Urban skylines vanished in an eye-watering haze. Parks and ball fields that normally fill with dog walkers and joggers were eerily quiet. Some beaches and pools closed. Cars slowed down, as visibility dwindled in the gloomy light of a dull orange sun.

A blanket of dense smoke from Canadian wildfires stretched across Ontario into the Midwestern and northeastern United States on Thursday, disrupting life for millions and prompting many to stay inside with the windows shut.

Others tried to limit the amount of time they spent outside, as air quality readings surged to dangerous levels in Chicago, Cleveland, Detroit, Minneapolis, Toronto and many places in between. The air in northeastern cities like Washington, Philadelphia and New York rated only slightly better but was still unhealthy and gray, with the acrid scent of a campfire.

In Chicago, Brandon Olsen, a 35-year-old digital advertising worker, took his dog, Juni, out for a short stroll in Portage Park on Thursday morning but said he planned to skip their evening walk.

“I walked outside and could feel it in my eyes,” he said. “I was not expecting that. It has a metallic taste and probably isn’t too good of an idea to be out here long.”

In New York City, Robert Vargas felt unsettled as he took his 9-year-son, Aiden, to Soundview Park in the Bronx on Thursday afternoon.

“I’m worried it could mess up his lungs,” Mr. Vargas, 47, a retail store manager, said as his son kicked along on a scooter. “Firefighters on Sept. 11 inhaled all that smoke and they started getting sick down the line.”

The bad air was the product of about 800 wildfires burning across Canada, particularly the roughly 180 that have been burning in Ontario, where the fires have prompted air quality warnings, road closures and some evacuations. More than 150 crews and nearly 50 firefighting aircraft were “working around the clock” to extinguish the fires, the premier of Ontario, Doug Ford, said on Thursday.

The smoke spread widely into the United States this week as the same high-pressure system known as a heat dome, which led to brutal heat in the Midwest and the Northeast, trapped the smoke and pushed it close to the ground. By the weekend, as another weather system pushes the hottest air away, the heat and the smoke are expected to ease in the Northeast, allowing for cleaner air during the World Cup final in New Jersey on Sunday. But in the Upper Midwest, closer to the fires, the smoke and the heat are likely to linger through the weekend.

Summer wildfire smoke — common in parts of the Western United States — may become part of the new normal in the Northeast and Midwest. As climate change drives global temperatures past record levels, it is making severe fire weather more common, drying out plants and soil and raising the chances of longer, more severe forest fire seasons, researchers have found. A similar cloud of smoke from Canadian wildfires hung over New York City in June 2023, delaying flights and prompting the cancellation of three big theater productions and two Major League Baseball games.

On Thursday, the Air Quality Index — which measures the density of five pollutants: ground-level ozone, particulates, carbon monoxide, nitrogen dioxide and sulfur dioxide — reached eye-popping levels in the Upper Midwest. An A.Q.I. of 50 or below represents “good” air quality; anything over 150 is considered “unhealthy,” and over 300 is “hazardous.” In the Western Upper Peninsula of Michigan, the A.Q.I. hit 785. In Toledo, Ohio, it was 775.

In Milwaukee, where the A.Q.I. was 460, Dave Sell drove to Bradford Beach, on Lake Michigan, with his dog, Yo Yo. The beach was mostly empty, with only about 15 people there, rather than the thousands who normally crowd it on a typical July day. The lake was clouded in a suffocating haze.

“You can’t even see the blue water,” Mr. Sell said. “It looks dingy and gray and brown, and the smell is gross.”

Wildfire smoke can exacerbate the symptoms of respiratory conditions like asthma. People with other chronic conditions, like diabetes and heart disease, also have a higher risk of getting sick, according to the Centers for Disease Control and Prevention. Children and pregnant women may also be at risk, according to the agency.

But anyone can feel the effects of wildfire smoke. Inhaling it can immediately cause coughing and wheezing, a headache, stinging eyes, a scratchy throat, sinus irritation, chest pain, a rapid heartbeat and fatigue.

Officials in many cities and states urged people to stay inside, if possible. Gov. Kathy Hochul of New York said it would provide more than 100,000 masks, some of which would be distributed at transit hubs, including Penn Station and Grand Central Terminal in Manhattan. New York City, where the A.Q.I. topped 150, made free masks available at some public libraries, which have also served as cooling centers this week.

Chicago closed its beaches and outdoor pools on Thursday, and a Major League Soccer game at Soldier Field was rescheduled for October. An outdoor concert by the rock band Creed near Minneapolis was rescheduled for September. Lakewood, Ohio, on Lake Erie, closed public pools, citing the unhealthy air quality.

“It’s terrible out here today, and I can tell it’s a little harder to breathe,” said Terry Pieh, 71, who took a morning swim in Lakewood before the pools closed. “But I had a little fun with it. The lifeguards take music requests for us when we do our morning swim, and I asked for ‘Smoke on the Water.’”

The United States and Canada have had a tense relationship since the start of President Trump’s trade wars, and four Republican House members from Michigan sent a letter to Prime Minister Mark Carney of Canada this week accusing the country of mismanaging the wildfires.

“Michiganders shouldn’t have to breathe Canada’s negligence,” one of them, Lisa McClain, said in a statement. Senator Bernie Moreno, Republican of Ohio, said he would introduce a bill next week to penalize Canada “for this atrocity.”

Mr. Carney, asked at a news conference in Ontario to respond to the criticism, said: “There must be a contribution from the United States in the fight against climate change.”

For others, the smoke was a reminder that American cities, for all their creature comforts, are not immune to the dangerously poor air quality that has become an inescapable part of life in other parts of the world.

As Raj Advani, 44, a software engineer who was walking his dog in New York City, put it: “This is how it is in Delhi every day.”

Reporting was contributed by Dani Blum, Dan Simmons, Nate Schweber, Robert Chiarito, Vjosa Isai, Ephrat Livni, Robert Jimison, Christina Caron and Sally Goldenberg.