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, Pheonix 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.

Read the full article about heatwaves by Alexandria Herr at Grist.