Last January, fires were raging across Los Angeles, smothering some 20 million people across the region in toxic smoke and ash.
LA residents worried the air was toxic, the soil contaminated, and the water poisoned. Questions swirled about the health risks created by the burns—and there were few answers at hand from city, state, or federal leaders.
Scientists from Los Angeles and around the country quickly scrambled into action as fires burned through the Pacific Palisades and Altadena. The priority, says UCLA physician and disaster researcher David Eisenman, was keeping people safe in the short term. But the fires also presented a moment to learn crucial missing information about the health effects of wildfires to help those affected and to better protect people's health from the inevitable next ones.
"This won't be the last wildfire that Los Angeles sees," says Eisenman. "Part of the community recovery process is to learn from what we experienced."
Researchers fanned out across the city to collect what data and samples they could. Doctors started thinking of ways to collect patient data to better understand the immediate and long-term health impact. They soon joined together to form a consortium that tied together 10 research institutions, developing a phalanx of research studies to explore some of the most pressing questions brought up by affected community members.
Some questions were simple, but frustratingly hard to find answers to, like: what was in the smoke? Other questions, like those exploring the long-term health impacts, will take years to untangle. But answers are beginning to emerge.
Extra-dangerous smoke
Wildfire smoke is dangerous under any conditions. Exposure to high smoke levels is linked to respiratory problems such as asthma and COPD, cardiovascular issues, and even dementia.
But from the first moment the Palisades and Eaton fires took hold last January, UCLA air pollution expert Yifang Zhu knew they were different. Because it wasn't just trees and plants burning: there was plastic from people's houses, and car batteries, and asbestos tiles — a "toxic soup" of air pollutants, she says.
What was in that soup, and how dangerous it might be to human health—that wasn't clear. Official air quality monitors in downtown Los Angeles, miles away from the heart of the fires in Altadena and Pacific Palisades, recorded high levels of lead and arsenic in the air during the burns. Researchers from Caltech and the Georgia Institute of Technology later measured lead concentrations in air samples both near and far from the fires. Lead levels, they found, were elevated, even miles away, signaling that smoke and ash from the burns spread the dangerous heavy metal widely.
But many scientists suspected the smoke and ash spread other toxic particles and gases widely, too — chemicals that standard EPA and state monitors didn't test.
"We need to test more than just what the EPA calls for. And the EPA has limited resources," says Kari Nadeau, an environmental health scientist at Harvard University and one of the leads for the new research consortium. "But as academics, we can test for hundreds of things all at once, which helps the community. Because what you don't know, you don't know, but it can still hurt you."
Before the fires, Zhu and her team had been getting ready to sample the air at Aliso Canyon, where a natural gas leak in 2015 had caused major health problems for nearby residents. When the fires broke out, the team pivoted, taking their sampling equipment as close to the fires as they could.
That opportunity was special: researchers are rarely ready to deploy at a moment's notice to capture samples during disasters such as the LA fires. The special circumstances let Zhu's team "set the stage about what's going on during this active fire burning all week," Zhu says.
Zhu's team set up air filters to capture the ash, and they captured air samples inside and outside homes in the Palisades and Eaton fire regions. In the air samples, they looked for more than 20 different volatile organic compounds—gases, many of which harm human health, and are likely to be produced by the fires. And while the fires were still burning, they measured high levels of benzene, a carcinogen, at their outdoor sites.
Lingering risks
The high benzene levels dissipated after the burning stopped, Zhu found. But other dangerous gases actually increased later on, especially indoors. A few health-harming gases, including toluene and carbon tetrachloride, became more concentrated inside people's homes a few weeks after the fire.
The message was clear. "The fire impact doesn't really disappear with the active flame," Zhu says. Homes themselves can absorb dangerous gases in the drywall, furniture, and other soft materials, releasing them for days and weeks after the smoke has dissipated. People need to know that their homes might be contaminated long after the fire is out, she says.
That wasn't the only lingering risk. Another research team started to look for a contaminant called hexavalent chromium, which can cause cancer, sometimes known as the "Erin Brockovich" contaminant, made famous by the movie of the same name. It can be produced when fires burn through certain types of soil or rock, as well as during industrial processes like welding. It's not often searched for after wildfires—but the researchers found it lingering in the air around cleanup sites long after the fires were out.
"It's actually one of those things that…makes you pay attention differently," says Joe Allen, an exposure scientist at Harvard University, who has been conducting ongoing research on building safety after the fires. And the contaminant was found in tiny particles so small that they can penetrate deep into people's lungs, bodies, and even directly to their brains.
"We've seen hexavalent chromium in soils after fires. I don't think anybody expected to see it in air. I don't think anybody expected to see it exclusively in the nanoparticle size range," Allen says.
Ash also contaminated people's homes, as well as soil and water across the region. The water impacts seemed to clear quickly, though longer-term effects are still being tracked. But levels of lead and other heavy metals inside people's homes and in the soil around them often remained high, even after clean-up was supposedly done.
"That is an ongoing question," says Allen. "Do we have enough funds to remediate all these properties, or are we just putting some people back into properties that are not properly cleared?"
Zhu was impressed by how much she and others learned about the dangerous smoke and ash. But she also worries they probably only scratched the surface. "We are only detecting things that our method allows us to detect. So even though we learn a lot from that, you know, I wonder what we missed," she says.
What does this all mean for people's health?
Scientists have known that in the short term, wildfire smoke exposure leads to more respiratory issues, such as asthma and COPD; increases the risk of developing dementia; and affects people's immune responses. But the full array of impacts, and the long-term costs of exposure, are still muddy.
"We know a lot about the health effects of wildfire smoke," says Allen. But "we don't know all that much about urban wildfire smoke. We certainly don't know what happens when you expose a population of 20 million people in the greater Los Angeles area to smoke like this, enriched in these toxic metals and other pollutants. "
The research is beginning to uncover some of the health impacts.
Cheng and colleagues collected data from the emergency room at Cedars-Sinai, one of the busiest in the region, and particularly close to the Palisades fire. In the 90 days following the fires, they saw a 24% increase in respiratory issues—and a 47% jump in heart attacks.
It was "very striking," she says. "This actually surpassed heart attack rates during January of all prior years, even during the worst years of COVID."
Abnormal blood tests also spiked, increasing by more than 100% over previous levels. That included unexpected blood sugar readings, signs of a disrupted immune system, and changes to people's metabolic profiles—signals, Cheng says, of bodywide stresses that could be precursors to many different health problems down the line.
"For a very large number of people who lived through these January wildfires, the wildfire exposures led to some kind of a biochemical or metabolic stress in the body that likely affected not just one, but many organ systems," she says.
The team is now tracking some of those patients, trying to understand what health issues their unusual bloodwork might have signaled coming.
The ER data is likely just skimming the surface, says Eisenman. Longer-term health problems, from heart issues to mental health stresses, are likely to linger or develop in the coming years.
Ongoing research will explore the different health outcomes for people who experienced different levels of smoke and toxin exposure. A UCLA-led study has enrolled over 4,000 people from across the city to follow their health changes long-term. Another study will focus on the specific health outcomes for those who stayed behind at their homes to fight the fires, giving them extraordinarily high smoke doses. The LA Fire Health study consortium is also tracking the long-term health impacts on firefighters and first responders.
Much of the emerging research is being supported by private philanthropy, says Eisenman. The wildfires happened just before the Trump administration began its campaign to tighten budgets for many of the science agencies that have historically funded post-disaster research, like the National Science Foundation.
"That gap was really filled in by the research community, who did ongoing and extensive and really thoughtful testing of air, of water, of soil, of debris for toxins, and really rapidly communicated those results back to the community," he says. But how to financially support the long-term future of some key studies, he says, is still uncertain, because many major federal research funding resources—like NSF and the National Institute of Health—have shifted priorities under the Trump administration.
How to protect yourself and your family
The biggest questions for the ongoing research, many of the researchers say, are about how best to protect yourself from similar fires in the future.
Allen says there are some clear lessons. Overall, the less smoke one inhales, the better. So while outside, he says it's crucial to wear an N95 mask, or even a respirator that can protect you from the fire's gases.
Indoors, keeping clean air is crucial, says Zhu. Using air filters, ideally HEPA-rated, can lower indoor pollution significantly. Carbon filters are particularly effective at removing the gases, Allen says. People can also install HEPA filters in a car's air-handling system to keep the air clean while they drive.
"You want to control what you can control," says Allen. So inside your space, clean up dust and ash thoroughly. Filter the air. And consider a low-cost air monitor to keep track of the air quality inside.
For people most impacted by the fires, Allen stresses that adequate clean-up of soil and buildings is critical. "It was a bit of the Wild West out there," after the fires, he says. A lack of standardized testing protocols and a hodgepodge of policies from different insurers "really harmed the survivor community."
That lack of guidance left many unsure whether their homes were safe to live in again, and many others were forced to go back to homes that were demonstrably still unsafe.
"We need more coordinated recommendations and rules to help people know whether their homes are safe," Allen says.
It will take years to get a full picture of the health impacts of the LA fires, many of the researchers say. But it's critical to learn from the tragedy, says Nadeau, the Havard environmental health scientist —to "be able to say, okay, in the future, here's what to do to protect your children or protect your elderly community against stroke," or lung cancer, or the myriad other risks from the wildfires that will, inevitably, come again.
Reporting for this story was supported by the Nova Institute for Health.
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