Metal toxin threat in wildland fires determined by geology and fire severity

Wildfires, intensified by climate change, present a growing global health concern due to smoke and dust inhalation. While the ecological role of fire is crucial, extreme wildfires severely impact both human and ecosystem health. Wildfire smoke, a complex mixture of gases and particulate matter (PM), depends on fire fuels and severity, triggering inflammatory pathways and DNA damage. Fine particulate matter (PM2.5) is particularly harmful due to its deep lung penetration. Studies show wildfire PM2.5 is more hazardous than urban or prescribed fire PM2.5, highlighting the need to understand the chemical composition and compound-specific health effects of wildfire PM. Many wildfire pollutants are respiratory carcinogens; organic aerosols and gases from incomplete combustion are well-studied. However, increased heavy metals in PM, including chromium in its hexavalent form [Cr(VI)], represent an under-recognized health risk. Metal release is often linked to structural burning in wildland-urban interfaces (WUIs), neglecting the widespread contribution of wildland soils and ash. Globally, landscapes with ultramafic and mafic geology, often near populated areas, have high soil Cr concentrations (200–60,000 mg kg⁻¹). While Cr(III) is relatively harmless, Cr(III) oxidation to toxic Cr(VI) accelerates at temperatures above 200°C, even from mineral-bound Cr(III). Studies show significant Cr(VI) generation in urban ash and topsoil after fires. The global distribution of fire-affected ultramafic and mafic areas (all continents except Antarctica) underscores the potential for widespread Cr(VI) exposure via wildfire dust and smoke. Post-fire erosion readily resuspend Cr(VI)-bearing dust and ash, posing acute respiratory risks. Cr(VI)'s toxicity, depending on PM size and solubility, includes pulmonary toxicity and translocation to other organs, increasing cancer risks. Even low airborne Cr(VI) concentrations pose significant lifetime risks. This study investigates the interplay between soil properties, fire severity, and Cr(VI) formation and persistence in post-fire landscapes, using recent California wildfires as a case study.

Existing literature demonstrates the significant health risks associated with wildfire smoke inhalation, particularly focusing on PM2.5 and its inflammatory and genotoxic effects. Studies comparing wildfire PM2.5 to PM2.5 from urban and prescribed burns highlight the comparatively greater harm posed by wildfire emissions. While research has documented increased heavy metals in wildfire PM and their potential contribution to cytotoxicity, oxidative stress, and lung cancer risk, the specific role and impact of chromium, especially Cr(VI), remains less understood. Previous work has shown the heat-catalyzed generation of Cr(VI) during fires, both in urban settings (from structural materials) and in wildland environments (from soil and vegetation). However, there is a lack of comprehensive research on the landscape-scale production and distribution of Cr(VI) in wildfire-affected areas with diverse geological substrates and varying fire severities. This gap in knowledge necessitates a detailed investigation into the environmental factors influencing Cr(VI) formation and persistence, ultimately informing strategies for mitigating the related health risks.

This study analyzed reactive Cr(VI) concentrations in burned and unburned soils from four California ecological preserves with varying ecosystems (grassland, chaparral, forest) and geologies (felsic to ultramafic). The 2019 Kincade Fire and the 2020 Hennessey Fire (within the LNU Lightning Complex) provided a natural experiment to examine Cr(VI) generation under different fire severities and durations. Soil cores were collected from burned and unburned areas, divided into intervals, and analyzed for Cr(VI) using a 10 mM K2HPO4/KH2PO4 extraction at pH 7.2. Total Cr concentrations were determined via energy-dispersive X-ray fluorescence (XRF) spectrometry. Bulk mineralogy was assessed using powder X-ray diffraction (XRD). Micro-X-ray fluorescence (µ-XRF) and µ-XANES were used to analyze the spatial distribution and speciation of Cr in wind-dispersible particles (<53 µm). Particle size distribution was determined using laser diffraction. Post-fire Cr(VI) persistence was assessed through resampling ten months after the fires. Statistical analyses (Shapiro-Wilk test, paired/unpaired t-tests, Mann-Whitney U test, Wilcoxon signed rank test) were performed using R to compare Cr(VI) concentrations across different geological substrates, fire severities, and sampling times. A global fire frequency map was created by combining a generalized geologic map with the GlobFire Database to visualize the potential for Cr(VI) exposure globally.

The study revealed a strong influence of geology on fire-catalyzed Cr(VI) formation. Cr(VI) concentrations were significantly higher in soils derived from metal-rich geologies (serpentinite) compared to those from felsic (rhyolite) or intermediate (mélange) rocks. Severely burned areas showed 6.5-fold higher Cr(VI) concentrations than unburned areas in serpentine soils, with concentrations reaching up to 13,100 µg kg⁻¹ in wind-dispersible particles. Reactive Cr(VI) persisted in surface soil and ash for nearly a year post-fire, even with minimal rainfall. The surficial soil-ash layer, highly susceptible to wind erosion, contained significantly higher Cr(VI) concentrations than deeper soil layers. µ-XRF and µ-XANES analyses confirmed the presence of Cr(VI) in wind-dispersible particles, with concentrations exceeding the US EPA screening level for residential soils (300 µg kg⁻¹). Fire severity was a critical factor: high-severity fires produced much greater Cr(VI) concentrations than low-severity fires. Extremely high temperatures (>800°C) could limit Cr(VI) production. The study also found that low severity fires could result in high Cr(VI) in the inhalable size fractions. Global maps illustrating the overlap of large fires and metal-rich landscapes showed a concerning global threat.

The findings highlight the significant and previously underappreciated contribution of wildfire-affected metal-rich soils and ash to Cr(VI) exposure. The interplay between geology and fire severity is crucial in determining Cr(VI) production, with metal-rich geological substrates greatly amplifying the risk. The persistence of Cr(VI) in the easily wind-dispersed surface layer poses a long-term respiratory health risk to both local and potentially distal communities. This study provides strong evidence to support the observed higher hazard of wildfire smoke compared to other pollution sources, specifically implicating the release and distribution of toxic metals. Future research should investigate Cr(VI) formation and mobilization in other geological settings, particularly lateritic soils found in tropical regions. The development of predictive tools that integrate geological data, fire severity predictions, and meteorological models is necessary for effective risk mitigation.

This study reveals a substantial threat to public health from wildfire-released toxic metals, specifically highlighting the role of Cr(VI) formation in metal-rich geological areas. The significant generation of Cr(VI) in wind-dispersible particles, combined with its persistence in the post-fire environment, emphasizes the need for increased awareness and improved mitigation strategies. Future research should focus on developing predictive models to assess risk and informing preventative measures, emphasizing the global scope of this emerging threat. Further research should investigate other toxic metals released by wildfires.

The study focused on a limited geographic area in California. While the findings suggest a global applicability, further research is needed to validate these findings in different climates and geological settings. The µ-XRF analysis had limitations in resolving particles smaller than 1 µm, potentially underestimating Cr(VI) in the most respirable particle fraction. The study primarily focused on Cr(VI) and did not comprehensively analyze other potentially toxic metals released during wildfires. The extent to which Cr(VI) is biologically available is not explicitly discussed.