This week we talk about urban fires, wildland fires, and AQI.
We also discuss PM2.5, chemicals, and plumes.
Recommended Book: Biological War by Annie Jacobsen
Transcript
On January 7, 2025, a fire began in the hills above Altadena, California, and surged through a community of homes, businesses, vehicles, and everything inside them. To someone watching the plume from a distance, it was indistinguishable from any other wildfire smoke. As it turns out, though, the composition of that smoke was a lot more dangerous than the emissions of a typical fire.
A study published in October 2026 found that, during the most intense period of structural burning, the smoke from the Eaton Fire carried far more lead and other hazardous material than smoke measured during a fire that burned mostly vegetation. The researchers estimate that roughly seven million people in the Los Angeles area were beneath air containing lead at more than eighty times its usual local concentration for a twenty-four-hour period.
Which is a striking number; though importantly, that doesn’t mean seven million people developed lead poisoning, or that a fire produced eighty times some established threshold of danger. The study measured the air and modeled where it went, it didn’t measure how much of that air ended up inside of people, and what happened inside of body that inhaled some of that air.
This finding does raise a practical question, though: when a wildfire moves into a city, do our standard ways of describing and responding to smoke give us enough information to make informed decisions about it, and the danger it potentially represents?
What I’d like to talk about today is the difference between a burning forest and a burning neighborhood, and what we still need to learn about protecting people downwind from this sort of disaster.
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Smoke is not just one thing, it’s a mixture of gases and tiny particles. And its nature is shaped by what’s burning, how hot the fire is, and what happens to the plume as it travels.
In a vegetation fire, the fuel might be trees, brush, grass, and other organic material on the ground. A fire moving through a built-up area burns those things, too, but also burns through roofing, insulation, wiring, cars, furniture, electronics, paint, and cleaning products stored in kitchens and garages. Some of those materials contain metals and synthetic chemicals that a stand of trees does not contain.
The location where homes and other development meet burnable vegetation is often called the wildland-urban interface. Which makes it sound like a narrow line on a map, but in practice this interface area contains neighborhoods where fire can make the leap from vegetation to buildings, and then from one building to another. It’s at this point that a landscape fire becomes, in part, a structure fire, and potentially a structure fire on a very large scale.
Researchers had reason to expect that structure fires would produce different smoke from wildfires. A National Academies report in 2022 described major gaps in our knowledge of what happens when the materials in a town burn alongside vegetation, though. Earlier EPA work found elevated metals in smoke associated with fires that destroyed buildings, but the aforementioned Eaton Fire gave us a more detailed look at how this mixture of smoke elements can change over a matter of hours.
Let’s take a moment to talk about how we measure this sort of thing. Most of us think of smoke, when we think of smoke, through the lens of an air-quality number. On a smoky day, a map might show that the air is unhealthy because of the presence of PM2.5, which are tiny particles that are small enough to be inhaled deep into the lungs. The PM2.5 number represents how much particle pollution is in the air, and it’s useful: decades of research link fine-particle exposure to breathing and cardiovascular problems, and the Air Quality Index tells people when to reduce their exposure, because the air is laden with such particles.
But PM2.5 describes particle size, not a single type of chemical. Two samples can contain the same mass of fine particles, the same amount of PM2.5 in the air, while having different ingredients—tiny bits of different sorts of things. An ordinary smoke reading won’t necessarily tell you whether those particles came from burning trees, a row of houses, or some combination of the two.
That was the opening for this new study. About eleven miles south of Altadena, in Pico Rivera, Caltech researchers had installed an advanced air-monitoring station that’s part of a twelve-site research network called ASCENT, which tracks not just the quantity of airborne particles but aspects of their chemical makeup. Importantly, it also collects measurements continuously, rather than gathering a sample every few days and then analyzing that sample later.
As wind carried the Eaton Fire’s plume past the station, the instruments recorded changes to the smoke composition hour by hour. The researchers then compared those readings with data from another station that had observed the Airport Fire, a 2024 Southern California fire that burned mostly vegetation.
The contrast was stark. According to the study, the urban plume had more than fifty times as much lead, around three times as much arsenic, and about seven times as much chlorine and bromine as the wildfire plume.
That composition changed with time, though. Lead levels rose sharply while structures and vehicles were burning in Altadena. Later, as the Eaton Fire moved into the mountains and burned more vegetation, its chemical signature shifted toward what the researchers saw in the mostly wildland fire. They could watch a single fire change from something more like an urban fire to something more like a forest fire, even though the smoke might have looked similar from the ground.
The research team also estimated how many people may have been underneath the lead-bearing plume when it was at its most dangerous. Their answer was roughly seven million during a peak twenty-four-hour period, when lead concentrations were more than eighty times the area’s usual background level.
Now, keep in mind that the instruments they used directly measured the air at a specific location, and that broader population figure is an estimate that comes from combining measurements with an assessment of how the plume spread. And “eighty times background” compares the event with what is normally a very low lead concentration in Los Angeles air. It’s not an eightyfold increase in an individual’s health risk, it’s an eightyfold increase in the presence of lead in the air, which can be absorbed or not by those breathing that air.
Being under a smoke plume also doesn’t mean every person breathed in the same amount, or in the same way. People were in different places, indoors and outdoors, and their buildings filtered air differently. The study did not test blood lead levels or follow the people who were exposed to determine later health outcomes, so this is a solid assessment of a potential source of danger, but not an indication of who actually got hurt, or even who was at risk of being hurt, because those other variables determine how exposed and vulnerable a given person is to this kind of environmental risk.
That said, lead is a neurotoxin, and long-term exposure is especially dangerous to children. Arsenic also has well-established health risks. These numbers support the need for further investigation, even if they fall far short of providing some kind of widescale post-disaster diagnosis.
Also worth knowing is that Altadena had many older homes, some of which may have contained lead-based paint. The prominence of lead in this plume doesn’t suggest that will always be the case for all fire-engulfed communities.
Of course, inhalation of smoke isn’t the only danger here.
Los Angeles County’s public-health department found elevated lead in a higher share of soil samples from intact properties downwind of the Eaton Fire. It cautioned that older pollu…
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