One of the most extreme heat waves ever recorded baked the American West last week, with 40 million Americans affected by temperatures soaring above 100 degrees Fahrenheit. Deemed a “mega-heat wave,” it broke temperature records over a century old. And it’s not over yet — this weekend is projected to bring another historic heat wave to the Pacific Northwest, with temperatures forecasted at about 30 degrees F above average, breaking 100 degrees F in Seattle, Portland, and Spokane. 

A mega-heat wave in the middle of a decades-long megadrought is the reality of climate change in the American West. These boiling temperatures come with major public health risks; heat waves are the deadliest weather phenomenon in the United States, even when compared to hurricanes and floods, causing an average of 138 deaths per year since 1991. Climate change is increasing that statistic; on average, more than a third of heat-related deaths globally are due to climate change. These effects are not equally distributed in the U.S. — due to the racist history of redlining and inequitable access to green space and trees, people of color are disproportionately affected by heat.

The most obvious public health risk of heat waves is the risk of heat exhaustion or heat stroke, especially for those who work outside, including agricultural and construction workers, people experiencing homelessness, and those living with poor ventilation or without air conditioning. But that’s not the only public health risk of heat waves. Along with heat also comes bad air quality, which poses its own dangers. 

As temperatures climbed across the West last week, so did pollution readings, including in Southern California, Texas, Phoenix, and Denver. In Phoenix, the Arizona Department of Environmental Quality advised that people limit their time outside as ozone pollution (commonly known as smog) reached levels dangerous for public health. The Texas Commission on Environmental Quality issued ozone warnings for six consecutive days in Dallas–Fort Worth.

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Ground-level ozone pollution forms when heat and sunlight trigger a reaction between two other pollutants, nitrogen oxide and volatile organic compounds — which come from cars, industrial facilities, and oil and gas extraction. High temperatures therefore make ozone pollution more likely to form and harder to clean up. Drought and heat also increase the risk of wildfire, which can make air quality worse as smoke drives up levels of fine particulate matter — also known as PM2.5, or soot. 

During heat waves, the air also becomes stagnant, trapping pollutants like ozone. “Everything – the pollution, the smoke, the ozone – gets trapped right here where we live, and it gets sealed in. It’s like a pot you put on a stove. It’s like putting a lid on that pot, and everything down here gets trapped,” meteorologist Chris Tomer said on local Denver news show FOX31 News. “The 100 degrees just keeps things kind of swirling down here, and we breathe it in. We’ll rebreathe it, days and days out.”  

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Both ozone and PM2.5 carry major health risks. Ozone can cause acute symptoms, including coughing and inflamed airways, and chronic effects, including asthma and increased diabetes risk. PM2.5 exposure can lead to an increased risk of asthma, heart attack, and strokes. Globally, long-term exposure to PM2.5 caused one in five deaths in 2018, including 350,000 deaths in the United States.

If you’re affected by heat and air pollution, the Centers for Disease Control and Prevention, or CDC, recommends drinking plenty of water, wearing wide-brimmed hats with light clothing, avoiding the outdoors and strenuous outdoor activity, learning the symptoms of acute heat-related illnesses, and checking on those at risk — including children, pregnant people, those who live alone, and the elderly. (The CDC’s guides are also available in Spanish.)