Ground-level ozone is getting worse. Here's what it means for your health.

Belching factory smokestacks and black smoke pouring from muscle-car tailpipes made the air unsafe to breathe in parts of the United States before legislation passed in the 1960s and the following decades helped reversed the trend. But now experts say a “climate penalty” is kicking in, with climate change worsening a key aspect of air quality: surface-level ozone. This ozone is especially damaging to the respiratory system, harming airways and making lungs susceptible to infection.

Air quality in the U.S. 30 years from now will be similar to that of the early 2000s—rather than the better air we have today, according to a report released in February by the research firm First Street. The causes of the bad air are rising ozone due to warmer temperatures along with climate-induced wildfires, which are expected to increase in frequency and severity. (The smoke from these fires increases tiny inhalable particulate matter known as PM2.5.)

Both will lead to more days of unhealthy air.

As surface-level ozone worsens, more than 400 counties in the U.S. that do not currently suffer any poor-air-quality days will experience them in the coming years, according to the First Street report.

Heat waves—already three times more frequent and lasting longer than in decades past—are especially conducive to troublesome ozone spikes.

“When you have high temperatures, a lot of sunshine, and stagnant air—which is common during a heat wave—that’s a perfect recipe for making surface ozone,” says Loretta Mickley, a senior research fellow in chemistry-climate interactions at Harvard University.

According to a study Mickley coauthored, parts of the country will see high levels of ozone trigger twice as many bad-air alert days by the 2050s—sometimes up to nine days a year.

Even now, ozone is making air quality worse in some locations than previously, says Jeremy Porter, a climate implications scientist at First Street. “This is almost always tied to climate change,” Porter says. (The organization’s Risk Factor website quantifies air quality risk, along with flood, fire, and other climate-related perils, for individual properties across the country.)

The U.S. government also highlights ozone’s role in its Fifth National Climate Assessment, released last fall. Ozone is one of the reasons “climate change is projected to worsen air quality in many regions, harming human health,” the report states.

Heat plays a huge role

Ground-level ozone is a colorless gas that forms when a mixture of precursor volatile organic compounds and nitrogen oxides interact with sunlight. While some of these precursors are emitted naturally from trees and soils, many come from human activity, particularly vehicles, power plants, industrial boilers, refineries, and chemical plants.

Ozone is linked to climate because air temperature is an important weather factor in driving ozone formation. “Hotter temperatures allow the chemicals to combine and produce ozone,” Porter says. Overall higher temperatures lead to more average days each year when ozone levels may be elevated as well as to more short-term spikes.

The National Climate Assessment report predicts severe ozone episodes will become more frequent in the Northeastern U.S. and California, while year-round ozone will rise the most in the Midwest and Northeast. First Street also believes pockets in the Great Plains, Deep South, and Gulf Coast are becoming ripe for year-round air issues.

Low-income and minority communities are especially at risk, according to the national assessment, because they tend to be located closer to the factories and oil refineries that emit many precursor chemicals.

“Everyone needs to pay more attention to ozone,” says Jeffrey Yanosky, a researcher at the Institute of Energy and the Environment at Penn State University. “Many people are largely unaware of potentially dangerous substances in the atmosphere and that the levels of those substances may be rising.”

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People with asthma will suffer most

Ozone, a compound made from three oxygen atoms, can also form in the stratosphere, where it plays a protective role because it absorbs a type of ultraviolet radiation—UVB—that causes skin cancer and harms marine life. Concerns rose in the 1980s when scientists realized that the ozone layer over Antarctica had thinned (dubbed the ozone hole) and allowed harmful radiation to pass through. This led to an international agreement, the Montreal Protocol, to phase out stratospheric ozone-harming chemicals such as chlorofluorocarbons.

(What is the ozone layer, and why does it matter)

But while stratospheric ozone is protective, high levels of ground-level ozone are detrimental to vegetation, animals, and people, which is why the U.S. Environmental Protection Agency labels it a “harmful air pollutant.”

Ground-level ozone is especially damaging to the respiratory system. It harms airways and makes lungs susceptible to infection, and many people report coughs or scratchy throats, and pain while breathing. These effects can occur even in those who are otherwise in good health. Yanosky recalls biking in Sacramento on a day that ozone levels were high and feeling his throat and chest burn during the ride.

People with asthma, emphysema, or chronic bronchitis are especially at risk. Deaths from respiratory illnesses are more common in areas with high levels of ozone, according to the EPA.

Margarita Torres, a 50-year-old medical education coordinator in Homestead, Florida, has suffered from asthma since childhood. Torres is increasingly concerned about how South Florida’s warmer weather and more frequent heat waves will likely exacerbate local ozone levels and her disease.

“With spring and summers longer and hotter, I worry about how climate change will continue to affect my asthma,” Torres says. “What’s going to happen to a person like me?”

(Air pollution kills millions every year, like a ‘pandemic in slow motion’)

Health gains touted by the EPA—a product of decades of regulations that make air cleaner—include preventing more than 100,000 hospital admissions and millions of lost work and school days in the past 50 years. These advances are now at risk of reversing, Porter says.

Certain trees, including the ponderosa and white pine trees, black cherry trees and red alder, are also readily damaged by too much surface ozone.

Reversing the trend

Converting many vehicles from fuel to electric will be especially important for improving air quality. Studies show that in many cities, the largest contributors to high-ozone levels are cars and trucks.

Other actions that limit the greenhouse gas emissions fueling climate change are also crucial for reducing troublesome air, including switching to renewable energy and encouraging methane reduction in agriculture and natural gas extraction.

People can track the level of ozone (as well as PM2.5) in their community on the EPA’s website, AirNow. When ozone is high, the EPA recommends staying indoors as much as possible.

If you must go out during an ozone spike, the EPA advises taking public transportation over driving a car, or at least limiting car engine idling to lower the amount of volatile organic compounds emitted. It’s also advisable to put off using household or garden chemicals that evaporate into the air, which can generate additional surface ozone.

“A lot of what will happen in the future depends on our approach to the emissions of the human precursors,” Mickley says. “Going forward we have a choice of what we see.”