A brutal heat wave will continue to swelter from the Midwest to Northeast on Wednesday, set to break more record highs as millions take to the outdoors to mark the Juneteenth holiday.

Eleven cities either tied or hit record highs on Tuesday and the extreme heat triggered air quality alerts in Connecticut, Massachusetts, New Jersey, New York, New Hampshire and Ohio.

On Wednesday, 74 million people are under heat alerts, with the majority impacting Michigan to Maine, as temperatures 10 to 25 degrees above average soar into the upper 90s to low 100s.

Hartford, Connecticut; Cleveland, Ohio; Detroit, Michigan; and Pittsburgh, Pennsylvania; could set records for high temperatures.

Heat index readings are expected to “peak from 100 to 105 degrees in many locations,” the National Weather Service said.

Several parts of the Midwest, the interior Northeast and New England could reach temperatures of 95 degrees Fahrenheit or above Wednesday, with Bangor, Maine, set to hit 106 on the heat index, a measure of how hot it feels that factors in humidity.

Dozens of more record highs are forecast to be broken over the next few days.

“One example: In Cleveland, OH (CLEVELAND/HOPKINS), the forecast high on Wednesday is 92°F. On average, the hottest day of the year there is July 14, when the normal high is 84°F,” the National Weather Service wrote on X.

Locals without access to reliable air conditioning are urged to find a a place to cool down as “record warm overnight temperatures will prevent natural cooling and allow the heat danger to build over time indoors without air conditioning,” the NWS said.

NBC News meteorologist Michelle Grossman warned early Wednesday that extreme heat is not to be taken lightly.

“It is dangerous — extreme heat is the No. 1 weather-related killer, so you need to take it seriously,” she said.

People in Syracuse, New York, were warned Tuesday not to open fire hydrants to get some relief from the heat. Fire Chief Michael Monds said in a statement that opening hydrants can affect firefighters’ ability to tackle fires and to do so would bring a civil penalty that comes with a potential $500 fine.

Conditions are expected to improve over New England this weekend.

Heat will also impact the desert Southwest where an excessive heat warning continues for Phoenix and Tucson.

Extreme heat has already been baking the Southwest for the past few weeks. On Friday afternoon, a 44-year-old mother from Pennsylvania passed out on the Hiline trail in Sedona, Arizona, and died. Her two young daughters and husband said she appeared to suffer heat exhaustion, the Yavapai County Sheriff’s Office said, noting no foul play is suspected.

Also in the Southwest, firefighters have been battling wildfires that sparked amid dry, hot conditions.

In New Mexico, at least one person has died and 1,400 structures have been lost in the South Fork Fire and Salt Fire — blazes that combined are now burning over 20,000 acres — prompting evacuations for thousands of people. As of Wednesday morning, those fires are 0% contained.

Meanwhile in California, the Post Fire is burning over 15,000 acres in northern Los Angeles County and Ventura County, and is 39% contained, according to Cal Fire. 

This article was originally published on NBCNews.com