Moshe Pomerantz’s job is to keep people cool. Doing so, it turns out, requires ample time in the heat.

As Chicago endures sweat-through-your-shirt conditions this week, Mr. Pomerantz is going about his routine, traveling from home to home, making sure air-conditioners are running well. Days like these are taxing in Mr. Pomerantz’s line of work, with emergency calls from homeowners who had no idea that their aging A.C. was careening toward its demise, along with all the usual maintenance appointments.

And so Mr. Pomerantz found himself atop a condo building in Chicago’s Ukrainian Village neighborhood on Tuesday evening, nearly 10 hours into a sweltering shift.

While examining wires and cleaning out the inside of a Carrier unit, the perks of Mr. Pomerantz’s job were evident. As he often does, he had a special vantage point on the city, with sweeping views of downtown skyscrapers a few miles to the east and of the sun glinting off a nearby cathedral’s golden domes.

But the paradox of Mr. Pomerantz’s work was also on display. To make sure his customer’s condo stayed in the 70s, he was perspiring through another hour in a heat wave that has gripped much of the central and eastern United States and sent Chicago’s temperatures spiking into the mid-90s.

“I just try and keep in the shade as much as I can,” said Mr. Pomerantz, who ranked Tuesday as his hottest day on the job in more than a year working for Shavitz Heating and Air Conditioning. You “can’t really do that on the roof,” he conceded, “but most other residences, you at least get a little time in the shade.”

Chicago might be more famous for its winter chill, but the city is no stranger to brutally warm days. When a hot stretch comes, Chicagoans cope by splashing in Lake Michigan, hunkering down at home or, to the Fire Department’s chagrin, illicitly turning hydrants into sprinklers.

After a June filled with mild days, this week’s weather was a reminder of just how fast Midwestern meteorology can shift, and of the risks that can bring. Unrelenting heat waves in 1988 and 1995 were linked to hundreds of deaths in Chicago.

Mayor Brandon Johnson implored residents during a news conference on Tuesday to “take steps now to protect yourself from the heat,” suggesting that they wear light-colored clothes and keep their homes’ shades closed. Local governments in the region opened cooling centers. And the National Weather Service warned that “the worst of the heat wave will continue through Thursday,” with high temperatures above 95 and heat indexes around 105 on Wednesday and Thursday.

Chicago was hardly alone in sweating. Much of the Midwest was enduring similarly steamy conditions, and the East Coast was bracing to take the brunt of the heat over the holiday weekend.

Weeks like this expose air-conditioning systems that had been limping along, said Bronson Shavitz, the owner of Shavitz Heating and Air Conditioning, which is based just outside Chicago in Skokie, Ill.

“When it was 80 degrees, problems are masked and the system was able to keep up,” Mr. Shavitz said. “But when you’re really putting it under full strain and full load, now the system can’t keep up.”

Mr. Shavitz said he expected this week to be one of his company’s five busiest of the year, with calls for emergency repairs layered on top of visits for routine maintenance. Technicians might be asked to take on extra work, and those who were on-call for the day knew they would be working.

There are rewards that come with doing that job on a hot day. On Monday, Mr. Pomerantz said, he visited a home where the air-conditioner had failed and it was 84 degrees inside. He managed to get it up and running again, just before the worst of the heat wave.

“Clients are always really, really thankful” in moments like those, he said.

Fixing up faulty air-conditioners often means working outdoors in the very heat everyone is trying to escape. Mr. Pomerantz said he focuses on staying hydrated, often sneaking a midday Gatorade. Mr. Shavitz said many of his employees wear wide-brim hats, lather on the sunscreen and sometimes even erect pop-up tents if they have a long day outdoors.

“They are working in some pretty tough conditions, whether it’s up an attic or baking on the roof,” Mr. Shavitz said. “Those guys, they’re worn out, they’re pretty exhausted by the end of the day.”

Mr. Pomerantz was not complaining. Yes, his job as a maintenance tech involves long stretches in the heat. But he also gets to spend time indoors, examining how A.C. units are functioning inside a home.

It is a luxury he did not have in his prior career. He was a roofer.

“I used to be in the heat all day,” Mr. Pomerantz said.