Is the ash and residue from the recent fires a danger to the fruits and vegetables in your garden?

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There is relatively low concern about potential contamination on produce in areas of wildfires, says Julia Van Soelen Kim, who was part of a team analyzing samples after the Tubbs Fire in Sonoma. Photo by Kazi Pitelka.

As the immediate shock of the Eaton and Palisades fires subsided, Angelenos walked outside to discover ash on the ground and dusting our yards. Then, we heard the warnings that urban wildfire ash contains countless toxic contaminants — lead, asbestos, microplastics, firefighting chemicals. 

Here at GoodFood, we've received countless questions about backyard gardens and schoolyard plots. Like you, we're curious and concerned. Are herbs and vegetables okay to eat? How should we clean them? Is the soil safe? 

Sadly, this isn’t the first time Californians have grappled with these questions. Julia Van Soelen Kim is the North Bay Food Systems Advisor for the UC Cooperative Extension North Bay, which includes Marin, Mendocino, Napa, and Sonoma. She led the studies assessing backyard gardens after the Sonoma Complex Fires in 2017.

In addition to this interview, Julia also shared a number of resources with us. 


These are the most important takeaways for fruit and vegetable safety after urban wildfires. Graphic courtesy of Julia Van Soelen Kim.

Evan Kleiman: You must be watching our situation from afar with so much empathy, with the knowledge that you have, having gone through it yourself. 

Julia Van Soelen Kim: Yeah, that's exactly right. We had a lot of similar questions coming our way after wildfires in Sonoma County and Napa County in 2017. The fires in LA brought me right back to that moment of crisis, of the deep hurt and loss of community and displacement of people across our community, and then these sort of secondary questions, secondary stress about, "Is the water safe? Is the air safe? Is the food safe?" And that really weighed heavily on many people's minds and hearts in the days and weeks after our wildfire.

Can you remind us of the similarities between the Sonoma Complex Fire and the recent Palisades and Eaton fires here in LA?

In many ways, they're very similar. When we had our 2017 wildfire, the Tubbs Fire, as well as a whole host of other neighboring fires happening concurrently, that was the first time that we had a significant urban wildfire in California that was fundamentally different in scale and in its urban nature than what had taken place historically. Thousands of homes and buildings were lost, and it was really in urban areas, right in residential communities and industrial zones, and that was something that was really unique and concerning.

A lot of the science prior to then had looked at potential contaminants from more common wildland fires, so we didn't have the science to really understand the impact of what an urban wildfire might have on community health.

Tell us about the research you did. How did you go about addressing these concerns and creating a different kind of science? 

Back in the days and weeks after the Sonoma County Fire, there were a lot of concerned community members who came together with UC Cooperative Extension and our master gardener program to really understand what the potential risk of produce exposed to smoke and ash might be. So we were able to mobilize people — community volunteers — to collect samples of leafy greens from 25 sites of community, school, and home gardens, and local urban farms across the county, using a really consistent sampling protocol.

We took samples of both washed and unwashed leafy greens in triplicate so that we could understand whether contaminants could be easily washed off. We focused on leafy greens because they have a large surface area and would therefore be exposed to sort of the most amount of air pollution. So we looked at collards and kale and other leafy greens.

In total, over 200 samples were taken and frozen for later analysis. Then we started the hard part of actually fundraising to get enough money to have those leafy greens tested for a whole host of contaminants. We weren't able to test all of the samples but in short, the results that we were able to have analyzed showed that there was relatively low concern about potential contamination on the produce. Later, we followed up with soil tests, and we also had relatively low concern about potential soil contamination. But also, there were some caveats there that I can go into later.

I think a lot of people are going to be relieved that there was relatively low concern. Before we get granular, was proximity a factor in your research? How far from the centers of the fire were you gathering samples? 

Yeah, we collected samples from across Sonoma County, and we did map out how close the samples were to the fire perimeter. It's a good point to underscore that what our study was looking at was the potential for soil and produce contamination from smoke and ash dispersed across our geography. It wasn't from sites that had directly burned or that were directly adjacent to burn sites. We're talking more about the ash that was picked up in the wind and in the air and dispersed broadly across the region. 

Let's get into the caveats you mentioned. What did you find? 

We found that produce samples did not have detectable levels of a whole host of contaminants, and that things that were commonly known and regulated contaminants were generally not present. No samples had detectable levels of lead or arsenic or mercury or chromium. All of that is really good news. 

Then, in terms of the soil, all of the soil that we tested and collected was of low concern, but there was one site that had some levels of detectable contaminants. We don't know if those were present before the fires or a result of the fires. That site was closer to the urban burn perimeter but we don't have reason to really be able to tie it to the urban burn. 

So actually, going forward, that's one of the things that we're really eager to have happen, is to have other communities replicate our study, using our tools and our protocols to better understand their own unique context and also to better continue exploring the topic.

We've been talking mostly about the ground, I mean people growing food directly in the earth. So many people use raised bed systems. Should people get rid of all that soil and start fresh?

 If we're thinking about best practices for soil safety for all of those gardens that were exposed to many days of a high air quality index, hazardous air quality and a dispersion of ash just on on the general environment, on houses and decks and patios and cars and gardens, then there are some really simple best practices for soil safety. 

The first is wearing a mask, if it's high AQI out. And if you're going to make be making any kind of dust airborne, if you're turning the soil, if you're sweeping, if you're hosing things down, you want to wear a mask to reduce inhalation of any potential ash or anything else that's in this soil. 

The second is containing the soil, so preventing soil from getting kicked up as you're raking and digging in wind. You want to create a barrier between the soil and the environment by mulching, and that can be with straw or wood chips or newspaper and cardboard. Any kind of mulching is going to be really beneficial to sort of cap the soil. 

And then to amend the soil with compost, to add clean soil, especially to raised beds, to help dilute any kind of contaminants and help break down any kind of organic organic chemicals in the soil. And then to use drip irrigation to try and reduce up-splash of water splashing soil onto the underside of leafy greens or produce that's growing in the garden. 

There is a concern for ingesting contaminants but that concern is actually greater around hand to mouth, as opposed to fruit to mouth or vegetable to mouth. Think about ways that you, your family, young children, can really practice good hand hygiene, washing your hands before and after harvesting, before and after working in the garden and minimizing the amount you're rubbing your eyes, licking your lips, that kind of thing, while you're outside. 

Then lastly, like I recommended earlier, it's always a good idea to test soil, even separate from a wildfire, to understand your own site's potential for having contamination in the soil, either naturally occurring or because of previous land use.

I want to ask you a question about soil testing. Are there any red flags we should be particularly aware of in a soil sample? 

l will direct people to UC Master Gardeners. UC Master Gardeners has lots of information on soil testing, including how to collect soil samples in a systematic way, how to send samples for analysis, and how to interpret results. 

I would say that generally speaking, and this is inclusive of remediation efforts after a wildfire to specific sites that burned, heavy metals can be used as a barometer for other potential contaminants. Looking at the presence of heavy metals is a good way to get a sense of the general safety of that soil, knowing that there are many other contaminants that would not have been tested.

Are fruit trees more resilient? Do they tend to not take up as much as leafy greens? 

This is our common sense hypothesis, which is to say that the more surface area something has that can be exposed to smoke and ash, and the closer it is to the ground and potentially contaminated soil on the surface of soil, the more likely that it could pick up residual contamination, either on its surface area or actually internal to the plant. Whereas, in contrast, things with a thick peel, tree fruits, for example, are generally going to be of lesser concern.

I really want to underscore that in a wildfire, there are so many things to worry about, and the safety of produce and the safety of soil isn't on the top of my list. I feel relatively at ease that produce safety, at least in our urban wildfire in Sonoma County in 2017, wasn't of significant concern. 

That's a relief. Thank you so much. 

Yeah, I'm very happy to say that. And just to reiterate, there are really common sense ways to reduce one's potential exposure. That's protecting yourself, paying attention to the potential for inhaling smoke, inhaling ash, for wearing layers and removing outer layers, taking off your shoes before you come inside. Anything you can do to keep the outdoors and the outdoor ash off of yourself and not coming into your home. And then, of course, rinsing, washing your hands and rinsing your produce before eating it.