There was no letup from the sweltering heat wave frazzling much of the eastern United States on Friday, with tens of millions of Americans under heat warnings.

Record-setting temperatures baked much of New England and parts of the Midwest on Thursday, with 65 million people under heat alerts. Heat indexes — which factor in temperature and humidity — hit between 100 and 110 degrees in some places. 

Parts of the Northeast were set to get some relief Friday, with highs in the 70s and 80s, according to the National Weather Service. But the temperature is forecast to rise in the mid-Atlantic, through the mid 90s Friday and perhaps into the 100s Saturday, “with record-tying/breaking temperatures possible,” the forecaster said.

Across the Ohio Valley, the NWS “Heat Risk” index was at level 4 — labeled “extreme,” the highest available — for the next two days.

“This rare and/or long-duration extreme heat with little to no overnight relief affects anyone without effective cooling and/or adequate hydration,” it said, adding that it was likely to impact “most health systems, heat-sensitive industries and infrastructure.”

In another update Thursday, the forecaster advised people to “drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors.”

Elsewhere, Gov. Greg Abbott issued a disaster declaration for 51 Texas counties after the first named storm of the hurricane season, Alberto, made landfall in Mexico bringing heavy rain.

What makes this heatwave so punishing, meteorologists say, is that nighttime lows are also stubbornly high, giving the body little time to recover from the day’s punishing conditions. 

The northeast in particular isn’t used to temperatures this hot at this time of year — with several calendar-day highs broken across Maine, New Hampshire and Pennsylvania — and the NWS warned that people without reliable air conditioning would wilt.

Commuters in the Thursday evening rush hour in and out of New York Penn Station were left sweating on trains with no AC after a malfunctioning circuit breaker caused a power outage, compounded by a brush fire in Secaucus, New Jersey, WNBC reported.

Amtrak warned that the high temperatures meant its trains would have to run at slower speeds throughout the week, predicting delays of up to an hour.

The immediate cause of this heat wave is the jet stream meandering northward and creating what’s called a “heat dome” over the eastern U.S.

But Americans are far from the only ones suffering in such conditions — with a monthslong heat wave in India killing more than 100 people and hundreds dying during the annual Muslim pilgrimage to Mecca amid scorching heat there.