An air quality index of 600 is a number most Americans never see. On Thursday, 16 July 2026, Detroit hit it. IQAir, the monitoring firm, ranked the city’s air the worst of any city in the world that day, and Reuters noted that the reading was double the level the U.S. Environmental Protection Agency calls hazardous.
The cause was smoke, and it came from a long way north. Most of what drifted over the Midwest and Northeast started in Ontario and other central Canadian provinces. Government data showed 858 wildfires burning across Canada that morning, 111 of them out of control, mostly in Manitoba, Saskatchewan and Ontario. About 5.9 million acres, roughly 2.4 million hectares, had burned so far this season. Ontario’s government asked Ottawa to prepare help evacuating remote northern communities, possibly with Canadian troops, Bloomberg reported. Ontario Provincial Police said 15 communities and their surrounding areas had already been cleared.
A lid on the Great Lakes
A stalled high-pressure system pinned the smoke close to the ground. Steven Freitag, a National Weather Service meteorologist in Detroit, said the high pressure held the haze low, and visibility in places dropped to half a mile.
Detroit was not alone. Chicago’s air quality index topped 300 on Thursday, the point at which the EPA rates air hazardous for everyone, healthy or not. In northeastern Minnesota, the Iron Range towns of Duluth and Hibbing recorded fine-particle concentrations of up to 900 micrograms per cubic metre, triple the hazardous threshold, according to the state’s Pollution Control Agency. Officials from Minnesota to New Jersey told people to stay indoors and handed out KN95 masks. Organizers cancelled or postponed a Major League Soccer match in Chicago, municipal swimming pools and nature camps in Minneapolis, and a Creed concert near the city.
The U.S. Weather Prediction Center counted roughly 64 million Americans dealing with high heat and dangerous air at the same time that day.
Why scientists are alarmed
Emily Fischer, an atmospheric chemist at Colorado State University, put it bluntly. “It’s basically a river of smoke pouring into the Midwest right now,” she said. She linked the episode to climate change: “This is the climate change that people breathe.” Drought across much of North America, a jet stream carrying smoke southeast and the trapped air all fed the event, Hindustan Times reported.
Michigan stayed under an air quality alert through Friday, CBS News reported. The smoke then moved east. A day after Detroit’s worst reading, New York City Mayor Zohran Mamdani said, “Air quality has reached unhealthy levels in parts of NYC due to smoke from Canadian wildfires.” Researchers add that the harm outlasts the haze. Yale School of Public Health scientist Dr. Yiqun Ma said the fine particles “can penetrate the respiratory tract and bloodstream,” triggering inflammation that can impair lung and vascular function.
Smoke of this kind regularly travels more than 1,000 miles, usually from west to east.
Sources
- AQI 600 in Detroit, floods in Texas, wildfires in California & Canada: What's fuelling America's weather chaos
- Wildfire Smoke Spreads To Detroit And New York
