Why Urban Wildfires like L.A.’s Release Such Toxic Smoke

What makes smoke from urban fires so toxic?

Who Allison Parshall edited by Dean Visser

Firefighters in Southern California are struggling Palisades and Eaton Firesthey have killed at least 25 people, burned 37,700 hectares and at least 12,000 structures. Smoke plumes are also visible from space.

Residents of many fire-prone areas—even those far from the wind—know it orange, apocalyptic haze of forest smoke in fact, these fires have become more common due to climate change. That smoke can contain a cocktail of chemicals linked to heart and lung disease and even cancer, ie. leading cause of death among firefighters. Here’s what makes wildfire smoke so dangerous.

No common pollutant

About supporting science journalism

If you like this article, please consider supporting our award-winning journalism subscribe. By purchasing a subscription, you’re helping to ensure a future of impactful stories about the discoveries and ideas that shape our world.

When trees, shrubs, and other organic matter are burned, they release carbon dioxide, water, heat—and, depending on the fuel available, various volatile compounds, gaseous pollutants, and particles. These small particles suspended in the air can include soot (black carbon), metals, dust, etc. If they are smaller than 2.5 microns, they can bypass our body’s natural defenses when inhaled, burrowing into the lungs and causing numerous health problems.

This fine particle is a common pollutant; it is also produced by motor vehicles and industrial plants, for example. But the kind in the smoke of wildfires can be even more dangerous. Researchers studying health outcomes in Southern California concluded that exposure to a particular matter, called PM, is less than 2.5 microns._2.5from fires to 10 times more harmful to human health Compared to exposure to PMs_2.5 from other sources. The researchers estimated that the particles produced by the fires were three to four times more toxic, but they still don’t know why.

A more dangerous fuel

As humans develop more and more land, we are increasing the number of points of contact between human settlements and increasingly flammable forests. This makes it more likely that a human-caused stray spark will start a fire, and the resulting fire will consume homes, offices, cars and other man-made infrastructure, expanding the types and amounts of toxic compounds that go up in smoke. . Paints, sealants, insulators, metals and many other types can release volatile organic compounds, gaseous pollutants and particles.

A 2023 study by Environmental Protection Agency researchers found that they were emission factors for some toxic compounds. 1,000 times larger in urban wildfires than in wildfires burning in forested areas.

Unexpected Chemistry

It is surprisingly difficult to predict what compounds someone is exposed to when they inhale smoke from wildfires. What’s in the smoke it depends on some factors: what burned (ponderosa pine, for example, or a car), at what temperature it burned (on fire or on fire?) and how far and how long the smoke has been. As the smoke ages, it is exposed to sunlight. This radiation produces nitrogen oxides (NO_x) and volatile organic compounds (VOCs), which usually start a complex set of reactions that produce another secondary pollutant: ozone, the main component of smog, which can damage the lungs.

And as smoke containing VOCs travels through other cities and settles, it can mix with even more local pollution in the form of NO._x—allowing for the creation of a greater amount of ozone. Research also suggests that VOCs and particulate matter — each of which can be toxic — can combine to make their respective health risks even worse.

(Read more about the scientists who fly planes through fires to find out what’s in the smoke)

Persistent risks

Even after the smoke from fires clears, it often leaves some of its toxic components behind. In a study conducted in Colorado following the 2021 Marshall Fire, researchers found high levels of polycyclic aromatic hydrocarbons (PAHs). they have been connected respiratory and developmental conditions, infertility and cancer in the ashes of homes that survived the fireas well as VOCs in air samples. Other researchers exposed glass, cotton, and a mechanical air filter to be burned in a laboratory and found HAPs. It remained above the background level for 40 days. Cleaning processes were 48 to 71 percent effective.

This persistent pollution can also cause health problems. In the aftermath of the Marshall Fire, many residents reported itchy or watery eyes, headaches, coughing and sneezing. This has led to some debate among homeowners and insurers as to exactly what it is damage caused by smoke from fires.

Some of the smoke has begun to clear around Los Angeles, and much of it was blown out to sea by the same Santa Ana winds that made the fire so disastrous in the first place. But the city is now bracing for stronger winds as the National Weather Service warns of “explosive fire growth” in the coming days.