pb) was strongly enriched relative to smoke from wildland fires, identifying burned urban materials as an important source, whereas most other metals remained within wildland-fire ranges. In particular, chromium abundance was consistent with the underlying geology. Size-resolved measurements showed that approximately 60% of Pb occurred in ultrafine particles (<0.25 µm). Electron microscopy identified Pb-bearing nanoparticles < 0.1 µm. These small particles can promote deep-lung deposition, translocation into the bloodstream, and crossing the blood-brain barrier. Our findings illustrate that WUI fire smoke is additive: It delivers an urban toxic-metal burden, dominated by Pb, on top of, and rather than in place of, the substantial metal load carried by wildland fire smoke.