What’s in Wildfire Smoke, and How Dangerous Is It?

By Matt Simon

The West Coast’s wildfire crisis is no longer just the West Coast’s wildfire crisis: As massive blazes continue to burn across California, Oregon, and Washington, they’re spewing smoke high into the atmosphere. Winds pick the haze up and transport it clear across the country, tainting the skies above the East Coast.

But what are you breathing, exactly, when these forests combust and waft smoke near and far? Charred trees and shrubs, of course, but also the synthetic materials from homes and other structures lost in the blazes. Along with a variety of gases, these give off tiny particles, known as PM 2.5 (particulate matter 2.5 microns or smaller), that weasel their way deep into human lungs. All told, the mixture of solids and gases actually transforms chemically as it crosses the country, creating different consequences for the health of humans thousands of miles apart. In other words, what you breathe in, and how hazardous it remains, may depend on how far you live from the Pacific coast.

When vegetation catches on fire, it releases a whole lot of carbon in many forms. The sooty stuff you can see is known as black carbon. The major components you can’t see are carbon monoxide—obviously very toxic—and carbon dioxide. When trees absorb carbon dioxide from the atmosphere, they sequester it in their tissues and release oxygen. When those trees catch on fire, that CO2 goes right back into the atmosphere.

Scientists have been sampling wildfire smoke in the atmosphere with a special plane loaded with a bevy of instruments connected to little tubes that stick out of the aircraft. “Basically, it's my laboratory,” says Rebecca Hornbrook, an atmospheric chemist at the National Center for Atmospheric Research. These instruments measure conditions like humidity and temperature, along with particulate matter and carbon dioxide, benzene, and formaldehyde—the last two are quite toxic. “By combining all that data together, we're able to get a really full picture of what's going on chemically inside the plume, both in the gas phase and in the particle phase,” she says.

Hornbrook has been exploring how the chemical components of wildfire smoke change the longer they remain in the atmosphere. Benzene, a highly flammable compound that easily evaporates into the air, can stick around for two weeks. Formaldehyde lasts only a day or two. Other components may last only a few hours, so Hornbrook can actually watch their levels decrease as she flies through a smoke plume. The smoke’s trip across the country takes four or five days, but it will then linger in the atmosphere on both coasts—and the most persistent components will survive the journey from coast to coast. “Some of those harmful chemicals remain in the smoke, remain in the atmosphere, far, far downwind from where they're emitted,” Hornbrook says. “Clearly the most toxic environment is very close to the fires where the concentrations are at levels that can be harmful.”

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As the smoke plume travels through the atmosphere, “the heavier particles are going to start to fall out as time moves on,” says Rebecca Buchholz, an atmospheric chemist at the National Center for Atmospheric Research. “But then those sticky, partially burnt carbon gases are going to start to coagulate and become more particles again. So you're losing particles out of the smoke, but you're also gaining particles as the air processes through time.”

Another atmospheric nasty we’re all too familiar with forms as well: ozone, which inflames your airways. “Ozone requires carbon-containing gases, nitrogen-containing gases, and sunlight,” says Buchholz. “And so the more processing time you have, the more ozone is going to get created in that smoke plume.”