Mapping the mechanistic evidence of wood smoke and wildfire studies in humans

Abstract

Background and Aim Wood smoke is a complex mixture of gases, chemicals, and fine particles generated by wood-burning and wildfires. We are conducting a cancer hazard evaluation of wood smoke for possible listing in the U.S. Report on Carcinogens. The database of cancer epidemiology studies for exposure to wood smoke, but not wildfire, is adequate to conduct an evaluation. As mechanistic evidence may inform the overall hazard evaluation, especially for wildfires, we mapped the published evidence by biomarkers for the key characteristic of carcinogens (KCC), focusing here on studies in exposed humans. Methods We systematically searched three bibliographic databases for relevant biomarker studies on wildfire and wood smoke exposure. For selected studies reporting on the three most common KCCs (genotoxicity, oxidative stress, immune biomarkers), we extracted information including specific KCC biomarkers (e.g., mutations), results, study design, and population, and presented the data in an interactive evidence map. Results Wood smoke: Most human studies (26) found positive associations with KCCs, consistent with the >70 non-human studies (in vitro and in vivo). Inflammation, inflammatory cytokines, and lipid peroxidation were the most common KCCs in humans. The studies were primarily intervention or cross-sectional in populations cooking or heating with wood or randomized studies of controlled exposure. Wildfires: Human studies (15) were largely positive for immune biomarkers but inconsistent for genotoxicity and oxidative stress, whereas non-human studies (17) were generally positive for all endpoints. Most studies measured KCCs in firefighters (13) before or after exposure (e.g., work shift or natural wildfire event). Conclusions Wildfires are increasing in frequency and intensity due to climate change, and exposure to wood smoke remains a worldwide concern. This mechanistic evidence will help inform the cancer hazard evaluations of these exposures. Ongoing risk-of-bias evaluations may explain heterogeneity in human wildfire studies. Keywords Wood smoke, wildfires, biomarkers, exposed humans