When Kate Gammon and her family fled their home in Santa Monica, California during the recent wildfires, she wasn’t sure what they would find when they returned.

Although they were not under an evacuation order, Gammon, who has asthma, told CBC News that the air quality was rapidly deteriorating and the fires were unstable, so they left on Jan. 8, when the fires were about six kilometers away. Four days later, she returned to find that the house had been spared from the flames, but was alarmed to see a layer of ash covering everything in her yard.

It only rains at night, she said. “They’re like big, white, chunky pieces of ash.”

Researchers now say that because of the sheer scale of these wildfires and the type of structures they burn, people returning home after the initial fire danger has passed may still face health problems from airborne compounds in the ash and smoke.

State firefighters are still working to put out the two largest wildfires in Altadena and Pacific Palisades, which have killed at least 27 people and destroyed more than 12,000 structures, including homes, schools and banks. On Thursday, Los Angeles County Health Officer prohibited any cleanup or removal of fire debris until hazardous materials inspections are completed.

Ashes and dirt in the remains of fires in residential and construction buildings may contain asbestos, heavy metals and other dangerous substancessaid a health official, noting that people can be exposed to the toxic substances by inhaling ash or fire dust particles, or through skin contact or contaminated drinking water supplies.

As an environmental journalist, Gammon is rather reported by the health hazards posed by ash, not only from the bushes, but also from thousands of charred buildings. That’s why he says he avoids touching the ashes and wears a mask outdoors.

Earlier this week, scientists published their findings about how forest fires burning both wild and urban areas can affect indoor air. They studied Marshall Fire 2021 in Boulder, Colo., where about 1,000 structures burned in eight hours, which could help shed light on urban fires like those in LA

Headaches after a fire, itching in the eyes

Colleen Reid, a professor at the University of Colorado, studied the health effects among people who returned to unburnt homes after that fire. She and her colleagues surveyed residents living near the burned buildings.

Over the course of about five weeks, they also measured the levels of volatile organic compounds (VOCs) in one house that survived the fires but was heavily affected by the smoke.

They found that higher-than-normal VOC levels persisted for nearly a month in the home before returning to typical indoor levels.

“People who said their home smelled different after the fire were more likely to have headache, dry cough, itchy, watery eyes, bad taste in the mouth and sneezing six months after the fire,” said Reid, an environmental epidemiologist.

WATCH | Ash from a forest fire, smoke remains in unburnt houses:

Harmful toxins from wildfires linger in surviving homes

As evacuees in Los Angeles begin to think about returning home, research suggests that the toxic effects of urban wildfires can linger for months in structures that survive the flames.

Reid and her team also surveyed people six months after the fire (642 respondents) and again a year later (413 respondents). Results included:

• 65 percent of respondents reported ash in their homes after the fire.

• Most respondents were less confident about the air quality in their home and neighborhood six months after the fire. Most returned their trust after a year.

Tips for cleaning up after a fire

Delphine Farmer, a Canadian scientist at Colorado State University, studies indoor air as part of the study experiment at the U.S. National Institute of Standards and Technology in Gaithersburg, Md., where she and her team analyzed the composition of particles inside a laboratory-turned-home after introducing smoke from a fire.

The farmer discovered it smoke is buried in painted surfacesit then slowly begins to return to the air, meaning residents are exposed to indoor pollutants long after the fire.

WATCH | The Science of Indoor Air After Wildfires:

To provide guidance for people who need to clean up after a fire, scientists are also studying how different air cleaners work and how surface cleaning affects air quality.

Based on her findings, Farmer’s cleaning recommendations include:

• Opening the window as soon as the smoke clears. The sooner you clear the smoke, the better.
• Wash fabrics such as bedding, clothing and curtains that may trap smoke.
• Vacuum floors. Wear an N95 mask while doing this as it can stir up soot or smoke residue.
• Mop the floors with soap and water and wipe down every surface you reasonably can, including walls if there was a lot of smoke in your area. Wash ceiling fans and other infrequently cleaned surfaces. Change the water as soon as it gets dirty.
• Clean out the ‘hidden spaces’ of your home such as attics, basements, closets and garages.
• If your HVAC system has been working, consider having it professionally cleaned. Change the filters on your home HVAC system or air purifiers and run fans all the time.
• Vacuum and wash the furniture as soon as possible. If you still smell smoke, consider deep cleaning or replace.