A freighter passes through the Detroit River as smoke fills the sky reducing visibility to Windsor, Ontario on Wednesday, as seen from Detroit. Paul Sancya/Associated Press

Dense smoke from wildfires burning in Canada continued to migrate eastward on Wednesday, swinging around a low-pressure area now moving offshore on the East Coast. In its wake, a channel of thick surface smoke draped across the Great Lakes and Ohio Valley, and increasingly over the Mid-Atlantic.

Detroit, Chicago, and Minneapolis were among the cities with the worst air quality in the world early Wednesday, according to IQAir. Unhealthy Code Red and Purple conditions stretched from eastern Iowa across Chicago and the lower Great Lakes region, then toward the Appalachian Mountains, according to AirNow.

Cities seeing Code Purple air quality, very unhealthy with an increased health risk for the general public, on Wednesday morning, included Chicago, Detroit, Cleveland, Pittsburgh, Milwaukee, Indianapolis, and Cedar Rapids in Iowa. Air quality alerts related to wildfire smoke were in effect for parts of 17 states, covering nearly a third of the American population.

The alerts stretch from Iowa and Minnesota in the west to New York, Maryland, Delaware, and North Carolina in the east. “Unhealthy air will continue today due to Canadian wildfires,” the National Weather Service in Indianapolis tweeted. “Limit outdoor activity and avoid outdoor physical activities, especially those that are sensitive to smoke.”

This round of Canadian wildfire smoke is perhaps a touch less intense than the one that swept across the Northeast in early June. Code Purple conditions on Tuesday in and around Milwaukee had a similar Air Quality Index at 246 compared to New York City at the peak of the last episode on June 7, when Queens reached 254.

Thus far, the daily value near Milwaukee is the highest in the United States from this batch. On June 7, Freemansburg in eastern Pennsylvania reached an AQI of 309 for the peak of that event. Additional Code Purples for a daily average seem likely Wednesday.

The AQI becomes problematic for sensitive groups at Code Orange. By Code Red, an AQI of 151 or above, the air can be unhealthy for everyone. Code Purple begins at an AQI of 201. Code Maroon readings indicating hazardous air quality start at 301.

Primary pollutants from wildfire at a distance are of the PM 2.5 variety, or carbon-based fine particulates of 2.5 microns and less in diameter. In Code Red or worse conditions, the best protection is staying indoors with the windows closed and going outside only as necessary while wearing an N95-type mask.

WHERE SMOKE IS AND WHERE IT IS GOING

A cold front passing through the Northeast United States during the summer brings at least a short reprieve from heat and humidity. This year, however, it is often a harbinger of smoke. The low-pressure area yanking smoke along for the ride is drifting offshore on the Mid-Atlantic. At the same time, a resurgence of high pressure across Quebec and Ontario will help keep the smoke on its current path.

Smoke was thickest around the lower Great Lakes region early Wednesday, and that patch will continue to drift toward the east and southeast through the day. A zone from near Detroit to Pittsburgh may see the worst of it, with Code Purple likely at times.

Wildfire smoke is also spilling over the Appalachian Mountains, with air quality headed downhill in Pennsylvania, Maryland, and Virginia. Air quality in the Mid-Atlantic is also likely to worsen through Wednesday.

Poor air is poised to remain over much of the same region through Thursday, though it will sink south slowly over the Great Lakes and perhaps diminish somewhat with time. Code Orange conditions are forecast from Georgia to the Mid-Atlantic on Thursday, as well as in spots west of the Appalachian Mountains.

A person rides a bicycle along the shore of Lake Michigan as the downtown skyline is blanketed in a haze from Canadian wildfires Tuesday in Chicago. Kiichiro Sato/Associated Press

TUESDAY TALLIES ACROSS THE NATION

Tuesday was a widespread Code Red day in much of the Upper Midwest, the Ohio Valley, the Great Lakes, and Ontario. The Milwaukee area reached Code Purple. According to data from the Environmental Protection Agency, many locations saw record high pollution for smoke particulates in data that go back to the 1990s.

For instance, the Milwaukee area recorded a 24-hour AQI, the average of the whole day, of 245, or its first Code Purple daily tally. It bested the old record for particle pollution of 165 in May 2001. The Grand Rapids area in Michigan had a daily AQI of 202, beating the old record of 141 in March 2003. The Davenport area in Iowa had a daily AQI of 185, a record that bested 155 in July 2008 and 2020.

The Detroit area reached a daily AQI of 184, the second highest on record, behind 190 in December 2006. Indianapolis hit an AQI of 182, the second highest on record, behind 191 in July 2014. This is the highest reading that was not related to fireworks. Dayton, Ohio, recorded an AQI of 172, beating 159 in December 2013.

More air quality records across the United States will probably be tested or surpassed Wednesday. The zones most at risk for that run from western New York and southern Ontario back through Ohio and into the Midwest.

A LOOK AHEAD AT THE IMPACTS

Unfortunately for Canadians dealing with the direct impacts, and Americans witnessing the fallout, the pattern that has fostered the firestorms looks probable to persist. With a record number of acres burned this year, the trend of record amounts of carbon sent into the atmosphere in June will probably continue into July. Forecasting smoke more than a few days in advance is a fraught exercise.

Looking past this weekend is nearly impossible outside broad pattern signals. We can conclude with reasonable confidence that high-pressure heat domes will continue to dominate both eastern and western Canada. In between and below the dominant heat domes, at least occasional storms may bring periodic rainfall to fire regions, notably those in eastern Canada.

There is no sign of an end to temperatures much above normal across Canada, which has been running hot for much of 2023 with accelerated warming from climate change. A resurgence of high pressure in the east may lead to significantly warmer than-normal readings in Quebec and areas around Hudson Bay by this weekend.

A storm track like the one that has repeatedly brought wildfire smoke to the Upper Midwest, Great Lakes, and Northeast this year may persist. This does not mean more episodes of smoke are a given, but it is a seasonal trend worth keeping in mind.