Stephen Cantwell remembers betting on horse races at a Las Vegas casino, drinking tequila in celebration of his wins and walking to a nearby restaurant for hibachi rice.

On his way there, Mr. Cantwell, 59, passed out on the sidewalk. The high temperature on that day, June 21, was 109 degrees, but the ground beneath him was most likely dozens of degrees hotter.

By the time he had regained consciousness in a hospital, the pavement had seared his skin, even through his clothing, he said. It gave him second- and third-degree burns to about 10 percent of his body — an arm, a shoulder, his left buttock and a part of his leg.

“It was very painful, absolutely,” he said, though he is recovering well after two weeks of treatment at the University Medical Center in Las Vegas.

As climate change pushes summer temperatures ever higher and for longer stretches, and with more Americans moving into rapidly expanding cities in the Southwest, more people are suffering serious burns from contact with hot outdoor surfaces. For some, the burns are so extensive that they prove fatal, according to burn experts.

In 2022, the Arizona Burn Center at Valleywise Health Medical Center in Phoenix, the largest burn center in the Southwest, admitted 85 patients for contact burns over the summer. Last year, as Phoenix sweltered through 31 straight days of temperatures above 110 degrees, that number climbed to 136 patients, 14 of whom died. This year, the center has already treated 50 patients, and four of them died.