Over the past 3 decades, fire season in the United States has become more severe as the annual number of fires and area burned increases. (1?5) This increase has been largely driven by climate change. (3,6?10) The average wildfire size has increased 4-fold during the 2000s (11) as compared to the previous 2 decades. In 2022 alone, 7.4 million acres were burned by nearly 65,000 fires. (12,13) The increase in wildfire size and frequency in the U.S. has coincided with the expansion of the wildland–urban interface (WUI), a region where houses and structures are interspersed among vegetation and forests. (14?16) Close proximity of vegetative fuel to structures elevates the risk of fire propagation, increasing WUI susceptibility to burning. (15,17,18) As of 2021, there were approximately 50 million U.S. homes in the WUI, with an expected increase of 1 million within 3 years. (19) From 1990 to 2010, the WUI grew by 41% in the U.S., (18) accounting for approximately 39% of all houses (14,17) and 10% of land area. (15) Between 1985 and 2013, approximately 69% of structures destroyed in wildfires existed in the WUI. (17) The destruction of structures at the WUI due to wildfires is a problem that is expected to increase as more people move into the WUI and climate change continues to progress, increasing wildfire activity. (20)

Wildfires have a direct impact on air quality. Wildfire smoke contains volatile organic molecules, fine particulate matter (PM2.5, PM10), ozone, aldehydes, sulfur dioxides, and other contaminants, (21?24) which have been linked to increases in overall mortality and respiratory morbidity. (25?32) Hospital admissions increase during wildfire activity (31,33,34) with respiratory admissions increasing 23–34%. (28,35,36) Repeated annual exposure carries an additional risk of long-term illness including elevated risk for developing lung cancer or brain tumors. (37) Wildfires also impact environmental health by destroying vegetation, (38,39) altering animal behavior, (40) and generating ash and atmospheric particulates. Following severe burns, slopes lose the vegetation that prevents erosion, (41) increasing vulnerability to debris flow landslides during rainstorms. (42) Erosion and wind events deposit ash onto soils (43?45) and surface waters, (46?51) thus contaminating water sources (52,53) and increasing sediment load. (54)

Wildfire ash and burned soils are often enriched in trace metals. (43,55,56) Metals can become volatilized at high temperatures during combustion and then condense, subsequently adsorbing to ash surfaces during cooling. (44,45) Studies have also shown that the conditions present during combustion can induce transformations in metal speciation across matrices, including soils, coal, and biomass. (57?61) These alterations to metal speciation can increase the mobility and toxicity of metals, (57,58,60) underpinning the urgency for quantification in environmental systems...

...The Marshall Fire, a WUI fire in Boulder County, was the most destructive fire in Colorado history. Over the course of 2 days, the Marshall Fire destroyed 1084 structures, primarily residential dwellings, and burned over 24 km2. In contrast to wildfires, which burn mostly vegetation, structures are highly concentrated sources of bulk metals, which are present in structural components such as support beams (Ni and Cr in steel), plumbing (Cu and Pb), wiring (Cu), electronics (Cu, Ni, Pb, and Cr), and paint (Cr, Cu, Pb). Nevertheless, exactly how the presence of bulk metals in materials subjected to WUI fires impacts the final metal concentration in ash is poorly understood. To date, there has been little research to determine the composition of ash generated from burned structures and its potential for environmentally and biologically relevant metal release.

Rapid expansion of WUI (18,71) and increase in wildfire activity (12,18,72) will lead to increased quantities of structure ash, and it is imperative to understand the impacts that this may have on human and environmental health. Metal mobilization is of environmental concern due to the persistence, mobility, bioavailability, and bioaccumulative properties of trace metals (43,63) which can be examined through the use of laboratory-based extraction procedures...