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New Stanford Research Reveals How Toxic Wildfire Smoke Can Be

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An orange, hazy sky amid burned trees is seen — the aftermath of a wildfire.
Scientists found that during a wildfire, areas with soil that contain high chromium levels can change from a benign form, known as ‘trivalent chromium,’ into a very toxic form called ‘hexavalent chromium.’ These particles in the air become a part of the residual ash, and the upper veneer of the soil is left behind with burn scars. (Dan Brekke/KQED)

Smoke from wildfires is even more toxic than previously thought, according to new research from a group of Stanford scientists.

Scientists and air regulators have long known the dangers of inhaling wildfire smoke, which can contain toxic gasses, fumes and particulate matter that can trigger asthma and heart attacks.

But this new research, published in Nature Communications, suggests wildfires can also cause the release of toxic heavy metals from the ground into the atmosphere.

KQED spoke with Stanford researcher and study co-author Scott Fendorf to learn more.

This interview has been lightly edited for clarity.


Brian Watt: Heavy metals, including chromium, naturally occur in our soils. Then, according to your research, when a wildfire occurs, it can trigger the release of those metals into the air. Can you explain that process?

Scott Fendorf: Sure. In the case of chromium, you have an extra concern that happens. So while a lot of the metals that we worry about can be in the fine particulate and are something that we haven’t necessarily been cognizant of, for chromium, not only do you have a metal, but you have a metal that during a wildfire can change from a benign form — what we call trivalent chromium —  into a very toxic form that we call hexavalent chromium. And just as a side note, if you remember the Erin Brockovich movie, that was all about hexavalent chromium.

You conducted your research on burn scars in Northern California: Napa, Sonoma and Lake Counties. What did you see there, and what are the implications for residents and first responders there?

We turned out to have this really great matrix to look at. And what we ended up finding out was that — not totally surprising to us — the areas that had high chromium in the soils ended up generating a lot more hexavalent chromium. And then, the other factor is that the more severe heating you get — meaning the higher temperatures and the longer the duration of that — the more hexavalent chromium ends up getting produced.

What that leads to is high levels of this toxin, hexavalent chromium, being a residual of the ash and the upper veneer of the soil being left in these burn scars. So if you are in a community that’s downwind of one of these burn scars, and you have dust coming in, that’s going to be bringing that hexavalent chromium into those communities.

It also means that if you’re coming back to those areas to do revegetation work, when you’re stirring up dust, that dust has all of that hexavalent chromium in it, and you’re getting exposed to that.

If these metals occur naturally in soil, is there a way to mitigate this toxic stuff released in a fire? 

We see only two mitigation potentials here, and there may be more, but these are what we recognize right now. The first one is really what you can do to protect yourself. And these are all the same things that we hear already: stay indoors; keep your windows and doors closed; if you go outside, wear an N95 mask. I’ll just go aside for a moment and say this certainly changes my risk calculation now. Back when we had these wildfires of 2019 and 2020, I was pretty cavalier, to be truthful.

Now, I would completely change my behavior in terms of how I would protect myself. The other one, though, is the possibility that control burns can really help be a mitigation effort, and I’ll explain why. Once you’ve done the controlled burn, then if we do have a wildfire, those areas still can burn in subsequent years, but they burn in much, much lower severity. So it ends up being this propagating win, if you will, that we keep the heavy metals in their more benign form rather than moving into this really toxic state.

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