Wildfire smoke delivers a 'double hit' to air quality

When wildfires rage, the skies turn dark, the air smells of ash, and people retreat indoors to escape the haze. Most of us think of the smoke’s visible particles – the kind that irritate eyes and clog lungs. But there’s a less obvious danger drifting in that haze: ozone.

Ozone is a reactive oxygen molecule, O₃, that can damage lungs and other sensitive tissues in the body.

Unlike the blanket of fine particulate matter you can see hanging in the air, ozone is nearly invisible and easy to overlook. Yet it can be just as harmful – sometimes more so – causing respiratory problems and aggravating existing health conditions.

Wildfire smoke and ozone levels

Researchers at the University of Utah studied how smoke from Western wildfires affects ozone concentrations. They found that smoke can substantially raise ozone levels – even in remote areas far from cities and industrial sites.

These are places where human-made sources of ozone’s main building blocks, such as nitrogen oxides (NOx), are minimal.

“The question I wanted to ask was, if we don’t have urban emissions, let’s say that we zero out all emissions, will we still have an ozone problem?” said lead author Derek Mallia, a research assistant professor of atmospheric sciences.

“This study suggests that we could remove all of the regional emissions from anthropogenic sources of NOx, but fires can still produce a large amount of ozone.”

The team’s findings reveal a troubling reality: wildfire smoke delivers a “double hit” to air quality – first with fine particles, then with elevated ozone. In the United States, smoke exposure is linked to an estimated 6,300 deaths each year.

How wildfires boost ozone

Ozone doesn’t come directly from fires. Instead, it forms when sunlight triggers chemical reactions between oxygen atoms, NOx, and volatile organic compounds (VOCs). VOCs are abundant in wildfire smoke, while NOx comes mainly from vehicles and industry.

Modeling ozone is tricky because so many factors influence it – wind patterns, temperature, cloud cover, and time of day all play a role.

To study the connection, Mallia’s team used coupled computer models, WRF-Sfire and WRF-Chem, to examine a record-setting smoke event in August 2020.

That month, fires burned across the West. California’s August Complex fire scorched more than 1 million acres and caused $12 billion in damage. Utah’s East Fork fire consumed 90,000 acres, and Oregon’s Lionshead and Beachie Creek fires together destroyed 400,000 acres.

Ozone spikes far from the flames

The study found that wildfire smoke increases ozone concentrations by an average of 21 parts per billion (ppb). “Ozone was roughly 20 to 30% higher because of wildfire smoke,” Mallia said. “That’s pretty big.”

Given that background ozone in the West is already elevated, the added boost often pushes levels above the U.S. Environmental Protection Agency’s health limit of 70 parts per billion.

The researchers also found that smoke can work against itself. When thick plumes block sunlight, ozone production slows, sometimes by up to 10 ppb inside the plume.

“You also have a lot of particulate matter, which is a pollutant, too, but it can block sunlight and therefore that will reduce the amount of sunlight available for ozone photochemistry. It can be substantial in some cases,” Mallia said.

“If you’re right over the fire, there’s usually enough smoke shading where it limits the amount of ozone. But if you get far enough away and the plume becomes relatively diffuse, it’s usually not thick enough to really limit ozone.”

Better models to forecast air quality

With climate change making fires more frequent and intense, the need for accurate models is growing.

The study suggests that tools like WRF-Sfire and WRF-Chem will be critical for forecasting air quality during wildfire season. But these models need refining to better handle the complex chemistry and weather effects inside smoke plumes.

The message is clear: as wildfires grow in number and strength, ozone pollution will remain a hidden but serious health hazard – one that can reach people far from the flames.

The full study was published in the journal Atmospheric Environment.

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