Urban Fire Smoke

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.

—

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.