Climate change is increasing the risk of wildfires which frequently extend into the wildland-urban interface (WUI). WUI fires burn a mixture of vegetation, structures and vehicles and there is a marked research gap regarding population exposures and health effects. Woodsmoke contains a toxic mixture of known and suspect carcinogens including but not limited to benzene, aldehydes, and polycyclic aromatic hydrocarbons (PAHs). However, wildland fire exposure monitoring has generally been limited to particulates for the general public and PAHs for firefighters. Firefighters, a high exposure group, are requesting participatory research to measure WUI fire exposures and effects and identify effective interventions. Silicone wrist bands can measure exposure beyond PAHs, and urine metabolomics can identify both exposures and effects. While cancer or other diseases caused by firefighting exposures can take many years to develop, metabolomic and epigenetic (microRNA and DNA methylation) endpoints can serve as sub-clinical biomarkers of toxicity. Interventions of firefighter interest include rapid provision of exposure data, improved personal protective equipment (PPE), more rapid dermal decontamination, and administrative controls. We hypothesize that: a) use of silicone wristbands and targeted urinary analyses (hydroxylated PAHs) will identify high-exposure settings and activities, and that untargeted metabolomics will reveal novel environmental compounds of concern; b) the urine metabolome and microRNAs will change acutely with exposures and cumulative exposures will be associated with long-term DNA methylation changes in firefighters; and c) interventions chosen by firefighters will significantly reduce exposures. We will test these hypotheses through evaluating firefighter exposures during WUI responses, measuring toxic effects, and evaluating interventions to reduce exposures. Our fire service research champions have enrolled Los Angeles County and Orange County firefigh