LA heat waves make the air worse to breathe. Here’s why

No, it’s not just in your mind. The heat waves really do make the air extra gross to breathe.

That’s because high heat helps accelerate a molecular reaction in the air, resulting in the production of one of the main ingredients in smog: ozone. This reaction is why the South Coast Air Quality Management District issued an ozone advisory for much of Southern California through 8 p.m. Thursday.

How sunlight creates ozone

Southern California has constant pollution from things like cars and factories — and now wildfires — so air pollution is never a single-cause issue. But if you browse air quality maps long enough, you may notice the outside gets extra unhealthy during a heat wave.

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You’re not seeing things. The South Coast Air Quality Management District is the air pollution agency responsible for regulating and monitoring air quality in our region, and its map measures the presence of contributors to poor air quality. Particulate matter (shown as PM 2.5 or PM10) and ground-level ozone (O3) — common pollutants — have been shown to rise when it’s hotter.

Ed Avol is a professor emeritus of USC’s Keck School of Medicine and was chief of its environmental health division. He said the pollution problem can be a photochemical one. Ultraviolet radiation from sunlight provides the energy needed to break the chemical bonds between certain gases in the air and re-connect as ozone.

“It’s sort of a double whammy because the sun is there, the solar radiation happens. But because the heat is there, it happens at a faster rate,” Avol said. “You get both those things to sort of help push it along.”

This compounds in the L.A. area because it has more of a chance to react with our high levels of pollution. And when we have multiple days of extreme heat, we get a “washing machine effect.” The air moves forward and back across the basin, essentially baking the L.A. area in bad air.

“The general soup sort of stays here and it just gets a little bit more congealed, a little bit thicker day by day,” Avol said.

What ozone does to your health

Screenshot of the SCAQMD hourly Air Quality Index, taken at 12:00 p.m. on July 8. Purple stands for "Very Unhealthy," red means "Unhealthy," orange means "Unhealthy for Sensitive Groups," yellow means "Moderate" and green means "Good."

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South Coast Air Quality Management District

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“There is a lot of evidence now on how heat and poor air quality together could have a worse impact on health than each one of them alone,” said Rima Habre, a USC associate professor of environmental health and spatial sciences.

You likely won’t notice breathing it in the short term because each person reacts differently, according to Habre.

“When you’re talking about individual people, it depends a lot on who that person is and their baseline vulnerability for things,” she said.

Ozone is a highly irritating compound to your airways and lungs. It can quickly lead to inflammation. So for example, Habre said people with cardiovascular conditions or any kind of underlying respiratory issue shouldn’t be outside when ground-level ozone is high.

Another factor is what happens alongside this reaction. When ozone is high, the mixture of particulate matter becomes more unhealthy, according to Habre. That could be more reactive and induce more harmful effects.

Bad air can trigger short-term issues, like an asthma attack, but it also negatively affects your health in the long term. Exposure is associated with lower lung function and lung development over time, and there’s a greater risk of other diseases.

“Like metabolic disease and cardiorespiratory disease,” Habre said. “So, it doesn’t just act immediately. People who are chronically exposed to it over long, long periods of time… we think it also affects poor lung health and development and growth of various systems.”