Episode 1: Data in the Immediate Aftermath

Julia Ehlert Nair: Welcome to the Caltech Science Exchange limited series, After the Fires. I'm Julia Ehlert Nair, a writer and science communicator at Caltech. In January 2025, nine fires erupted across the Los Angeles area. The Eaton Fire in Altadena was one of the largest, with flames burning just a few miles north of Caltech's campus and even closer to the Jet Propulsion Laboratory, or JPL, which Caltech manages for NASA.

"This truly came up so quickly. So many folks up here really didn't even have time to grab anything. They had to just jump—here comes one of these gusts we're talking about. These things are so strong. I have covered a lot of fires in my time out here in southern California and I have to tell you I have not experienced one like this in terms of the gusts."

"And so much destruction there—over 7,000 structures destroyed by the Eaton fire that's in Altadena and some of the other foothill communities. The majority of those structures are homes."

The devastation touched our entire community. Hundreds lost homes, many more were displaced, schools and other gathering places were destroyed--and so the Caltech community came together, supported one another, and raised and distributed much-needed funds. Caltech folks were also in a unique position to do something else.

Jessica Neu: But as scientists, we believe that knowledge is power.

Julia: Researchers across fields, from geology to chemistry to social science, jumped into action, collecting and monitoring data almost as soon as flames broke out. Some literally in their own backyards. They deployed sensors to collect information on air quality, tested samples of soil and ash from the burn zone and beyond, predicted where post-fire rains could raise the risk of debris flows, and contributed to conversations about community resilience and rebuilding. In this limited series, find out what Caltech and JPL scientists are learning, as they're learning it. In this episode, you'll hear about the air and soil data scientists collected and analyzed during and after the fires and how that can help us understand potential health risks. Unlike wildfires up in the mountains of LA, the Eaton Fire happened at what we call the "wildland-urban interface." In other words, where human-built environments meet nature. A lot of houses, businesses, and cars burned, which created the potential for toxic materials to be released in the air. Here's Caltech grad student Haroula Baliaka.

Haroula Baliaka: So unlike conventional wildfires that primarily burn grass and trees, the Eaton Canyon and Palisades fires were structural urban fires that burned significant portions of the built environment where painted surfaces, pipes, plastics, and even the structures themselves became the fuel.

Francois Tissot: And the thing with Altadena, which is also where my house was, 90% of the houses were built before the 1970s before the lead paint was banned, which means that there is a very high chance for lead to be released during the fires.

Julia: And that was Caltech geochemist Francois Tissot. So what exactly was circulating in the air after the fires? For that, we turn to Sina Hasheminassab, a scientist at JPL who has been making sense of outdoor air quality data since the fires started. This data comes from air sensors on the ground, plus from remote sensing, which are measurements taken from spacecraft orbiting Earth.

Sina Hasheminassab: The Palisades fire, since it was closer to the ocean, it generally had a smaller impact on the air quality in the downwind regions as opposed to the Eaton fire, which was situated in the northern part of the Los Angeles basin. And given the strong offshore winds that we had during that period, it's facilitated this spread of smoke to a larger areas and impacted larger communities downwind.

Julia: Sina's focus is measuring air pollutants called PM 2.5. JPL scientist Jessica Neu explains.

Jessica: So PM stands for particulate matter, and that just means solid and or liquid particles that are suspended in the air.

Julia: 2.5 refers to the size, less than 2.5 microns.

Jessica: And just for reference, dust that's visible to the human eye is about 25 microns in size.

Julia: So, 2.5 microns is pretty small.

Jessica: So some examples of particulate matter are things that are produced by fires are things like black carbon, nitrate. For normal wildfires, we wouldn't see trace metals, but in urban fires like we had in this situation, you'll see trace metals like lead, arsenic, cadmium.

Julia: Sina found that if you look at the levels of particulate pollution over time, using instruments placed across Greater Los Angeles, you see a big spike on New Year's Eve because of all the fireworks. Then, of course, there was an even bigger spike, more than three times that size, when the fires broke out. But it's not all bad news.

Sina: Now focusing on the fire period starting the late evening of January 7th, we see that a number of monitoring sites recorded highly elevated PM 2.5 levels well into the hazardous region. Moving forward, we see that the two nearest monitoring sites to the Eaton fire, specifically Caltech and JPL, continued to record highly elevated PM 2.5 levels, but then after that, especially after January 9th, we see a gradual decline in PM 2.5 levels across all of the monitoring sites such that by midday on January the 12th, we see that the PM 2.5 concentrations we return back to pre-fire levels. And since then, the concentrations of PM 2.5 remained within the typical ranges.

Julia: When it rained in the weeks after the fire, that also helped bring down the PM 2.5 levels.

Sina: Another thing that's evident from this map is that PM 2.5 levels clearly decreased with distance from the fire.

Julia: Measuring total PM 2.5 levels gives us important information. But if we want to know what those particles are made of—and if it's dangerous—we need different tools. For that we go back to Haroula Baliaka, who has been working on a relatively new collaboration called ASCENT.

Haroula: ASCENT is a nationwide collaborative effort to provide continuous and high-resolution measurements of PM 2.5 specifically focusing on the species that comprise this particulate matter. So, we've been measuring since 2023 at the 12 sites across the U.S. So, lead is typically found in older batteries, older paints, pipes and soil. Chlorine is found in plastics, household chemicals and even pools. And black carbon is a product of incomplete combustion of fuels, and Los Angeles is notoriously known for its bad air quality. So even day-to-day we can find black carbon attributed to cars, fireplaces or wood stove cooking. So, preliminary results here show that PM 2.5 lead actually peaked at 0.5 micrograms per cubic meter on the 9th of January of 2025 at the ASCENT site. This is on average was around a hundred times higher than the typical range. For chlorine, we have a peak at about 13 micrograms per cubic meter, a value 40 times higher than the typical levels and black carbon was about eight times the average concentration.

Julia: By the second half of January, average measurements for all of these materials had fallen back to normal levels. But let's keep in mind that what's normal for LA air now, hasn't always been that way.

"What? It's happening again, the city's disappearing. Boys? I got the answer. What makes the city disappear? Smog. What makes your eyes water? Smog."

"Here we are. You should be able to see the mountains. You should be able to see our beautiful city. But you can't see anything. And it's destroying not only our health, but our economy. From 50-70% of the junk that's in the air comes from our own cars."

Before regulations like the Clean Air Act and the phaseout of leaded gasoline, airborne lead levels in LA were consistently much higher than they are now. So even though the fires temporarily degraded air quality , the air we're all breathing today still has far less lead than what Angelenos were breathing 40 or 50 years ago. But what about the particles that settle—the ash and dust that those of us who live near the fires have come to know all too well? Francois Tissot, who we met earlier, uses fancy instruments like spectrometers to study substances from meteorites to human blood. And he lives in Altadena. So, naturally, he's been out there sampling and measuring the whole time.

Francois: Urban fires can release heavy metals such as cadmium and lead either as vapors or fine particles. And these will be transported by the winds and then deposited depending on the trajectory of the fire plume. And so a question that is on everybody's mind is were heavy metals released by the Eaton fire? How much of these metals were deposited? So, my group and I, with help from colleagues have done some preliminary work. We've gone and sampled about 10 ash samples throughout Pasadena, and then we've looked at Caltech, which is about three to four miles south of the fire region. And we've taken a hundred dust samples inside four different buildings to look at the concentration of heavy metals on desk and windowsills, trying to assess how much of a danger that is.

Julia: The clips from Francois that you'll hear on this show are from a talk on January 31st. Since then, his team has collected samples from more than 50 houses around Altadena, Pasadena, and LA, so he'll have more data to release in the coming weeks and months.

Francois: So, here we will focus on lead, cadmium, arsenic, and chromium as four important heavy metals that people might be familiar with.

Julia: And the reason we're probably familiar with them is that they're all hazardous to our health if we are exposed to them enough.

Francois: We did the sampling about one week after the fire started and then we went into the lab, released the metals from the wipe and measured them on a mass spectrometer. So what we see immediately is that for lead and cadmium in ashes we have about 10 times more lead than in typical soil while the dust, fine dust and the particulates show enrichment going from a hundred to a thousand times more lead than you would find in the dust coming from your garden in a normal day before the fire. Now those lead levels are elevated, but how does it compare with the EPA dust lead clearance levels that have been recommended for health reasons? Although we can clearly see that the fire has released lead and we can see an enrichment in the samples we have taken in the Caltech buildings, a lot of the values are still below the EPA window seal clearance threshold.

Julia: And there are steps people can and have taken to minimize their exposure to these metals.

Francois: What we can see is that the cleaning, which by the way is just a very simple cleaning with a wet wipe, a wet cloth, the cleaning removes about 90% of those heavy metals because they tend to be highly soluble. After one cleaning, we are well below the EPA dust clearance level in the Caltech buildings that we have tried. And the cleaning was done like anyone would clean their own house. We recommend of course, that you wear protective equipment when you do the cleaning, put on gloves, put on goggles, put on an N95 mask, and then wet wipe all the surfaces that you want to restore to a usable state. And of course, for the bigger particle, everything that is ash, a HEPA vacuum will be needed to minimize exposure to the dust.

Julia: So, it's not fun, it's not convenient, but cleaning surfaces with wet wipes seems to be truly effective. That's what I did in my house! But what can you do when the ash mixes with soil and grass?

Francois: If you have a garden that you're growing things into, the general recommendation is if you have the option to remove the first six inches and replace it with fresh soil, do that. It's mostly the grass-root vegetables that will uptake lead and other heavy metals. So things that grow outside the ground like tomatoes, if you rinse them, they should not have elevated lead inside of them. That has been found to be relatively safe. as far as kids playing and the lead making it into the ground. Most of the ashes we've tested so far don't show extremely elevated lead, just a factor of a few compared to a regular soil. So, once the lead will make it into the soil, it will at most double the amount of lead that is present, which means that you don't run a much higher risk.

Julia: It's been nearly two months since the worst of the fires and smoke. It will take far, far longer than that for the communities impacted to recover and rebuild, but we are all taking the first steps as best we can. Clean-up in Altadena is well underway with most of the hazardous materials removed by the Environmental Protection Agency, and as debris is removed and new construction begins, those of us who live locally will face more difficult decisions about how to keep ourselves safe. I'll leave you with some advice from Sina about how he thinks about air quality and personal risk in these times.

Sina: So I would start by saying that AQI is still a great resource here. That is sort of the official air quality information that's put out by regulatory agencies. Despite its caveats, we can still use that to get a good sense of air quality at the regional level.

Julia: Quick aside on those caveats: air quality index, or AQI, is calculated based on near real-time levels of some regional pollutants, specifically ozone, carbon monoxide, nitrogen dioxide, as well as overall particulate matter. But AQI does not directly measure the chemicals that Haroula and Francois talked about earlier on the show, like lead, that we're all concerned about during extreme events like urban fires. Still, Sina says, AQI is a good proxy.

Sina: I also want to take this opportunity and recommend people to check out air quality forecasts that South Coast AQMD puts out on a daily basis. They also put out hourly forecasts, which is really useful for planning your daily activities. Another thing that I encourage people to keep an eye on is the air quality advisories that AQMD puts out. At the more local level, there are some other resources that I would like to encourage people to consider. One is the EPA's Fire AirNow, which aggregates data from regulatory monitors and a number of low-cost sensor networks. They calibrate the data from low-cost sensor networks and bring all of the data into the same scale.

Julia: These low-cost sensors, which you might see for sale, are equipped to measure PM 2.5, the overall particulate matter that we've talked about, but be skeptical of the ones that claim to measure VOCs, or volatile organic compounds. VOCs can be things like benzene found in gasoline and toluene that's found in paint and paint thinners. You really need more specialized equipment to measure those, and you might not get reliable results from the at-home instruments.

Sina: Finally, at the very personal level, I want to encourage people to trust their common senses. That's very important. If you smell smoke, if you see plume of dust or ash, or even if you hear activities that may lead to resuspension of ash and dust, such as construction, such as leaf blower, those are the moments that you should trust your common sense and take actions regardless of what the AQI says or any of the other websites that I just mentioned. So I believe with the combination of these tools and resources, everyone can be very well aware of the environment and the air quality that they're being exposed to.

Julia: As I record this, Sina and his JPL colleagues are setting up a monitoring site in Altadena. Haroula, along with a group of Caltech faculty and students, have deployed a number of low-cost sensors around Altadena to be able to detect changes in air quality during the debris clean-up process. You can follow their progress at breathe.caltech.edu/phoenix.

For now, thank you for listening. I'm Julia Ehlert Nair signing off for After the Fires, a limited series from the Caltech Science Exchange. Subscribe wherever you get your podcasts to listen to future episodes on the fire-flood cycle, how communities recover from fires, and more research updates from Caltech. And visit scienceexchange.caltech.edu for explainers on the LA fires and other topics like AI, quantum science, and earthquakes. This episode was produced by the Caltech Office of Strategic Communications and Caltech Academic Media Technologies, in partnership with the Keck Institute for Space Studies, Resnick Sustainability Institute, and the Linde Center for Global Environmental Sciences.