A heat wave blamed for as many as two dozen deaths settled over portions of the East Coast as the National Weather Service issued an extreme heat warning for much of the Mid-Atlantic, saying Friday would be the hottest day in the region.

The highest heat index values — how hot it feels — could reach 115 degrees in some locations, the weather service said.

“These triple-digit temperatures are forecast to remain in place across the eastern U.S. through Saturday,” it said.

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The weather was expected to cool off slightly to the mid-90s by Sunday, according to the National Weather Service.

The warnings came as the heat wave broke 55 record highs and tied 60 more records in portions of the Midwest and the Ohio Valley, forecasters said.

Millions were also being warned to avoid strenuous activity and exercise outdoors after the National Weather Service issued a number of code red air quality alerts — meaning air pollution levels are considered unhealthy for the general population — in a handful of cities, including Baltimore and Washington D.C.

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The heat has affected many outdoor activities, prompting one Minneapolis movie theater marquee to read, “We have AC. Who cares what’s playing,” and one Canadian couple to forgo their original plans while visiting the Twin Cities.

Scott Hoffort and his wife, Colleen, of Saskatchewan, Canada, arrived in Minneapolis Sunday with their camping gear, planning for a vacation spent mostly outdoors at a campground. Instead they’re now doing “more touristy” things.

“We went from camping in a tent to staying in hotels, so that we could get air conditioning,” said Hoffort said. “We spent a fair amount of time in the Mall of America yesterday because of the heat.”

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About 100 people, primarily teenagers, were overcome Thursday by the heat at the Vans Warped Tour in Camden, New Jersey, where more than 12,000 people gathered at the outdoor concert, authorities said.

“I just, I guess got overheated and I got really dizzy,” 17-year-old Maureen Meckly, who attended the concert, told CNN affiliate KYW. “I had to grab onto her (friends’) shoulder to tell her I was passing out.”

In Wildwood, New Jersey, tourists and residents beat the heat by heading to the ocean.

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But for some working hot food concession stands at Morey’s Piers in Wildwood there was little relief.

“Sometimes it’s so hot, I can’t even breathe,” Erik Perez, who works at Curley’s Fries, told KYW.

In Newark, New Jersey, Mayor Cory Booker announced that facilities were available in every ward to help keep citizens cool.

“It is imperative that our residents drink plenty of fluids, stay indoors in a cool climate, and avoid strenuous activity,” Booker said in a statement. “I urge our senior citizens and Newark residents who lack air conditioning to come to one of our cooling centers.”

In New York, Con Edison called on customers to stay cool and not waste energy.

“Store owners who leave doors open with the A/C running could be subject to fines from the city,” ConEd said.

The first confirmed heat-related death in Kansas City was declared Thursday, but it stemmed from a death last month. A 57-year-old man was found dead in his home on June 5, according to Dan Ferguson, who works for Jackson County.

In addition to the man’s death, there are 13 other possible heat-related deaths in Missouri — the youngest was a woman in her mid-30s, and the oldest were two women in their mid-70s, said Jeff Hershberger, spokesman for the Kansas City Health Department.

It may take six weeks to several months for officials to process toxicology tests to determine whether all 13 died of heat-related causes, Hershberger said.

In Wisconsin, a 65-year-old man from Fountain Prairie was confirmed by health officials to have died from heat-related causes Thursday, according to CNN affiliate WKOW. Columbia County Medical Examiner Angela Hinze said the man had underlying medical conditions that were made worse when he was helping a family member outside with housework.

In Oklahoma, four heat-related deaths have been confirmed since May, said Cherokee Ballard, a spokeswoman for the state medical examiner. Three of those occurred in the past 30 days, including a 3-year-old boy in a car in Norman and a 69-year-old man from Blackwell, she said.

An additional eight Oklahoma deaths may be related to the heat, she said, with most occurring in July.

CNN affiliate WPXI reported Thursday that a child accidentally locked in a hot car was rescued in Spring Hill, Pennsylvania. The girl’s mother locked her keys in the car and immediately called for help, the report said. The girl was not hurt.

The high heat is also taking its toll on animals.

In South Dakota, 1,500 head of livestock have been lost to the heat, Larry Olsen of the Farm Service Agency told CNN affiliate KSFY.

Dozens of cattle died south of Harmony, Minnesota, CNN affiliate KTTC reported. And on a cattle farm just across the border in northern Iowa, an estimated 100 cattle died, the report said.

At the Brookfield Zoo just outside Chicago, staff kept water misters in exhibits and gave some animals huge blocks of ice filled with meat and fruit.