Wildfire smoke toxicity worsened by heavy metals in soil, flame intensity

The job of wildland firefighters is grueling; long treks into the wild and countless hours of manual labor on the job take their toll. Because of this, gear is often reserved for the bare essentials like flame-resistant clothes, hard hats, and tools to cut a fireline.

Urban firefighters, on the other hand, are outfitted like armored tanks with gear that’s nearly triple the weight of what the wildland firefighter carries. The most obvious visual difference in their gear is a breathing apparatus, meant to protect structural firefighters from smoke. Despite this, cancer remains the largest killer of urban firefighters, in part because of the synthetic materials that burn inside buildings and release toxic chemicals into the air.

A self-contained breathing apparatus (SCBA) is a device worn to provide an autonomous supply of breathable gas in an atmosphere unsafe for breathing — which structural firefighters often encounter.

Development of a wildland fire respirator. Two versions are being tested, with the filter being carried on the chest hip. Department of Homeland Security photo.
Development of a wildland fire respirator. Two versions are being tested, with the filter being carried on the chest hip. Department of Homeland Security photo.

A breathing apparatus or mask hasn’t historically been a staple of wildland firefighters’ gear, though some have been in testing for years. The added heavy carry capacity is one reason, along with the assumed lack of toxic chemical inhalation, since the fire’s burning in a natural area free from synthetic materials.

That assumption isn’t true, according to new research from Stanford University. Wildfire can actually create cancer-causing toxic heavy metals depending on where they burn and the severity of the flames.

“Soil-and plant-borne chromium is of particular concern,” the research team told WildfireToday. “Altered by fire, chromium is transformed into its toxic hexavalent state. We show that fire severity, geologic substrate, and ecosystem type influence landscape-scale production of hexavalent chromium in particulates during recent wildfires.”

The Stanford team researched soil and ash gathered from the 2019 Kincade Fire and the 2020 Hennessey Fire within the LNU Lightning Complex for their study. At the burn scars, the team measured the levels of chromium 6, which is known by most as the toxic chemical from the 2000 film Erin Brockovich, and they found dangerous levels of it in certain areas of the fire.

The chemical was present in heightened amounts where the soil had a greater concentration of metals from the area’s geology and had also been severely burned. Areas that weren’t on metal-rich geologies, or that had burned at a low severity, had either non-detectable chromium 6 levels or very low levels not of concern.

“Up until now, for wildfires at least, we’ve worried a lot about the fine particulate exposure … what we’ve been blind to is that those ultra-fine particles can differ in composition,” researcher Scott Fendorf said. “Even in wildfires that are completely removed from any dwellings, with certain geologies and certain vegetation types which are pretty common, we can see that the particles have these toxic metals in them.”

The team’s findings may not only help define the health risks wildland firefighters face in certain wildfires, but may also help in understanding what risks nearby populations may experience when inhaling air downwind of wildfires. In areas that experienced dry post-fire weather, chromium 6 was found to last on the soil’s surface in wind-dispersible particulates for up to a year after the fire was extinguished.

Researcher Alandra Marie Lopez hopes to further her research for this study and use the findings to examine what levels of chromium 6, if any, are found on landscapes post-prescribed burning. Additionally, the team hopes to use the research to create a risk analysis map to determine which areas and geologies after severe burns pose the greatest risk to human health.

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