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Why Do Heat Waves Make Ozone Pollution So Much Worse?

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Boy, is it hot? The gnarly heat wave we’ve been dealing with since last week isn’t quite over yet—and it doesn’t just cause us to sweat and complain. Events like these usually bring awful ozone pollution, too. And that’s stuff people need to worry about, especially as they’re outside enjoying their Fourth of July.

With the sun beating down hard, heat indexes are reaching well into the 100s across the U.S. this week. That’s resulted in cities declaring Ozone Action Days across the nation, which only happens when the air quality reaches unhealthy levels. On Monday, 72 cities declared an action day, according to Weather Underground. These advisories are continuing through the July 4th holiday in at least five cities across the Northeast and Midwest.

Ozone is a gas that, in our upper atmosphere, protects life on Earth from harmful ultraviolet rays. Near the ground, it’s a pollutant that forms from nitrogen oxides and volatile organic compounds. These chemicals spew out of vehicle exhaust and refineries, and they react to form ozone in the presence of sunlight.


“Like most chemical reactions, that chemical reaction comes faster in warmer temperatures,” explained Jeff Masters, a meteorologist with Weather Underground, to Earther. “So you’re going to crank out the ozone the hotter the temperature is
. That’s why ozone is a summer problem.”

At high enough levels—above, say, 70 parts per billion—ozone pollution can cause muscles in the lungs to contract and increase the chance of lung infections. Although the summertime pollution is especially dangerous for vulnerable individuals (those with asthma, the elderly, and children) risks can be present even for healthy people, per the Environmental Protection Agency.


This week, ground-level ozone pollution is bad. In fact, an air monitor in southeastern New York saw its air quality enter the purple range with respect to ozone on Monday, Masters said. That’s the highest tier on the Air Quality Index (AQI) and means everyone is at risk from the air. AQIs that combine ozone and particulate matter are registering more moderate levels in the northeast today, except for Philadelphia, where the air quality is currently unhealthy.

Unfortunately, ozone pollution events like we’ve seen this week may be a sign of our times. And not just because the heat isn’t going away. The emissions aren’t either.


Under the EPA Administrator Scott Pruitt, emissions regulations are dwindling. Those regulations are what will ultimately keep the air safe from ground-level ozone. And from the greenhouse gases that warm the atmosphere, resulting in more hot days that are likely going to exacerbate ozone pollution.

Until things cool down, maybe keep your morning runs off the streets and head to the safety of the treadmill at your air-conditioned gym instead. If you’re enjoying a gluttonous Fourth of July barbecue, maybe take that hot dog indoors?