Along With Devastation, LA Fires Bring Health Hazards in Smoke and Ash

Here's what you need to know about toxic fallout from a major wildfire

By Juliet Grable

January 26, 2025

Photo by Marcio Jose Sanchez/AP

Smoke rises from the Castaic region, seen behind a neighborhood in Santa Clarita, California, on January 22. | Photo by Marcio Jose Sanchez/AP

The wildfires that ravaged the Los Angeles area have caused untold trauma, affecting hundreds of thousands of people. The Palisades and Eaton Fires alone have killed 28, consumed over 37,000 acres, and destroyed nearly 16,000 structures. 

Not only are these fires taking a devastating emotional and financial toll, but they’re also exposing thousands of people to hazardous smoke and ash. As urban conflagrations such as these become more common in a warming world, learning about how smoke and debris can impact your health is just as important as knowing evacuation routes and what to put in a go-bag

Even smoke from “conventional” forest and wildland fires is a complex mix of water vapor, gases like carbon monoxide and nitrogen oxide, and volatile organic compounds such as formaldehyde. A large component is particulate matter, made up of organic and inorganic bits of varying sizes. Of special concern is PM 2.5, which refers to particles that are 2.5 microns or smaller. These tiny particles are especially dangerous because they can penetrate deep into the lungs and even into the bloodstream.

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“It’s very clear that when the PM 2.5 concentration goes up, people die,” says Anthony Wexler, director of the Air Quality Research Center at UC Davis. Spikes in PM 2.5 concentration are consistently followed by an uptick in hospitalizations and cardiovascular-related deaths like heart attacks and strokes. This is true regardless of the source of the pollution, says Wexler. 

When a wildland fire moves into suburbia and starts consuming houses, vehicles, appliances, and the like, “lots of metals are being burned and lofted into the atmosphere,” says Wexler. “Some are really toxic.” Plumbing pipes release copper; car batteries emit lead; pressure-treated lumber contains chromium, copper, and arsenic.

“Transition metals” like copper are especially dangerous when they are inhaled, says Wexler. “Then you’ve got the plastics, a lot of which contain chlorine.” Chlorinated compounds, including polyvinyl chloride, are known toxins. PVC is a ubiquitous building material that’s found in pipes, windows, flooring, siding, decking, fencing, and more. When it burns, PVC may emit several toxic gases, including phosgene, a chemical warfare agent.

The pollutants a fire produces depend in part on how hot a fire burns, says Sheryl Ehrman, dean at Charles W. Davidson College of Engineering at San Jose State University.

“When you have perfect combustion, all of the hydrocarbons are going to CO2 and water,” she explains. When a fire smolders, it produces gases like methane and carbon monoxide and compounds like formaldehyde and polycyclic aromatic compounds, or PAHs. Some of these are known to cause cancer, disrupt hormones, and aggravate lung conditions. 

The smoldering phase can continue long after a flame front passes, says Ehrman. Firefighters “mopping up” after a fire and residents who return to check on their homes may all be exposed to hazardous smoldering fuels.

Chamber experiments in the lab have studied how woody material burns under different conditions. Now, says Ehrman, “the research community is really trying to understand, What are those effects when you’re moving away from natural materials to more synthetic materials?” Another newer area of research is understanding what happens to smoke and soot as it “ages.”

As smoke drifts, compounds in smoke interact with other chemicals and UV light. As more people are learning firsthand, impaired air quality isn’t limited to the vicinity of an active fire. One potential downwind pollutant is ground-level ozone, which can trigger muscles in the airway to constrict and cause wheezing and shortness of breath and spur asthma attacks and aggravate other respiratory conditions. Children are especially at risk.

Yifang Zhu, a professor at the Department of Environmental Health Sciences at UCLA, recommends using Watch Duty. This app tracks the progress of wildfires. Its maps include overlays for the Air Quality Index, wind speed and direction, and evacuation zones, providing “a more comprehensive view” of dynamic situations.

Then there’s the ash.

On January 15, the Los Angeles County Health Office issued an order prohibiting “the cleanup or removal of fire debris at fire damaged and burned properties” located in several Critical Fire Areas until a government agency can complete a hazardous materials inspection. The order does not prevent people from retrieving personal property from burn zones, but warns that “fire debris, ash, and dirt from residential and structural fires may contain asbestos, heavy metals, and other hazardous substances.”

Understandably, people want to return to homes and businesses and try to recover valuables. But unless the item is of irreplaceable personal significance, it’s best to let it go, says Wexler. “The ash is pretty toxic, bad stuff,” he says. “Stirring it up is increasing what’s in the air, which is increasing the risk to people who are stirring it up and the neighbors.”

Buildings that don’t burn may still be contaminated. When evacuated residents return to their homes, they often find a fine layer of ash covering yards, driveways, and vehicles, and a smoky “campfire” odor lingering inside.

Newer studies are attempting to quantify the risks. After the 2021 Marshall Fire destroyed nearly 1,000 homes in a neighborhood in Boulder County, Colorado, researchers installed sensors inside a home adjacent to the burn zone and measured concentrations of 50 volatile organic compounds, or VOCs, for five weeks following the fire.

They found that finishes and products inside the home can act “like a sponge,” absorbing and releasing gases from smoke over time. Concentrations of several chemicals were much higher indoors than outdoors, and though concentrations dropped quickly, they also lingered longer than the researchers expected. 

Another study suggests that dangerous compounds can linger inside homes long after the smoke clears. Researchers measured PAH concentrations on glass, cotton, and air filter media for four months following exposure to wildfire smoke. Though concentrations decreased over time, they remained elevated over background levels for over a month. The good news? Cleaning the glass with household cleaners and laundering the cotton significantly reduced these concentrations.

Zhu recommends wearing gloves and a mask and using “wet methods” like a mop and damp cloth when cleaning surfaces indoors. 

A body’s signature reaction to smoke can come in the form of a constantly runny nose, sinus headache, and general feeling of malaise. But, Wexler urges, don’t rely on signs and symptoms to tell you air is unhealthy. “Much of this stuff you can’t see; sometimes you can’t smell it either,” says Wexler. “If you inhale chromium 6, which is a carcinogen, you may not know for years.” 

Instead, monitor outdoor air quality using a reliable source such as the EPA’s AirNow website or Purpleair.com. These sites use the color-coded AQI scale to rank air quality from good (green) to unhealthy (red, purple, and maroon). PM 2.5 is the primary driver of AQI scores, and while sensors directly measure individual pollutants, “the concentration of all pollutants from a fire correlate very well with PM 2.5 and with each other,” says Zhu. In other words, if a region is in the green and you don’t smell smoke, you can safely assume the air is free of nasty pollutants.

When air quality is poor, stay indoors with the windows and doors closed and the air purifier on. Make sure your filter uses activated carbon, which captures both particles and gases. If you don’t have a purifier, consider a DIY option called a Corsi-Rosenthal Box. “You can go to Home Depot and buy a box fan, four high-end furnace filters and a roll of duct tape and make a particulate filter for your house for $50 or $60,” says Ehrman. When constructed with MERV-13 filters, the homemade purifier even removes viruses from the air.

If you must go outside when air is unhealthy, wear a N95 mask. In any case, avoid exercise when skies are smoky. “I like to say you have permission to be a couch potato,” says Wexler. “More exercise means you’re inhaling more, which means you’re being exposed to more smoke and more pollution.”

Even if we collectively stopped emitting greenhouse gases tomorrow, more of these devastating fires will be common in our lifetime. Knowing how to protect ourselves is important, and that includes knowing where to turn in times of crisis—which is especially tricky during a time of rampant misinformation and disinformation and eroding trust.

“When you have the conditions that bring the wildfire smoke into heavily populated areas, it serves as a wake-up call,” says Ehrman.