Temperatures throughout the Midwest and Plains exceeded 100 degrees. The heat index, a measure of temperature plus humidity, climbed as high as 120 in some places. The National Weather Service issued heat warnings for such cities as Chicago, Cincinnati, Dayton, Ohio, and Tulsa, Okla.
Scientists say global warming is to blame for the scorching heat but there's a twist: the warming may have come sooner — had it not been for pollution, reports CBS News correspondent Bob Orr. Before clean air rules, coal-burning factories and cars using leaded gasoline produced dense blankets of smog that actually repelled some of the sun's harmful rays.
"It shielded us from some of the sunlight that was coming in," climatologist Jay Gulledge tells Orr. "And that reduced the amount by which the earth was warming at the time."
Cheryl Harriston struggled to stay comfortable as she handed out fliers supporting an increase in the minimum wage at an intersection in Columbus, Ohio.
"I have my water, my hat, and I stand in the shade a lot," Harriston said. "And, when I feel that cool breeze, I really take a minute to appreciate it."
The Midwest could get some relief by Wednesday, but the worst of the heat was expected to drift into the Northeast on Tuesday, bringing scorching temperatures to New York, Washington and Boston.
NFL teams closely monitored players for signs of heat-related illness. The heat prompted the Chicago Bears to cancel morning practice at training camp in Bourbonnais, Ill. On Sunday, the Tennessee Titans let defensive tackle Albert Haynesworth leave practice early with dizziness.
Chicago resident Tony Tesfay, 43, left his basement room at a halfway house first thing Monday and rode his bicycle to one of the city's cooling centers — air-conditioned recreation centers and other buildings that were opened to the public to prevent a repeat of 1995, when a heat wave killed 700 people in Chicago.
"I was pedaling slow, not too hard, so I could keep hydrated," he said. "It took me about 15 minutes. It wasn't too bad."
In California, the sweltering heat that punished the state for two weeks subsided, but the number of confirmed or suspected heat-related deaths climbed to 164 as county coroners worked through a backlog of cases.
Cities across the Midwest urged neighbors to check on the elderly and disabled. Utilities expected to set records for power usage and asked customers to conserve electricity to prevent blackouts.
In Chicago, officials made available a special telephone line to request checks on vulnerable neighbors and friends. The Department of Human Services and police responded to nearly 50 such requests by early Monday. The city's Department of Aging also telephoned more than 300 senior citizens to offer help, such as rides to cooling centers.
The Cook County medical examiner's office reported two heat-related deaths Monday. Both victims were men in their 50s or 60s with heart disease. In Oklahoma, authorities reported two more deaths that happened over the weekend.
In Wisconsin, sheriff's deputies put a high priority on responding to calls about disabled vehicles. "When it's 100 degrees and you've got kids in the car, that's not good," said Waukesha County Sheriff's Lt. Thom Moerman.
Burlington County, N.J., offered free fans to poor people and the elderly.
The weather posed special risks for people with outdoor jobs, such as construction workers and delivery drivers.
Jerry Wall, who collects coins from parking meters in Tulsa, Okla., said he tries to work in the shade of buildings whenever possible. But "there's no good way to do it on days like today," he said.
In the Minneapolis suburb of Bloomington, Minn., a youth swim team cut practice short because the water temperature rose to 80 degrees, about 10 degrees above normal. "When the water is a lot hotter, you get more fatigued a lot easier," said Jenny Bussey, 17.
Cleveland's mayor said city recreation centers would be open Monday and Tuesday to provide relief from the heat. The mayor of Akron, Ohio, opened four cooling centers.
"So many of us live and work in air-conditioned environments, we may not realize how dangerous this oppressive heat truly is for those who do not," Mayor Donald Plusquellic said.
By the evening rush hour, authorities closed a bridge spanning the Cuyahoga River in Cleveland because the heat had caused the steel to expand and prevented parts from fitting together properly.
In Nebraska, high temperatures, a drought and strong winds combined to feed enormous wildfires near the Panhandle town of Harrison.
Chicagoan Danita Winfield, who does not have air conditioning in her apartment, planned to visit the city's 24-hour cooling station Monday night.
In the meantime, she said, "I sit out in the front of the complex for a while until I really get tired, then I go in the house and make a pallet on the floor because we do have a little breeze that comes through the window."
In Terre Haute, Ind., the heat was a concern for many players attending the season's first Indianapolis Colts practice. Two-time MVP Peyton Manning said players have taken the danger seriously ever since the death of Vikings tackle Korey Stringer, who collapsed five years ago from heat exhaustion in training camp.
"So guys try to be smart about it," Manning said.