The 2025 Eaton and Palisades fires in Los Angeles (LA) exemplify destructive wildfires that increasingly threaten cities across the globe. The dangers during the post-burn cleanup phase from major urban wildfires are still being discovered. Here we report that airborne chromium bearing nanoparticles (diameter<56 nm) were found in the LA wildfire debris cleanup zones, a unique finding implicating the fires as a source of nanoparticle metals. The airborne chromium was predominantly in the carcinogenic +6 oxidation state two months post-fire with average concentrations of 13.7 ± 6.2 ng m^-3, below the NIOSH workplace exposure limit of 200 ng m^-3 but above the US EPA screening levels for indoor air (0.1 ng m^-3 for cancer; 3 ng m^-3 for non-cancer effects). Model calculations indicate that chromium containing nanoparticles traveled 10–15 km downwind from the cleanup zone. Caution and health surveillance is warranted for nearby residents given that nanoparticles can easily cross cell membranes and circulate throughout the body.

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