CHICAGO, United States (AFP)— Smoke from Canada’s worst-ever wildfires was severely impacting air quality Wednesday across Ontario and at least 15 US states, with monitors warning that over one hundred million people face potentially unhealthy conditions.

Chicago, Detroit and Cleveland recorded some of the worst air quality in the United States, with residents told to stay inside or limit outdoor activity as smoke blanketed huge swaths of the country just weeks after communities suffered similar disruptions from Canada’s hundreds of active forest fires.

Canada’s most populous city Toronto was a 10 out of 10 — “high risk” — on Environment Canada’s air quality health index, while Swiss-based monitoring company IQAir pegged it as the worst air quality of any major city in the world.

Alerts were issued from Ontario to northern US states Minnesota and Michigan, across to New York and down to the southeastern states of North Carolina and Georgia, the latest dangerous air conditions in much of North America’s Great Lakes and Mid-Atlantic regions.

The air quality alerts come as much of the US South and Midwest bakes under a brutal heat wave that is affecting several million Americans, with the National Weather Service issuing a heat index forecast as high as 115 degrees Fahrenheit (46 Celsius) Wednesday in northern and central Texas.

The suburbs of Chicago, whose metropolitan area is home to more than nine million people, posted a “very unhealthy” air quality index, or AQI, of 285 mid-day Wednesday, according to AirNow.

US President Joe Biden’s Air Force One touched down in the Windy City Wednesday “through a thick layer of smoke and haze,” according to a White House pool report, ahead of an economic speech there.

He then flew by presidential helicopter on a short trip to the speech location, offering a bird’s-eye view of the dangerous air conditions.

“Air Quality in Chicago is still very unhealthy today. Please limit time outdoors,” Chicago’s emergency management office post on Twitter.

AirNow showed the Detroit area, with 4.3 million people, recording a “hazardous” AQI touching 306, before dropping off slightly.

An AQI of 301 or above reflects “emergency conditions” that are likely to affect everyone, according to the US Environmental Protection Agency.

The wildfires — the largest ever recorded in Canada — have raged for two months, darkening Canadian and US skies with smoke and haze that contain tiny dangerous particles considered especially harmful for people sensitive to pollution.

“Unhealthy levels of smoke are expected for a wide swath of the Midwest today, the US National Weather Service said.

“Wildfire smoke from Canada will reduce air quality over parts of the Upper/Middle Mississippi Valley, Great Lakes, Western Ohio Valley, Central Appalachians, and Mid-Atlantic, prompting Air Quality Warnings over the area.”

In New York City, where noxious haze three weeks ago disrupted flights and forced the cancellation of outdoor events, officials on Wednesday warned that air quality is expected to deteriorate again this week.

New York’s Metropolitan Transit Authority said it would offer free KN95 masks at its subway and train stops.

The state of Pennsylvania also declared a “Code Red” on air quality for Wednesday.

The wildfire smoke has drifted across the Atlantic Ocean and over European countries including Portugal and Spain.

But air quality there remained mostly fair Tuesday, “because most of the smoke that reached Europe was higher in the atmosphere, where it is less likely to affect human health,” according to US space agency NASA’s Earth Observatory.

The wildfire smoke was also bearing down once more on the US capital Washington, a situation that NASA scientist Ryan Stauffer, who studies air pollution and the ozone, called “absolutely brutal.”

Scientists say human-induced climate change is helping drive the increased rates of wildfires, heat waves and intense weather systems.