Even as firefighters douse the deadly LA wildfires, a more insidious danger remains. Chemicals from the ashy residue of thousands of burned homes and cars, scorched plastic pipes and even lifesaving fire retardants have blanketed parts of the region.
And that may jeopardize the immediate health of people living near burn zones for months to come. Long-term health consequences are also possible.
“There’s a general misperception that after the flames go out the hazard has gone away,” says Joseph Allen, who directs the Harvard Healthy Buildings Program. Wildfires’ aftermath may expose people to toxic chemicals and harmful particles in the air and water both outside and inside their homes, Allen and other experts warn.
The impact from the LA wildfires is still being assessed. But lessons learned from previous wildfires and from lab experiments can point to ways to return air and water supplies back to prefire safety.
Hazards linger after wildfires
Ash, soot and other pollutants that settle out of smoke may get stirred up and resuspended by wind and as people move about, Allen says. “These emissions are not necessarily captured by the regional air quality monitoring,” he says. So even if your city’s air “looks good or healthy or green, that doesn’t necessarily indicate that the air quality is good around your home.” That’s especially a problem for those living close to burned areas but may be an issue kilometers away, too.
Exactly what may be in the air depends on the fuels that fed the fire. Smoke from burning vegetation is full of fine particles and chemicals, including ozone, sulfur dioxide, polycyclic aromatic hydrocarbons and other volatile compounds that can be hazardous to health.
Increasingly, urban areas are burning too, especially as development snuggles up next to wildlands. So heavy metals are another health worry.
“Urban wildfires represent a unique challenge because of the types of pollutants that are generated,” Allen says.
When buildings and automobiles burn, heavy metals, such as lead and copper, get dispersed. Older homes may release asbestos fibers. Plastics and electronics give off noxious chemicals and metals. All of that can contaminate air, soil and water. Ash collected after the Lahaina, Hawaii, fire in 2023 contained high levels of arsenic, lead, antimony, copper and cobalt.
Some worry that the chemicals used to fight the fire pose a risk, too. Among the many iconic images of the LA fires were low-flying planes dumping fire retardants that painted neighborhoods red.
The main component of fire retardant “is ammonium phosphate, which is basically just fertilizer,” says Daniel McCurry, a water quality scientist at the University of Southern California in Los Angeles. “The red color comes from iron oxide, which is more or less rust.” Gum or other thickeners may be added. Those are “not so harmful,” he says.
Even ammonium phosphate itself is not “too concerning from a human health perspective,” McCurry says. But he and colleagues found high levels of heavy metals like chromium and cadmium in some commercial fire retardants, including an earlier formulation of the product used in LA, the team reported in October in Environmental Science & Technology Letters.
Breathing in or coming in contact with large amounts of the form of chromium called chromium six,can cause health problems including skin irritation and damage, and irritation of the eyes, nose, throat and lungs. Prolonged or repeated exposure may raise the risk of cancer. Short exposure to high levels of cadmium can cause flulike symptoms, including fever, chills and muscle pain, and may damage the lungs. Low levels of exposure over time can lead to kidney, bone and lung diseases.
The metals found in the fire retardant McCurry tested may be contaminants from the mine where the company, Perimeter Solution, gets its phosphate, he says. The U.S. Forest Service now uses a newer version of the product that, in standard safety tests, is less toxic to fish. McCurry has not tested the new formulation for heavy metals.
Water can be contaminated after wildfires
Drinking water and water pipes and tanks may be unsafe after a fire, says Andrew Whelton, an environmental engineer at Purdue University in West Lafayette, Ind. He has advised many communities about protecting their water supplies after wildfires.
Chemicals may be introduced to drinking water when plastic pipes or other plumbing components burn or get too hot, or when smoke is sucked into the system. Low water pressure and breaks in pipes may also allow microbes to get into the water. Some cancer-causing chemicals such as benzene, released when plastics burn, can seep into plastic pipes and gaskets and leach out slowly, making water unsafe for a long time.
Levels of chemicals in the water determine whether it is safe for drinking, bathing, laundry or doing dishes. “With low levels of contamination in water, we can bathe in it, but we can’t drink it,” Whelton says. “But if it’s highly polluted, then just having it touch our skin or [smelling it] can make us sick.”
Wildfires create indoor contamination
Recently scientists have learned that wildfire smoke isn’t just a problem for outdoor air quality. It pollutes indoor air and surfaces, too.
Some still-standing homes close to or within a burn zone may no longer be habitable, because of a build-up of toxic chemicals, Whelton says. “What I’ve seen is those homes have to be stripped down their studs and then rebuilt.”
That’s because “homes act like a trap when they’re exposed to a lot of smoke,” says Joost de Gouw, an atmospheric chemist at the University of Colorado in Boulder. “A lot of ash stays behind inside homes, and also a lot of odors linger for weeks.”
Take what happened after the Marshall fire on December 30, 2021, in Boulder’s suburbs. Hurricane-force winds whipped two small grass fires into an inferno that destroyed 1,084 homes and some commercial buildings, including a hotel. It forced the evacuation of more than 50,000 residents and killed two people as it consumed the towns of Louisville and Superior.
About 11 hours after the fire started, a snowstorm quenched it and scrubbed the outdoor air clean, says Colleen Reid, a health geographer at the University of Colorado in Boulder. As a result, Reid, de Gouw and colleagues could study how smoke permeated homes near the burn zone without continued contamination from outside air.
In a survey about six months after the fire, residents reported that when they returned to their homes, many found that ash had creeped in through doors, windows, and stove and dryer vents. Ash deposited on countertops and tables, beds, furniture, carpets, walls, in vents and was even found in boxes stored in closets in one case.
The ash came with health concerns. “People who reported that they found ash in their home after the fire were more likely to have headaches,” Reid says. And “people who lived closer to more burnt structures were more likely to report a strange taste in their mouth and headaches.”
More than 60 percent of survey respondents said their homes smelled differently a week after the fire, likening the smell to ashtrays, campfires or chemical fires, the researchers reported in ACS ES&T Air in December.
“People who said their home smelled different when they returned … were more likely to report headaches, sore throat, dry cough, itchy or watery eyes and strange taste in their mouths, than people who said that their homes did not have that smell change,” Reid says. Those symptoms may be due to volatile organic compounds, or VOCs, but the researchers can’t make that link because they couldn’t measure the compounds in all the homes, she says.
In companion studies, Colorado researchers measured particles and VOCs in homes close to the burn zone, including a heavily smoke-affected home in Superior. The team put a large instrument in that home and measured the chemicals for about a month.
VOCs, including benzene, toluene, naphthalene, furan, furfural and guaiacol were high 10 days after the fire when the researchers began measuring. Levels of benzene and toluene inside the home rivaled those seen in Los Angeles in the 1990s — a period of especially bad air pollution when more than half of the days each year had unhealthy or hazardous air quality.
In Colorado, “during the fire, these homes were exposed to very high smoke levels, and they acted like a sponge,” de Gouw says. “They soaked up a lot of these smoke compounds into different materials inside the home.” Drywall, furniture, carpeting and other materials were permeated with high levels of VOCs and slowly released the chemicals over time.
Concentrations of the VOCs dropped rapidly in the first five days of the study and then declined steadily, returning to outside levels by about five weeks after the fire, the team reported.
In other places where fires burn longer and that don’t get snow or rain to wash ash and smoke out of the air, it may take longer for indoor and outdoor air quality to return to healthy levels. And not all chemicals dissipate at the same rate, says Delphine Farmer, an indoor and atmospheric chemist at Colorado State University in Fort Collins. “There are some VOCs that clear out within a few days. There are others that are going to be more on the order of months. So just waiting five weeks isn’t going to be enough,” she says.
Air cleaners can help speed recovery, but only while they’re running, tests on fire-impacted houses in Colorado found.
The researchers built a Corsi-Rosenthal style box with four pleated air filters coated with activated carbon to catch both particles and VOCs. Within an hour after turning the cleaner on, levels of benzene and toluene in the house dropped close to those measured outside. When the device was turned off, levels rebounded almost to those recorded before the test.
Opening windows also helped temporarily drop VOC levels, but that’s not likely to work in places where outdoor air quality is bad.
The best method for getting rid of harmful smoke chemicals is cleaning, Farmer and colleagues found. Vacuuming, mopping and wiping down horizontal surfaces in a test house reduced smoke VOCs better than air cleaners and kept the air cleaner longer than opening windows, the researchers reported in Science Advances in 2023.
But to be effective, cleaning probably needs to be even more extensive. Chemicals from smoke attach to surfaces of all kinds, walls and ceilings included. “These gases don’t care what room they’re in,” she says. “And they don’t care whether the surface is horizontal or vertical or upside down.”