Heat-related emergency room visits spiked this week in regions of the United States that had been hit the hardest by the heat wave, according to data from the Centers for Disease Control and Prevention.

Across swaths of New England, the Midwest and the Mid-Atlantic States, there were “extremely high” rates of heat-related illnesses this week, the C.D.C.’s heat and health tracker showed. The tracker only had data available through Thursday.

The data used emergency room visits associated with the heat to determine the rise in heat-related illnesses, showing which areas had visits that exceeded the 95th percentile of what is typical there. The numbers were based on a scale of per 100,000 visits.

In New England, the number of visits climbed from 57 per 100,000 on Monday, the first day of the heat wave, to 833 on Thursday — the highest rate in the country all week. The region is less acclimated to high temperatures this time of year, and places like Boston and Hartford, Conn., saw record temperatures of 98 and 97. Caribou, Maine, reached 96 degrees this week, tying the highest temperature ever recorded there.

Much of the Midwest also had higher-than-usual heat-related emergency room visits, reaching a peak of 628 visits on Wednesday. Chicago hit a record high of 97 degrees on Monday.

Areas around New York and New Jersey also saw a surge on Tuesday, Wednesday and Thursday, going up to 459 heat-related emergency room visits. New York City reached temperatures it hadn’t seen in almost two years, going up to 94 degrees in Central Park on Friday.

Deaths resulting from extreme heat have been on the rise in the country in the past few years. The C.D.C. recorded about 2,300 heat-related deaths in 2023, up from approximately 1,700 deaths in 2022 and about 1,600 in 2021.

Global warming has been making heat waves hotter, more frequent and longer lasting. And the longer a heat wave, the more health risks people face as each additional day of extreme heat further strains the body.

Temperatures in New England fell on Friday, but parts of the Midwest and Mid-Atlantic are expected to continue to bake through the weekend.