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“The heat index is how hot does it feel, and with more humidity the worse your body is at cooling itself down,” Mr. Strong said.

Police and fire departments sent out warnings to parents and pet owners reminding them to not leave children or animals in hot cars.

Even historic re-enactors at Colonial Williamsburg in Virginia were doing what they could to stay cool Monday, spokesman Jim Bradley said.

“In the warmer months, they tend to go to some of the lighter materials,” Mr. Bradley said of the historic costumes. “Even wool tends to be breathable because it’s a natural fabric. One of the advantages of natural fabric is it wicks perspiration away from you and actually helps you cool off.”

Mr. Bradley said employees go through a training course each spring on how to identify signs of heat stress in fellow employees and in guests. Visitors, he said, will be happy to know that many of the buildings on the property are also air conditioned to help preserve antiques housed inside.

“When you think about it, 200 years ago was way before air conditioning,” Mr. Bradley said. “People dealt with the heat. It was just as hot and frankly just as humid.”