What Are All These Ground-Level Ozone Alerts?

If you’re of a certain age, you probably remember the hole in the ozone layer. Like Joseph Kony and Livestrong wristbands, the obsession over O3 now feels like a cultural artifact, thanks to ozone depletion being one of the rare success stories of international environmental cooperation. Since the world banned chlorofluorocarbons under the Montreal Protocol in 1987, the holes over the North and South poles have steadily recovered .

Today, if you hear about “ozone” at all, it’s much more likely to be from an air quality alert on your phone. Unlike the stratospheric ozone that we were all so concerned about in the 1980s and 1990s, which makes up a protective layer around the planet that insulates us from the sun’s cancer-causing ultraviolet rays, “tropospheric” or “ground-level” ozone is mainly man-made. In fact, when people throw around the word “pollution,” what they’re probably talking about is ground-level ozone, which is created by a chemical reaction between nitrogen oxides (highly reactive gases produced by burning fuels) and volatile organic compounds (organic compounds that easily evaporate under normal environmental conditions and can be found in vehicle exhaust as well as scented personal care products like deodorants, lotions, and bug sprays), plus sunlight. This chemical reaction usually occurs when cars, refineries, power plants, and other industrial sources emit pollutants into the environment during a hot, clear day. You probably know the result by its other name: smog.

Ozone is a climate issue not just because it is yet another concerning consequence of burning fossil fuels. According to some estimates, high levels of ground-level ozone pollution could grow in frequency by three to nine additional days per year by 2050 because of the gas’s close relationship with intense sunlight and high temperatures . While ozone dissipates fairly quickly once those conditions go away, it can build up while they last. Hot days, which are increasing in the U.S., also coincide with weak winds and stagnant air — conditions that allow ozone to accumulate in one place.

When the temperatures start to rise, here’s what you need to know and what you can do to protect yourself and others from ozone pollution.

How worried should I be when I get an air quality alert for ground-level ozone?

Different pollutants cause concern at different concentrations. The Air Quality Index is designed so that, in theory, a level of “100” corresponds to the point at which people in sensitive populations might start to be affected by the pollutant in question. (To learn more about how the AQI is calculated, you can read our explainer here ).

That said, “The evidence has clearly been increasing that lower levels of ozone — levels well below the current standard of 70 parts per billion — are causing more health impacts,” Katherine Pruitt, the national senior director of policy at the American Lung Association, which is campaigning to strengthen the standard to 55 to 60 parts per billion, told me.

As Pruitt explained, ozone is a caustic irritant and can corrode metals . Breathing it in can cause inflammation in anyone, “from vulnerable children and elders to even the fittest elite athletes,” Pruitt said, adding that it is, “at some level, like getting a sunburn on your lungs.” Anyone who spends time outside is vulnerable to ozone, but the more sensitive groups — including children; the elderly; people with asthma, chronic heart disease, and other diseases; and pregnant women — are at a higher risk. They might already be paying more attention to the AQI levels in their area, and will potentially notice that they need to slow down and limit exertion during “yellow” or “orange”-level ozone events.

In the short term, ozone pollution can cause coughing, shortness of breath, and a lowered immune response, on top of aggravating any preexisting lung conditions or diseases. But Pruitt stressed to me that “living in places that have high levels of ozone day in and day out, for months and years, can cause respiratory diseases, nervous system disorders, metabolic disorders, reproductive problems, and mortality. It’s not just a cough and a wheeze on one bad air day.”

Where are the worst places in the country for ozone?

Ozone requires two main ingredients: the burning of fossil fuels and other chemicals, and sunlight. While ozone concentrations can be high in communities with a lot of industry and freeways nearby, ozone is “not really so much a roadway problem; it’s more of what we call an ambient air pollutant,” Pruitt said. Ozone can travel far away from where it was produced, in other words.

There are some rules of thumb, though. The places with the highest emissions and most appropriate atmospheric conditions for ozone pollution are “increasingly the western U.S. and the Southwest,” Pruitt said. The top four worst cities for ozone on the 2024 State of the Air report by the ALA were all in California, led by Los Angeles and Long Beach.

Since the passage of the Clean Air Act in 1963, other regions of the country have been doing much better, including the Southeast, mid-Atlantic, and Northeast. (Bangor, Maine, had the cleanest air in the report.)

What can I do to protect myself?

Because ozone is so strongly related to sunlight, it does not cause indoor air pollution to the same extent as wildfire smoke (which, if you’re keeping score, is a PM2.5 pollutant ). “Because it’s so reactive, it gloms onto your furniture and your walls and stuff, once it gets inside,” Pruitt said of ozone. To protect yourself, you can just stay indoors and run your air conditioner.

But what if you want or need to go out? Because ozone is a gas rather than a particle, HEPA filters and face masks won’t protect you. Instead, Pruitt said that you can time your errands, tend to your garden, and exercise when the sunlight is the weakest — mornings, especially, tend to be less demanding on the lungs during ozone events.

What can I do to protect others?

The Clean Air Act of 1963 requires the Environmental Protection Agency to review the national ambient air quality standards for ozone (as well as several other pollutants) every five years. “It almost never actually does it every five years” though, Pruitt said. “Sometimes advocates have to sue them to get them to move things along.” The EPA completed its last review in December 2020, with the Trump administration maintaining the 70 parts per billion standard set in 2015. Attacks on the Clean Air Act would likely resume if Trump retakes office .

Aside from agitating for stricter clean air standards, there are measures you can take to protect others from ozone events. The simplest is not to contribute any more nitrogen oxides and volatile organic compounds to the environment than you otherwise have to when ozone levels are high. Avoid driving or idling your car; top off your tank during the coolest parts of the day , such as after dark; minimize your electricity use; and set your air conditioner no lower than 78 degrees.

In the long term, reducing ozone pollution will mean “choosing greener products for cleaning and personal care, so that we’re not producing volatile organic compounds,” Pruitt told me. The National Oceanic and Atmospheric Administration previously found that in New York City in 2018, “about half” of the ambient volatile organic compounds it measured were produced by people, not vehicle exhaust. (Here’s a guide to reducing VOCs from your rotation.)

Additionally, “transitioning to zero-emission technologies so we're not burning fossil fuels” will help limit ozone pollution, Pruitt said. The difference can be pretty significant: A study from the University of Houston published earlier this month found that by switching to electric vehicles , New York and Chicago could prevent 796 and 328 premature pollution-related deaths per month, respectively. Counterintuitively, the study found that more EVs on the roads could increase mortality in Los Angeles due to a corresponding increase in secondary organic aerosols caused by complicated dynamics between nitrogen oxides and volatile organic compounds and the city’s unique geography. “This underscores the need for region-specific environmental regulations,” the authors said.

Read more: This Is How You Die of Extreme Heat