Toxic Ultrafine to Nanoparticulate Materials in Wildfire Smoke

Alireza Namayandeh , Jonathon Howell, Sebastian Mergelsberg, Charlie Lamb, Frida Daniela Garcia Ledezma, Alexander Honeyman , Kevin Rosso, Scott Fendorf

Wildfire smoke contains metal-laden ultrafine particles (less than 0.25 µm) systematically overlooked in air quality monitoring, representing a hidden hazard. We show that these ultrafine particles dominate smoke composition, accounting for >60% of particle mass and carrying toxic metals including chromium, nickel, and titanium at sizes down to 5 nm, small enough to penetrate lung barriers and enter the bloodstream. Unexpectedly, ultrafine metal abundance is decoupled from conventional PM₂.₅ measurements but correlates strongly with underlying geology, revealing that burned lithology controls smoke toxicity. With wildfires intensifying globally and smoke exposure affecting hundreds of millions annually, current exposure assessments based solely on PM₂.₅ mass miss the most hazardous fraction of smoke. Our findings necessitate the incorporation of size-resolved metal composition into air-quality forecasting and public health protection.