Combustion Byproducts of Individual Materials Characteristic of the Wildland–Urban Interface (WUI) Display Unique Chemical and Toxicological Profiles That Are Masked in Aggregate Samples

Residual ashes from wildland–urban interface (WUI) fires pose chemical hazards to water bodies adjacent to burned structures. However, the full extent of the pyrogenic contaminants present in postfire residues remains underexplored. To address this knowledge gap, this study employed a bottom-up approach by combusting representative WUI materials, both individually and in mixtures, followed by water extraction of the resulting ashes. Chemical profiling of the aqueous leachates via mass spectrometry revealed complex mixtures of both organic and inorganic contaminants, with compositions that varied by material type. In vitro Escherichia coli bioassays of the bulk leachates further demonstrated toxicity associated with specific WUI materials, particularly polyvinyl chloride (PVC). For PVC, this toxicity was not fully explained by the inorganic fraction alone, suggesting the presence of unidentified bioactive organic contaminants. Additionally, antagonistic effects were observed in WUI material mixtures, which appeared to mask the demonstrated toxicity of PVC. This variability in the chemical and toxicological fingerprints across individual materials and within mixtures complicates efforts to generalize risk after WUI wildfires. Overall, characterizing the chemical profiles and bioactivity of laboratory-generated ash leachates from specific WUI materials provides a valuable complement to top-down observations of field-collected samples and informs understanding of postfire contaminant profiles.