DETROIT (AP) — Nature’s oven was on high Tuesday for millions of people in the Midwest and Great Lakes states as intense heat and humidity baked the regions with no immediate relief before the misery shifts to the eastern U.S.

The National Weather Service was blunt: Conditions were “dangerous” as the heat index, a combination of air temperature and humidity, exceeded 100 degrees Fahrenheit (37.8 degrees Celsius) in some areas. It warned about a risk for heat-related illnesses, especially among people without air conditioning.

Detroit’s air temperature was in the high 90s, the Weather Service said, and could even reach 100 at some point through Thursday. The city said a dozen recreation centers were open, some until 11 p.m., for people to cool off. Big chunks of Michigan, as well as Illinois, Ohio, Indiana, Kentucky and much of Iowa, were under an extreme heat warning.

The Northeast, including New York City and Boston, will next feel major heat through the Fourth of July holiday. Norristown, Pennsylvania, 20 miles (32 kilometers) from Philadelphia, canceled a Saturday parade because of the weather.

Philadelphia declared a heat emergency, Wednesday through Saturday, and said 50 cooling centers will operate with extended hours. The city said visitors will find misting tents, water refill stations and medical stations at the free World Cup fan festival at East Fairmount Park.

When the heat’s on, people adjust

The Chicago History Museum offered free admission to state residents who wanted a cool space Tuesday. Roads in a few places in Illinois buckled under the heat. When the surface has no room to expand in the heat, it can rise and crack.

At 9:15 a.m., window washer Stephen Mason, 72, was wiping mayflies off glass at a Detroit convenience store. He got an early start to avoid the worst conditions of the day, but it was already 85 (29.4 C).

“It’s the only way to beat it. But it’s already starting to cook out here,” Mason said.

Adam Schubatis, 36, a runner who was shirtless in Detroit’s Indian Village neighborhood, said he was cutting his route to 6 miles (9.6 kilometers).

“I know where all the drinking fountains are,” he said. “My wife thought I was crazy. She offered to pick me up if I got tired or if there was anything I was doing that wasn’t safe.”

A dog’s day can be ‘miserable’

In Milwaukee, more than 100 firefighters spent hours controlling a fire at a school in the extreme heat. Fire Chief Aaron Lipski said crews were rotating and shedding their thick coats while they rehydrated.

Overnight low temperatures were in the 70s (21-26.7 C) in many places, which doesn’t give the body a chance to cool down naturally, said Dr. Kisha Davis, health officer for Montgomery County, Maryland.

“The heat stress will compound day after day,” said Davis, who added that alcohol and soda, popular holiday drinks, are dehydrating.

In Jackson, Mississippi, where the afternoon heat index exceeded 100 F (37.7 C), massive fans tried to keep about 100 dogs cool at an old mattress warehouse that serves as a shelter for Community Animal Rescue and Adoption.

“It’s miserable. … We try to keep everybody hydrated back there. Dogs and people,” said kennel manager Raquel Burney.

George Liller, 64, was a hero in Grosse Pointe Park, a Detroit suburb. He added air conditioning to his home, so he offered a window unit for free on Facebook — extension cord and remote control included.

“It was probably on my porch maybe 15 minutes,” Liller said. “That air conditioner was given to me. I thought, ‘Somebody needs it.’ I know how it feels to be in an old house when it’s this hot.”