What to Know About Wildfire Smoke and Air Quality

(Bloomberg) — As fires burned across LA this week, worried residents have been looking at measures of air quality to help determine how safe it is to be outside.Smoke from wildfires can travel thousands of miles and affect the quality of the air, making it particularly dangerous for the very young and elderly, as well as people with medical conditions that affect their breathing and circulation. 

One way to understand how wildfire smoke is causing dangerous air pollution is the Air Quality Index scale, often used by weather apps. But that isn’t the only way to measure it — and relying solely on that might be misleading. Here’s what you need to know about air quality, AQI and wildfires. 

It’s the US Environmental Protection Agency’s official way of communicating how safe the air is to the public. It’s a scale of numbers and color-codes which corresponds to the concentration of five pollutants — ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide. 

The scale is based on the EPA’s standards for the level of pollution someone can be exposed to over a 24-hour period without it damaging their health. This scale ranges from “good” to “hazardous.” The EPA says that in the broadest terms, readings at or below 100 are satisfactory. As wildfires sweep across Southern California, readings of over 300 have been recorded — meaning the air is hazardous to breathe. 

The most dangerous and widespread pollution from a wildfire is particulate matter. These are tiny particles of soot or dust in the air, which can come from everyday pollution sources like gas cars and stoves as well as from wildfires. PM2.5 are the tiniest particles and PM10 are larger. 

That is what government-run websites like AirNow as well as unofficial apps and monitoring networks are typically measuring. They won’t necessarily default to PM2.5, says the California Air Resources Board — so check you’re looking at the right thing. Short-term effects include irritation to the eyes, throat and lungs, as well as coughing and shortness of breath. Longer term, it’s linked to health problems like asthma and heart disease. 

What’s going on with the LA fires and air quality?

Fires can also mean people are exposed to ash particles that are too large to be picked up by air quality monitors, says Scott Epstein, air quality assessment manager at the South Coast Air Quality Management District, which regulates air pollution in the Los Angeles area. The agency offers air quality ratings for the region, but this is only based on five pollutants — ground-level ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide.

“Our instruments are designed for essentially the pollution that you can’t see,” he says. “We can’t measure ash,” but it’s possible to see with the naked eye, and smelling smoke means ash is probably close by, he said. 

Wildfires in urban areas release other pollutants as well, says Anthony Wexler, director of the Air Quality Research Center at the University of California Davis. “The wildfire situation is different because there is lots of other stuff in the atmosphere that can be quite toxic.”Modern homes contain metals, plastics and other materials that are released into the air when burned. The most harmful are metals like copper and chromium as well as chlorinated compounds, like PVC, which is found in things like plastic pipes. 

Unlike particulates, there’s no good real-time way to test for these pollutants, and you won’t find information about them on AQI websites or apps, but “if the particulates are high in a wildfire smoke situation, everything is high,” says Wexler, “so the gases and all the toxic stuff are going to be there too.”

Why are some fires worse for air pollution than others?

Wind is a big factor, says Epstein. In the current blazes, the same offshore winds that have caused the fires to spread so quickly have also blown smoke out to sea, away from many people, reducing the air quality impact. In 2020, fires near Los Angeles had a much greater effect on the city’s air quality, because the wind direction changed and began to blow smoke onshore. 

Conditions can differ even in neighborhoods that are quite close to each other, depending on the direction of the wind. In some cases, there might be visible smoke high in the atmosphere but not at ground level, making conditions look worse than they actually are. 

“Our worst smoke events that we see are when we have strong offshore winds that start the fires because they’re dry and hot, and then we get an onshore switch that pools all the smoke within the region,” Epstein says. 

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