Dangerous and historic wildfire smoke pollution event engulfs the U.S. and Canada » Yale Climate Connections

by Jeff Masters, Yale Climate Connections
July 17, 2026

As climate change bakes forests across North America, dense smoke from dozens of out-of-control wildfires burning in northern Minnesota and adjacent portions of Ontario is blanketing tens of millions of people with hazardous pollution.

The fires are bringing the worst air quality on record to much of the Great Lakes, mid-Atlantic, and Northeast United States. Pollution from small particles called PM2.5 — the fine particles less than 2.5 microns in diameter are the primary air pollution killers — has been far into the “Hazardous” range across five states since Wednesday morning.

The award for worst air in the nation yesterday went to the city that has in the past billed itself as a climate haven: Duluth, Minnesota. The city’s 24-hour air quality index, or AQI, for PM2.5 particle pollution hit 934, over three times the threshold for “Hazardous” pollution. This shattered Duluth’s previous all-time AQI record of 159 set July 20, 2021. EPA pollution records go back to 1999.

Because of the huge number of people affected, and since this is occurring at the same time as a severe humid heat wave, this extreme and widespread pollution event — which will be followed by many months of repeated wildfire smoke incursion into the U.S. — will undoubtedly cause hundreds and perhaps thousands of premature deaths. The only comparable wildfire smoke event affecting this portion of North America occurred in 2023; a 2025 study blamed that event for 33,000 premature deaths in the United States, 8,300 in Canada, and 23,000 in Europe. According to the EPA, a premature air pollution death is one that occurs on average 14 years before a person would have otherwise died.

The climate change connection to the wildfires

As the climate warms, fire danger increases, mostly because the atmosphere gets “thirstier” – more water vapor can evaporate into warmer air. This results in more water vapor evaporating from plants, which dries them out and creates an increased risk of large and intense fires that can generate huge smoke plumes. Most of the fires grew out of control under extreme heat conditions made up to five times more likely by climate change (Fig. 1). According to Climate Central, the heat that helped fuel these fires would have been “highly unlikely” to have occurred in a world without climate change.

Five states recorded “Hazardous” air quality on Thursday

On Thursday, July 16, portions of Ontario and five states — Minnesota, Wisconsin, Michigan, Illinois, and Ohio — experienced 24-hour levels of PM2.5 with an air quality index in the “Hazardous” (brown) range. According to the U.S. Environmental Protection Agency, these conditions necessitate health warnings of emergency conditions, with the entire population more likely to be affected. Purple “Very Unhealthy” air was observed in three other states — New York, Pennsylvania, and Indiana. This level of pollution triggers a health alert, and everyone – not just people with vulnerabilities – may experience more serious health effects.

According to air pollution scientist Ryan Stauffer, yesterday’s air pollution event blows away the previous most extreme wildfire smoke event in this region — in June 2023 — for extremity. During the 2023 event, only about three EPA monitors, all in Pennsylvania, recorded a 24-hour AQI in the hazardous range. But on Thursday, 50 official EPA monitors recorded a 24-hour AQI in the “Hazardous” range (Fig. 2), plus an additional nine monitors in Ontario. According to rankings at iqair.com, Detroit was the most polluted major city worldwide for most of Thursday, with Chicago bumping Detroit out on Thursday night. On Friday morning, the top five most polluted cities in the world were all in North America: Detroit, Chicago, Washington D.C., Toronto, and New York City.

On an hourly scale, some truly extreme AQI readings above 1,000 were recorded Thursday in Minnesota, Wisconsin, and Michigan. The “Hazardous” (brown) range is for an AQI in above 300, so these readings were more than three times beyond the “Hazardous” threshold. The most extreme readings occurred in northern Minnesota downwind of the fires burning in the Boundary Waters park, where an AQI of 3,567 was measured by a purpleair.com sensor.

How the trouble started: record heat and a record-strong high-pressure system

The wildfire event began on Monday, when the strongest upper-level ridge of high pressure ever observed in the north-central U.S. baked the region. All-time record heat exceeding 100 degrees Fahrenheit (37.8°C) was observed, worsening existing moderate to severe drought conditions. Thunder Bay, Ontario, on the north shore of frigid Lake Superior, hit 39.5 degrees Celsius (103.1°F), smashing its all-time heat record by over 2°C. To the north, Armstrong hit 40.7°C (105.3°F), the hottest temperature observed in all of Ontario since the great Dust Bowl heat wave of July 1936.

Strong winds moved in with the heat, fanning multiple wildfires that feasted on the dry fuels, which featured plenty of dead trees from a spruce budworm infestation and a 1999 derecho event that felled thousands of trees. Extreme fires with a rapid rate of spread resulted and created at least two massive pyrocumulus clouds — giant thunderstorms spawned by the heat of intense fires that reached the stratosphere. Tomer Burg has an excellent thread explaining the meteorology that led up to the wildfire event:

The current situation: over 20 large fires out of control

As of Friday, there were 193 active fires covering 1.7 million acres (673,000 ha) in Ontario, with the vast majority of these considered to be “out of control,” according to the Canadian Interagency Forest Fire Centre. Of the 22 large fires in over 2,000 acres burning nearest the U.S. border, 18 were receiving a full fire-fighting response, and four were not being fought. In the U.S., the National Interagency Fire Center reported six large fires in northern Minnesota, all 0% contained, covering 60,000 acres.

The forecast: relief coming by Saturday for the worst-affected states

Thunderstorms moved over the Minnesota/Ontario fire area Friday morning, bringing up to two inches of rain. This water will help firefighting efforts, but the fire danger index in Ontario is still high to very high, and the thunderstorms may have sparked additional fires.

The cold front accompanying this storm will bring westerly winds that should flush the worst of the smoke out of Chicago and Wisconsin by Friday afternoon, and out of Michigan by early Saturday morning. However, a renewed invasion of smoke is predicted for Michigan and Wisconsin Saturday night into Sunday, bringing more “Hazardous” AQI conditions. Another pulse of smoke is predicted to move deep into the central U.S. on Wednesday, perhaps reaching Arkansas and Missouri.

We can anticipate that some of the major smoke-emitting fires in Minnesota/Ontario will continue to burn for an extended period, with some lasting until the first snows come in October. The long-range fire assessment (Fig. 3) calls for above-average fire risk over much of the forested areas of northern Canada and the western U.S. this summer, and we should anticipate frequent bouts of poor air quality from wildfire smoke across much of North America. July and August are usually the peak months of fire season, which typically extends well into September. A potent North American Monsoon is now bringing heavy rains to parts of the southwest U.S., so the overall fire risk in that region may decrease to average levels by August.

Many lightning-caused fires occurred in the Pacific Northwest yesterday. With hot, windy weather expected to move in during the coming week, some of these fires may well become significant smoke producers.

Climate change predicted to further worsen wildfires

The number of people in the U.S. who experienced at least one day each year with smoke-related fine particle pollution levels at three times over the EPA standard has increased 27-fold over the last decade, and we can expect climate change to significantly worsen wildfire smoke problems in North America in the coming years.

For example, the frequency and magnitude of extreme wildfires around the globe have doubled in the past 21 years because of climate change, according to a study published last year in the journal Nature Ecology & Evolution. Rising temperatures have ushered in an era of hotter and drier weather, lending the right conditions for wildfires to erupt, the researchers found.

Here are some additional resources on climate change and wildfires:

Related

15 sources of wildfire smoke forecasts for North America

Silent calamity: The health impacts of wildfire smoke

How to protect yourself from wildfire smoke

Climate change made deadly Los Angeles wildfires 35% more likely: new attribution study

Bob Henson contributed to this post.

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