Nearly 60 million people were sweltering under heat alerts on Wednesday as a late-summer surge of extreme temperatures blanketed much of the eastern half of the United States.

Record heat was expected to impact the mid-Atlantic states with temperature highs of around 100 degrees Fahrenheit (37.8 Celsius) in and around the capital Washington, the National Weather Service said, though the heat wave was expected to be short-lived as a cooler and damper front moves in from Canada.

"However, much of the Ohio and Tennessee Valleys will feel a couple more days of high temperatures topping into the upper 90s at the hottest locations," added NWS.

In Baltimore, the mayor's office issued a "code red extreme heat alert" and posted on X the locations of five cooling centers opened to support the city's homeless population. Authorities were also distributing cold water to those without shelter.

The Centers for Disease Control and Prevention advised people who are outdoors in regions where the heat threat is considered "major" to stay in the shade, take breaks, and confine their activities to the coolest parts of the day or evening.

It was also important to carry a bottle of water, limit intake of caffeine, and monitor urine color for signs of dehydration, the agency advised.

Climate change has been extending the heating season across much of the United States, increasing both the frequency and intensity of heat waves.

Heat is the leading weather-related killer in the US, even though heat-related illnesses and deaths are largely preventable through outreach and intervention, according to the Environmental Protection Agency.

A report published this week in the Journal of the American Medical Association found heat-related mortality rates in the United States increased between 1999 and 2023, especially during the last seven years.

But the true number of heat-related deaths is probably still being underestimated, the authors wrote, due to potential misclassification of causes of death and a lack of data on vulnerable populations.