January 13, 2025
3 minimum read
Why is urban wildfire smoke toxic?
Wildfires burning in cities release a combination of toxic and unpredictable compounds into the air.

Smoke billows over a destroyed home in the aftermath of the Eaton Fire on Saturday, January 11, 2025, in Altadena, California, USA. Firefighters have made some progress in extinguishing the deadly fires that have scorched Los Angeles, but the cost of damage continues to rise each year. Entire neighborhoods were reduced to ashes.
Kyle Grillot/Bloomberg via Getty Images
Southern California firefighters are battling the Palisades and Eaton fires, which have killed at least 25 people and burned a total of 37,700 acres and at least 12,000 structures. Plumes of smoke can be seen from space.
With climate change making wildfires more common, residents in many fire-prone regions, as well as those far downwind, are no longer familiar with the orange, apocalyptic plumes of wildfire smoke. I’m here. Such smoke can contain an unpredictable mixture of chemicals linked to heart and lung diseases and even cancer, the leading cause of death among firefighters. This is why wildfire smoke is dangerous.
No common contaminants used
About supporting science journalism
If you enjoyed this article, please consider supporting our award-winning journalism. Currently subscribing. By subscribing, you help ensure future generations of influential stories about the discoveries and ideas that shape the world today.
When trees, shrubs, and other organic matter burn, they release carbon dioxide, water, heat, and, depending on the fuel available, a variety of volatile compounds, gaseous pollutants, and particulate matter. These small airborne particles can include soot (black carbon), metals, dust, and more. If they are less than 2.5 microns in diameter, when inhaled they can bypass our body’s natural defenses and penetrate deep into our lungs, causing a variety of health problems.
Such particulate matter is a common pollutant. For example, it is also produced by automobiles and industrial plants. But the types of substances found in wildfire smoke can be even more dangerous. Researchers studying the health effects in Southern California concluded that exposure to a specific substance smaller than 2.5 microns, called PM, is to blame.2.5those caused by wildfires were up to 10 times more harmful to human health compared to exposure to PM2.5 From other sources. Researchers estimate that particulate matter from wildfires is three to four times more toxic, but they don’t yet know why.
more dangerous fuel
As humans develop more land, the number of points of contact between human settlements and increasingly flammable forests increases. This makes it more likely that an errant human-made spark will start a fire, and that the resulting wildfires will burn through homes, offices, cars, and other man-made infrastructure, increasing the variety and amount of toxic compounds in the smoke. will be higher. . Paints, sealants, insulation, metals, and more can emit many types of volatile organic compounds, gaseous pollutants, and particulate matter.
A 2023 study by researchers at the Environmental Protection Agency found that urban wildfires have emission factors for some toxic compounds that are more than 1,000 times higher than fires that occur in forested areas.
unpredictable chemistry
It’s surprisingly difficult to predict what compounds people will be exposed to when they inhale wildfire smoke. What’s in the smoke depends on several factors. It depends on what burned (a ponderosa pine, for example, or a car), the temperature at which it burned (were the flames burning or smoldering), and how far and how long the smoke continued to burn. I traveled. As the smoke ages, it is exposed to sunlight. This radiation can hit nitrogen oxides (NO).Ă—) and volatile organic compounds (VOCs) that trigger a complex series of reactions that typically produce another secondary pollutant: ozone, the main component of smog that can damage your lungs.
Additionally, if smoke containing VOCs travels and settles in other cities, it can mix with even more local pollution in the form of NO.Ă—– Gives an opportunity to form a larger amount of ozone. Research also suggests that VOCs and particulate matter, each of which can be toxic, can combine to exacerbate the health risks of each.
(Read more about scientists flying planes through wildfire plumes to uncover what’s in the smoke)
residual danger
Even after wildfire smoke clears, some toxic components often remain. In a study conducted in Colorado after the 2021 Marshall Fire, researchers found high levels of polycyclic aromatic hydrocarbons (PAHs). linked VOCs in ash and air samples from homes that survived the fire are linked to respiratory diseases, developmental disorders, infertility, and cancer. Other researchers have exposed glass, cotton, and mechanical air filters to smoke in the laboratory and found that PAHs persist above background levels for 40 days. The effectiveness of the cleaning process ranged from 48 to 71%.
This lingering contamination can also cause health problems. In surveys following the Marshall Fire, many residents experienced itchy or watery eyes, headaches, coughing, and sneezing. This has led to some controversy between homeowners and insurance companies over what exactly constitutes wildfire smoke damage.
Some of the smoke has begun to clear around Los Angeles, but much of it was blown out to sea by the same Santa Ana winds that fanned the deadly blaze in the first place. However, the National Weather Service has warned that “explosive fire growth” could occur in the coming days, and the city is now bracing for more strong winds.