Thick smoke from a rash of wildfires in Canada and northeast Minnesota has swept over the Great Lakes and Northeast, leading to dangerously unhealthy air quality from the upper Midwest to the mid-Atlantic states and New England.

This more widespread smoke event began Wednesday from the northern Great Lakes into the Northeast, including Boston, New York City and Toronto.

Now it has draped itself over the entire Great Lakes as far south as Chicago, Cleveland and Pittsburgh. Each of these cities is reporting code purple "very unhealthy" air quality, according to the EPA's air quality index.

(MORE: Wildfire smoke tracker maps)

Late Thursday morning, Detroit, Minneapolis and Chicago were the three most polluted major cities in the world, according to calculations from IQAir.

Here are the latest smoke forecasts

Air quality alerts have been issued from Minnesota to the East Coast, from parts of Virginia to New England, not only for smoke particles, but also for ground-level ozone pollution more typical in summer. You can see them in gray on the map below.

(MORE: The Weather Company Labs: active wildfires & fire weather tracker)

Friday

Parts of the upper Mississippi Valley not near the active fires, including the Twin Cities, may see some modest improvement in ground-level smoke Friday, according to the latest computer models.

However, thick, unhealthy smoke is still expected from northeast Minnesota to Michigan and Ohio.

In the East, the smoke plume should continue to sag southward where code red air quality is possible in Washington, D.C. and Baltimore, and could linger in Philadelphia and Pittsburgh.

(WATCH: What air quality index forecasts mean)

Weekend, next week

There is some short-term good news.

Showers and thunderstorms are likely Saturday afternoon and evening in the Ohio Valley and much of the Northeast, helping to clean the air of smoke particles in those areas.

Unfortunately, model forecasts suggest two more southward plunges of smoke could happen starting Saturday in the northern Great Lakes. By Sunday, that smoke could spread as far south as the Ohio Valley and possibly again invade parts of the mid-Atlantic and Northeast.

Then, another round of smoke could nosedive southward through the Midwest beginning Tuesday and could make it into parts of the South mid-to-late next week.

What you need to know to stay safe

Wildfire smoke is hazardous to breathe for several reasons.

It contains microscopic "particulate matter" much smaller than the width of a human hair that can lodge deep into the lungs and then enter your bloodstream.

It can lead to short-term effects, such as coughing, shortness of breath, a scratchy throat, runny nose and burning eyes.

Prolonged exposure to dense smoke can increase the risk of chronic lung issues and increase inflammation that could lead to heart attacks and strokes.

If you're in these affected areas in the Midwest and Northeast, check your air quality forecast before heading outside.

You can do that on The Weather Channel app by going to your local forecast, then tapping the "Breathing" button. Or, from your local forecast page on weather.com, click on the "Air Quality" button at the top.

The index is color-coded from green (good) to maroon (hazardous).

If your air quality forecast is....

• At least code orange: More sensitive groups such as those with respiratory or heart conditions, the elderly, children and those pregnant should avoid staying outside more than necessary.
• Code red, purple or maroon: Everyone should limit their time outdoors. Consider exercising indoors and if you need to walk your pet outside, do so as briefly as possible.

(MORE: How air pollution affects you)

Why the smoke?

Plumes of smoke generated by larger wildfires are steered by winds from the ground to the top of the smoke plume.

On Monday, triple-digit heat, winds and low humidity triggered a major flare-up of wildfires that had been burning since May in northeast Minnesota, prompting a temporary closure of the Boundary Waters Canoe Area and a peacetime emergency declaration from Gov. Tim Walz.

The map below shows locations of 800 active Canadian wildfires, most numerous from the Northwest Territories to northern Saskatchewan, northern Manitoba, western Ontario, northern Quebec and Labrador.

In this case, the heat dome responsible for the recent record heat in the northern U.S. will get squashed into the Carolinas, setting up a northwesterly wind flow in the upper levels of the atmosphere.

That will allow a pair of cold fronts to sag into the Midwest and Northeast.

Northwest winds behind those fronts will tap smoke from those wildfires not just in western Ontario and northeast Minnesota, but also potentially some lofted smoke from additional fires in northern Canada.

So, while these weaker cold fronts bring relief from heat and humidity, they may also usher in some wildfire smoke, both aloft and at the ground. This is a scenario that has occurred several past summers this decade and is a pattern we were concerned about this summer during a developing, strengthening El Niño.

(MORE: This $65 DIY air filter builds in just 15 minutes)

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.​