The heat wave that has scorched much of the American South and Southwest is now spreading through the Midwest, bringing temperatures above 100 degrees, dangerous conditions for millions of people and pleas from state and local officials to avoid spending time outdoors.
The extreme heat and humidity was expected to spread misery across the region for several days, meteorologists said, warning that there was also a risk of tornadoes in Indiana and Michigan. In cities like St. Louis; Wichita, Kan.; and Kansas City, Mo., temperatures could be 10 to 20 degrees above normal, and heat index readings, which consider both temperature and humidity, will reach into the 100s.
And the blistering weather arrived just as another health menace swept in: Canadian wildfire smoke that has once again fouled the air over parts of the Midwest.
Public health authorities in Detroit encouraged residents to go to libraries and recreation centers to avoid the double whammy of high heat and unhealthy air.
Experiencing both skyrocketing heat and humidity and the smoky air from wildfires at the same time is not something that people in the middle of the United States are accustomed to, said Christina Floyd, the acting chief public health officer in Detroit.
“That’s not normal in this region,” Ms. Floyd said. “The norm in the summer is high heat and humidity. But when you add that particulate air matter, that’s the unique situation. Most people are just not equipped to be in that kind of environment.”
The planet has warmed about 2 degrees Fahrenheit since the 19th century and will continue to grow hotter until humans essentially stop burning oil, gas and coal, scientists say. The warmer overall temperatures contribute to extreme-weather events and help make periods of extreme heat more frequent, longer and more intense.
In Detroit, Ms. Floyd said that she was especially concerned about older people and children with asthma, hypertension or any respiratory condition.
The heat wave has hit especially hard in parts of Kansas and Missouri, where temperatures reached 100 degrees on Wednesday.
Capt. Ray Mattas, a spokesman for the Emporia Police Department in eastern Kansas, said that the lobby at police headquarters was open to anyone who needed a place to cool off for an hour or two. Pets were welcome, too, he said.
The air in Emporia felt as though it was 104 degrees on Tuesday, according to the heat index, he said. Wednesday has been nearly the same.
“You walk out of the air-conditioned building and — you ever open an oven door really fast?” he said. “That’s what it feels like.”
Police officers in Emporia were being given breaks to hydrate if they were working outdoors for long periods, in situations like the scene of a car crash, where heat can radiate painfully off the pavement.
Little relief was expected in parts of Kansas this week, even in the middle of the night, officials said, with overnight low temperatures in the 80s.
“It can be a pretty serious deal,” said Brandon Drake, a meteorologist for the National Weather Service in Topeka, Kan. “The stress on your body builds up if you’re never getting away from the heat, if you don’t have air-conditioning. Then you’re still hot, even during the overnight period.”
Forecasters said that the high heat and humidity would continue throughout the week but would probably shift to the Northeast and Mid-Atlantic region by the weekend. Temperatures on the East Coast could reach into the upper 90s on Friday.
In some Midwestern states, even the height of the corn crop can make the discomfort worse for residents: The tall, mature plants of late July release excess moisture into the air in a process called evapotranspiration — or more colloquially, corn sweat.
In Milwaukee County, residents were encouraged to visit public pools and splash pads, or one of the county’s eight beaches along Lake Michigan.
Cook County, Ill., with more than five million residents, announced the opening of additional cooling centers beginning on Wednesday, when heat indexes were expected to rise to 105 degrees. Camp programs and activities across the state were forced to move indoors: In Lisle, Ill., a suburb of Chicago, a softball camp was relocated to a gym because of the poor air quality outdoors.
The conditions posed a threat to livestock and pets as well. During a heat wave last summer, thousands of cattle in Kansas died from exposure to high temperatures, resulting in gruesome scenes of carcasses lined up along the edges of fields.
A.J. Tarpoff, a veterinarian and associate professor at Kansas State University, said that farmers were far better prepared to face heat-related threats this summer. than they were last year. They even have a new tool that researchers designed to help them monitor animal comfort, using weather forecasts and other data.
For weeks, farmers have been ready with strategies to help animals survive the heat, like using larger water tanks in the fields and moving animals to different locations when necessary. It is also helpful, Mr. Tarpoff said, to feed cattle in the slightly cooler evenings rather than during the day, since the animals produce heat when they digest food.
“Last year was a very specific event that was the worst-case scenario,” he said. “That is not unfolding this year.”
