WASHINGTON, D.C. — If you’re outside during a heat wave in Texas, the shade of oak trees can be tempting. But when it’s really hot, levels of lung-irritating ozone and other air pollutants can become worryingly high in these areas, new research shows.

When stressed by heat, trees release chemicals. These can mix with molecules from car exhaust to make toxic air pollutants. And this can happen even where you can’t smell any car exhaust. Atmospheric chemists at Texas A&M University in College Station shared their new findings here, on August 18, at the American Chemical Society fall meeting.

People expect to find air pollution in and around cities. There, vehicles cough pollutants out of their tailpipes. But at Texas A&M, “the air smells beautiful,” says Renyi Zhang. The campus is about 100 miles away from the closest big city. Here, he notes, the air is usually safe to breathe.

But that may not be true during heat waves. Zhang’s team collected data on campus. During a month-long event in August 2024, daytime temps hit 32˚ to 41˚ Celsius (90˚ to 106˚ Fahrenheit). And it wasn’t just the heat that made these days uncomfortable. Pollution ramped up, too. Levels of lung-irritating ozone got especially high. It rose nearly to a level deemed hazardous to breathe.

That was a surprise, the researchers now report.

A super-sensitive “nose”

Previous studies had shown a link between heat and excess ozone. But much of that work had been done in big cities, with lots of pollution from traffic and smokestacks. Zhang wanted to see how heat might affect pollution in a more suburban setting, like College Station.

Bianca Aridjis-Olivos, a graduate student in his lab, set up air-monitoring equipment atop the university’s meteorology building. It’s surrounded by grand oak trees. It seemed a good location. It’s high above passing cars and other expected pollution sources.

The instruments she used are “like a super-sensitive nose,” she notes. By changing how they operate, she could alter “what scents the nose could detect.” Besides ozone, they could pick up nitrogen oxides from combustion (including gasoline-powered vehicles).

They also could detect isoprene (EYE-so-preen). It’s a natural hydrocarbon released by trees, especially oaks. By itself, isoprene is not toxic. But in the presence of sunlight, it can mix with nitrogen oxides to cook up ozone (O₃), which is toxic to our lungs.

Once all her equipment was ready, Aridjis-Olivos waited for the next heat wave.

It hit in August 2024. From August 5 to September 3, she sampled different gases and particles from the air 24 hours a day.

As the temps rose each day, so did levels of ozone, oxygenated volatile organic chemicals (some of which are toxic to human airways) and acid-rich nanoparticles. Ozone levels, in particular, climbed as the heat rose. Then they stayed high throughout the hottest part of the day. That in itself was no surprise. Trees ramp up isoprene release during heat waves. Plus, heat speeds up the chemical reactions that produce ozone.

The surprise was just how high the ozone levels climbed. If you just looked at the levels of other pollutants, like the nitrogen oxides from cars, you might think the air was safe to breathe.

And while a heat wave feels miserable, says Zhang, “exposure to both extreme heat and high ozone can be very dangerous.”

How might these data be used?

This research shows how “climate change is making air pollution harder to control,” says Russ Dickerson. An atmospheric chemist, he works at the University of Maryland in College Park. Air pollution is a leading cause of death, he notes. The World Health Organization reports that today 99 percent of people worldwide are exposed to air unhealthy enough to pose a risk to our lungs.

Clearly, trees play a role here. But chopping them down would be the wrong conclusion, he adds. “Trees are generally good. But when you mix them with car and truck exhaust, bad things happen.”

“Studies like this can help us improve our air-quality forecasting models,” says Aara’L Yarber. An atmospheric scientist, she works at Howard University in Washington, D.C. With such computer models, she says, “We can better predict bad-air days during heat waves.” And that should help protect people’s health.

Data from this study are not limited to modeling sites like College Station, Aridjis-Olivos says. Being “surrounded by vegetation, but also some industry and traffic,” this region is similar to what’s found many places, including in many developing nations.

What’s the take-home message?

If you can’t avoid a heat wave, how can you protect yourself from air pollution that may come with it? Stay indoors when temperatures and ozone levels are highest, typically from noon to 4 p.m., says Zhang. Air-conditioned buildings (with windows closed) are best. Their cooling equipment contains filters that typically remove some pollutants.

If outdoors, stay as far away from busy roadways (and traffic pollution) as possible. Remember to check your local air quality index when making plans, Yarber says. “Kids are especially sensitive to air pollution,” she notes, “because their lungs are still growing.”

At home, Dickerson recommends getting a low-cost HEPA filter from a hardware store. For breathing “cleaner air — averaged over a day — put a HEPA filter in your bedroom,” he says. As an alternative, you can even duct-tape five furnace filters together. Then direct air through it with an electric fan. Remember to keep the windows closed.

There are also ways to improve outdoor air quality. For instance, communities can work to limit becoming “heat islands.” Two obvious ways: Increase shaded area, and limit how much heat-absorbing concrete they have.

Anything that reduces the levels of nitrogen oxides helps. They’re toxic by themselves. Plus they boost ozone to dangerous levels during heat waves. So walking, biking and taking a bus or train instead of driving can help limit traffic pollution. Switching from gasoline-powered to electric vehicles (EVs) also helps, because EVs don’t emit exhaust. Even better if the power to charge them comes from green sources, such as solar or wind.

A long-term strategy for improving air quality, says Dickerson, is “combating climate change itself.” The hope, he says, is that this will mean “heat waves happen less often, are less severe and are shorter.” And one great way to address climate change? Planting trees.