A hidden wildfire pollutant causes thousands of excess deaths per year, satellite data shows

When you think of air pollution from wildfires, you probably picture the thick plumes of smoke and ash that waft into the atmosphere during a blaze. And if you've lived in an area that’s been enveloped by these emissions, you know to stay inside or wear a mask when the light tints red and gets hazy.

But this thick cloud isn't the only component of wildfire smoke that carries a health risk. Now, new research based on satellite data helps quantify the impact of an "invisible" wildfire pollutant: ground-level ozone. It would appear the yearly human cost of this hidden consequence lies in the thousands.

Over the past few decades, climate change — primarily driven by human activities like burning coal — has turned wildfire smoke from an occasional, regional-specific concern to a major source of air pollution in the U.S. Since the 1990s, the area burned by wildfires in the country each year has roughly doubled. This means the amount of pollution released by these fires is on the rise, too.

Researchers have therefore been scrambling to quantify what risk all that smoke poses for human health. So far, however, most of these efforts have focused on fine particulate matter, or PM2.5. This is made up of tiny bits of ash, dust, carbon or other material less than 2.5 microns across that get released into the air from fires or other sources, like industrial emissions. Scientists know that high PM2.5 exposure is hazardous to human health — it can exacerbate conditions like heart disease and asthma, and even damage lung tissue directly.

But PM2.5 is not the only type of pollutant that fills the air during wildfires. The blazes generate a complex cocktail of compounds, including ground-level ozone, one of the main ingredients in smog. Like PM2.5, ozone can mess with people's lungs and cardiovascular systems. But the two pollutants have very different pathways to formation. While fine particulate matter is made of charred bits flung directly into the atmosphere by wildfires, ozone forms after the fact, when nitrogen oxides and volatile organic compounds interact with light.

"It's what we call a secondary pollutant," Minghao Qiu, an atmospheric scientist at Stony Brook University and co-author of the new study, told Space.com.

While the health impacts of PM2.5 from wildfire smoke are pretty well-documented, fire-generated ozone has been overlooked. That’s a problem, Qiu says, because "high ozone days don't necessarily coincide with high PM2.5 days."

To help determine the effects of smoke ozone on health, Qiu and his colleagues looked at nearly 20 years of satellite data, meteorological records and ozone measurements. Unlike fine particulate matter, ozone pollution is not visible to the naked eye, but scientists can detect it in the ultraviolet spectrum.

The researchers found that certain regions of the U.S. were more likely to accumulate ozone from wildfires than others; states like Texas, Louisiana, Arkansas, Mississippi and Florida were at particular risk. They also estimated that wildfire-derived ozone was responsible for 2,045 excess deaths, on average, per year across the U.S. — nearly 16% of all deaths attributed to wildfire smoke.

That number is also increasing. The estimated deaths from smoke ozone in 2006 alone was around 100; by 2023 it was close to 10,000.

This outcome appears to be undermining gains made from regulations around ozone emissions under the Clean Air Act. While overall ozone-related deaths in the U.S. have been trending downward for the last two decades, smoke ozone is starting to push those numbers back up.

This study is a good start for establishing risk analysis for ozone, says Qiu, but there's still a long way to go before researchers fully grasp the health impacts of wildfire smoke. For example, wildfires often release heavy metals like lead into the atmosphere, along with aromatic hydrocarbons and other pollutants. More research is needed to determine how these compounds affect mortality — and how they might compound with each other. "We don't fully understand the impacts on health when you are exposed to all those chemicals together," Qiu says. He and his colleagues are already working on follow-up studies.

But future work may be hamstrung by federal funding cuts. Much of the data used in the new study was originally collected by satellites and monitoring stations operated by NASA and the National Oceanic and Atmospheric Administration (NOAA). Under the current Trump administration, NASA faces a proposed 47% cut to its science budget in 2027. NOAA faces a 26% reduction, focused largely on eliminating climate monitoring programs. Without these crucial projects, it will be much harder to disentangle the health costs of wildfire pollution, let alone predict future fire risk.

The study was published on April 29 in the journal Science Advances.