For the second time this summer, a thick, orange haze has settled over huge swaths of North America — and this time it has pushed several major American cities to the very top of the world's worst air quality rankings. More than 115 million people across the Midwest, Northeast, and Mid-Atlantic have been placed under air quality alerts as smoke from hundreds of Canadian wildfires continues to pour south of the border.
Nearly 900 Wildfires Burning Across Canada
According to the Canadian Wildland Fire Information System, 858 active wildfires were burning across Canada as of July 16, the vast majority in the province of Ontario, with 113 of those fires classified as completely out of control. The smoke plume from those fires has been unusually persistent because two weather systems — one over the U.S. and one over Canada — have remained largely stationary for about four days, effectively trapping the smoke over the same regions rather than allowing it to disperse.
Which Cities Have It Worst
The rankings have shifted daily, but the pattern has been consistent: American cities dominating the list of the world's most polluted places. On Thursday, Minneapolis overtook Detroit for the worst measured air quality of any major city on Earth, according to monitoring group IQAir. Detroit had earlier reached an air quality index of nearly 570 — a level IQAir classifies as 'hazardous' once it crosses 301. Chicago ranked third worldwide, with Toronto not far behind in sixth place. Milwaukee recorded the most extreme single reading of the event: an AQI of 644, more than double the city's previous all-time record of 300, which had stood since 1987.
The East Coast Gets Its Turn
By Friday, the worst of the smoke had shifted toward the East Coast. Pennsylvania's Department of Environmental Protection escalated its alert from Code Red to Code Purple — the second-most-severe tier on its air quality scale — while Washington, D.C. was placed under a Code Red alert meaning the air was unhealthy for the entire population, not just sensitive groups. All of New York State, including New York City, along with parts of New Jersey, were also placed under air quality advisories, with parts of the city measuring AQI levels above 200 — categorized as 'very unhealthy.' Massachusetts, Vermont, New Hampshire, and Maine all reported hazy, brownish-yellow skies as the plume pushed into New England.
What the Smoke Actually Does to Your Body
Wildfire smoke is dangerous largely because of a pollutant called PM2.5 — fine particulate matter small enough to travel deep into the lungs and even enter the bloodstream when inhaled. Health officials say the particles can trigger shortness of breath, coughing, dizziness, and fatigue, and can seriously aggravate existing conditions like asthma, bronchitis, heart disease, and diabetes. Children, older adults, pregnant people, and anyone with existing heart or lung conditions face the highest risk. Meteorologists have also flagged a specific added danger this week: the smoke is arriving at the same time as an ongoing heat wave in parts of the affected region, and the combination of extreme heat and poor air quality is considered particularly hazardous because both conditions independently strain the cardiovascular system.
How to Protect Yourself
Health authorities are recommending that residents in affected areas stay indoors as much as possible, keep windows and doors closed, and run air purifiers or HVAC systems with high-efficiency filters where available. For anyone who must go outside during an active alert, wearing a properly fitted N95 mask is advised, since standard cloth or surgical masks do not filter out fine particulate matter effectively. Outdoor exercise and strenuous activity should be avoided entirely during the worst periods of an alert.
When Will the Air Clear?
Meteorologists say relief is on the way, though not immediately. A pair of cold fronts is expected to push into the Midwest and Northeast over the weekend, and while those fronts will also usher in some fresh smoke initially, they are ultimately expected to bring rain and storms that should help wash the existing smoke out of the atmosphere. Some improvement was already being observed in the Twin Cities by Friday. Longer term, climate scientists note that fossil-fuel-driven warming is increasing the odds of prolonged, intense wildfire seasons like this one, meaning events of this scale may become less of an outlier in the years ahead.
Frequently Asked Questions
How many Americans are affected by the wildfire smoke? More than 115 million people across the Midwest, Northeast, and Mid-Atlantic have been placed under some form of air quality alert.
Which US city has the worst air quality right now? Rankings have shifted daily, but Minneapolis, Detroit, and Chicago have each held the top spot on IQAir's worst-air-quality-in-the-world list this week, with Milwaukee setting an all-time pollution record.
Why is Canada having such a bad wildfire season? The majority of the nearly 900 active fires are burning in Ontario, with more than 100 classified as out of control. Persistent, stagnant weather patterns have kept the resulting smoke trapped over the same regions for days at a time.
When will the air quality improve? Meteorologists expect cold fronts moving through this weekend to bring rain that helps clear the smoke, though some areas may see brief additional smoke before conditions improve.