NEW YORK (AP) — Tens of millions of people in the northeastern United States sweated in dangerously high heat Wednesday, with temperatures that felt like 100-plus degrees and prompted emergency measures including school closures and extra breaks for star players wilting at the U.S. Open tennis tournament.

Authorities on Long Island say heat appeared to be a factor in Tuesday’s death of an 11-year-old girl found in a vehicle with the windows closed. The heat index was past 100.

In Massachusetts, nearly two dozen school districts canceled classes or sent children home early Wednesday in response to heat that reached 98 degrees, breaking the Aug. 29, 1953, record of 96. With the added high humidity, it felt like 107 degrees, according to the National Weather Service.

“Our schools were simply not designed for the sustained heat and humidity we are experiencing since many do not have air conditioning or sufficient cooling systems,” said Mike Morris, superintendent of the Amherst-Pelham Regional School District, in a message to parents.

School districts in Pennsylvania, New York, Connecticut, New Jersey and Virginia also had early dismissals Tuesday and Wednesday. Philadelphia already announced a Thursday early dismissal.

At Bath Iron Works in Maine, the scorching temperatures didn’t keep shipyard workers from toiling away inside a 610-foot destroyer, the future USS Lyndon B. Johnson, that’s under construction.

With the steel soaking up the heat and no air conditioning, temperatures climbed inside.

“It’s hot, sweaty, sticky, damp,” said welder Neal Larsen. He said the temperature routinely gets above 100 inside on hot days.

In 12 Northeast states tracked by the weather service, the elderly without air conditioning were urged to go to cooling centers.

The Maine Department of Environmental Protections says ground-level ozone concentrations would reach unhealthy levels along the coast Wednesday and are asking residents to avoid outdoor activity.

At the U.S. Open in Queens, an extreme heat policy kicked in, allowing for 10-minute breaks between sets from the heat-absorbing courts.

“Hydrate, hydrate, hydrate,” said spectator Susan Clarke, of Larchmont, New York.

“Water, water, water — cold water in the face,” chimed in her sister, Pam Fitzgerald, of Stamford, Connecticut.

She also suggested sitting in the shade, “but everybody else is sitting in the shade so we haven’t had any luck there.”

Weather service meteorologist Bob Oravec said either heat advisories or excessive heat warnings have been issued across the region; the latter applies to temperatures that feel like over 105, while the advisories apply to lower figures.

In either case, he said, “the effects on the human body are the same: It’s tough to be outside.”

Temperatures are expected to ease into the 80s Thursday.