Why those in L.A. whose homes were spared in wildfires could still face serious health risks

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

Though they weren't under an evacuation order, Gammon, who has asthma, told CBC News that the air quality was quickly worsening and the fires were volatile, so they left on Jan. 8, when the fires were about six kilometres 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 dotting everything in her yard.

"It's just raining down on us at night," she said. "They're sort of like big, white, chunky pieces of ash."

Researchers now say that because of the massive scale of these wildfires and the types of structures that are burning, people returning home when the initial fire hazard is over can still face health concerns from airborne compounds in the ash and smoke. 

Firefighters in the state are still working to put out the two largest fires 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, the Los Angeles County Health Officer prohibited any cleanup or removal of fire debris until hazardous materials inspections are completed.

Ash and dirt in debris from residential and structural fires may contain asbestos, heavy metals and other hazardous substances, the health officer said, noting that people can be exposed to the toxic substances by inhaling ash or particles of fire dust, or through contact with the skin or contaminated drinking water supplies. 

As an environmental journalist, Gammon has previously reported on the health hazards posed by ash, not only from brush, but also from thousands of charred structures. That's why she says she avoids touching the ash and wears a mask outdoors.

Earlier this week, scientists published their findings about how wildfires that burn both wildland and urban areas can impact indoor air. They studied the 2021 Marshall Fire in Boulder, Colo., where about 1,000 structures burned in eight hours, which could help shed light on urban fires like those in L.A. 

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

Over about five weeks, they also measured levels of volatile organic compounds (VOCs) at one home that survived the fires, but was heavily impacted by the smoke.

They found higher-than-normal levels of VOCs persisted for nearly a month inside the home before returning to typical indoor levels. 

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

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

Delphine Farmer, a Canadian scientist with Colorado State University, studies indoor air as part of an 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 home-turned-laboratory after they introduced wildfire smoke.