Some 141 million Americans over nearly 1 million square miles were under a heat alert on Wednesday, the result of a heat "dome" that's only slowly moving away from the central U.S. — and into the East Coast.
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The National Weather Service said 22 deaths in recent days were potentially heat related.
"This heat is dangerous on many levels," NWS Director Jack Hayes said in a statement. "Temperatures and humidity levels are high, the heat will be prolonged, and very warm temperatures overnight won’t provide any respite. All of these factors make this an unhealthy situation, especially those in the upper Midwest who are not accustomed to such heat."
Hospitals in Wichita, Kan., treated 25 heat-related illnesses, according to the Weather Service report. In Des Moines, Iowa, 16 people were hospitalized because of the heat.
In Minneapolis, dozens of fans at recent Minnesota Twin games have been treated for heat issues, even though the club did take extra precautions such as providing free water stations and having first aid and guest service staff on hand to monitor crowds.
Day after day of high temperatures and humidity with no relief overnight was taxing the region.
"It's just draining, physically draining," said Chris Vaccaro, a Weather Service spokesman.
At least 27 states were under some sort of heat warning, watch or advisory.Video: Heat 'dome' effect explained (on this page)
An atmospheric high pressure ridge hanging over the Midwest is blocking moisture, causing a buildup of heat that acts more like a dome than a heat wave. The ridge will weaken by the weekend, permitting cooler temperatures and some rainfall in the Midwest.
In Philadelphia on Wednesday temperatures were forecast to reach 93 degrees. Raleigh, N.C., could see 99 degrees, and it could be 95 degrees in Washington, D.C.
At 11:30 local time, it was already 91 degrees in the shade outside the Chicago Board of Trade Building in the Loop.
"It's hot today, but it's going to be hotter later in the week" on the East Coast, said Jim Keeney, a meteorologist with the Weather Service.
In Pittsburgh, some residents said they had plans to stay cool as temperatures rose.
Retired police records clerk MaryAnn Dean was heeding the experts' advice to stay indoors.
"I'm not one that likes the real hot humidity," said Dean, 59. "I'm just staying in the air conditioning."
Meredith Brown planned to head to the pool with her dog to escape the heat.
"My dog is kind of going a little crazy with it," said Brown, 29, as she walked her pooch. "It's a little hard for him."Video: Heat makes subway 'hell on earth' (on this page)
Cities take precautions
With temperatures expected to approach 90 degrees over the next several days, Boston Mayor Thomas Menino reportedly issued a heat advisory and warned residents to take it easy.
"The hot weather has now become steamy weather as high temperatures and humidity arrive this week," Menino said in a statement. "In these conditions, we must remember to stay cool and hydrated and continue to keep an eye on our neighbors."
Additionally, the city will open cooling stations at community centers and extended pool hours to 9 p.m., the statement said.
In New York City, where the humidity is building, temperatures are expected to near 90 degrees on Wednesday. The city's Office of Emergency Management is offering residents the ability to search for a cooling center near their location.
In the central U.S., two men appeared to have died while tending to yards. The core body temperature of a 65-year-old man who died on Monday while mowing his lawn near Wichita, Kan., was 107 degrees, Butler County Sheriff Kelly Herzet said.
"It's tragic," Herzet said. "People need to stay in when it's this hot, or drink plenty of liquids. They need to know their limits."
It was a similar story about 65 miles away in Blackwell, Okla., where a 70-year-old man last seen walking down a street pushing a lawnmower was found unresponsive.
He died at the hospital. His body temperature was at least 108 degrees, according to police.
"This is a guy you would see out all the time in all different weather," said Blackwell Fire Chief Tom Beliel. "It's just unfortunate."
High temperatures were also responsible for an alarming spike in deaths of illegal immigrants trying to cross into the United States, according to the U.S. Border Patrol. It did not say, however, exactly how many had died due to weather.
"Cases of dehydration and heat exhaustion are way up," said supervisory agent Dan Milian, with the Rio Grande Valley sector in Edinburg, Texas.
As a result, the patrol deployed search and rescue units in the south Texas brush country.
The heat also set new peak records for electricity usage.Slideshow: Major Heatwave (on this page)
Xcel Energy, which serves 1.64 million customers in North Dakota, South Dakota, Wisconsin and Minnesota, broke a demand record on Monday with 9,504 megawatts of power used, said Tom Hoen, a company spokesman. The old record set in August 2010 was 9,100 megawatts.
In Omaha, Neb., flood control work along the overflowing Missouri River was halted due to the heat, as officials worried that filling sandbags was too strenuous.
In Illinois, the second-largest corn and soybean producing state, the heat and humidity were not yet damaging crops, according to University of Illinois agronomist Emerson Nafziger. But a lack of rain was cause for concern.
"Corn is holding on so far," Nafziger said. "We're starting to get a little worried right now from a water standpoint."
Msnbc.com staff and Reuters contributed to this report.