As wildfires continue to burn in Canada, the Great Lakes region is seeing thick smog and record high air quality issues caused by smoke from the blazes.

Swaths across Canada, particularly in the Quebec province, began battling fires weeks ago. So far, more than 9 million acres of land have burned and over 20,000 people have evacuated, Canadian officials said.

Hundreds of miles away from the fires, in one of the smokiest areas, Chicagoans woke up to the worst air quality in the world on Tuesday morning, deemed “very unhealthy” by the Environmental Protection Agency. Nearby areas, including Detroit and Minneapolis, are also plagued with smog and poor air quality.

The midwestern smoke is reminiscent of the scene in New York City earlier this month when the skyline turned a dark, orange hue that people liken to Mars, and people complained of a strong cigar-like smell.

Smoke from the wildfire crisis traveled south from Canada to the U.S. in early June where it affected the northeast, the Great Lakes, and the mid-Atlantic. Philadelphia and other cities took measures like canceling outdoor school activities at the time.

Government officials warn that a cold front moving toward New York could bring unhealthy levels of smoke to the area again overnight on Tuesday, going into Wednesday.

“As we closely monitor the changing forecast, New Yorkers should be prepared for the potential return of smoke from the Canadian wildfires,” Governor Kathy Hochul said in a statement. “I encourage everyone to remain vigilant, especially if you are vulnerable to air pollution, stay up to date on the latest information and take steps to protect yourself.”

Here’s what to know about the smoke:

Why smoke can travel so far

Tom Kines, a senior meteorologist at AccuWeather, explains that the smoke people are seeing was able to spread all the way from Canada because of how copious and ongoing the fires are. “These fires are still burning up there. They’re still putting smoke into the sky, and as long as you have a source of smoke it’s going to be an issue,” Kines says. Canada’s wildfires have been exacerbated by extremely dry conditions and warm temperatures. There were 493 active fires in Canada as of Monday, at least 259 of which were deemed out of control.

Read more: What Wildfire Smoke Does to the Human Body

The smoke’s trajectory follows the wind, which is currently traveling north to south, and as a result, bringing smoke from fires in Canada, towards the U.S. “All the smoke that gets deposited up in the air gets caught in the wind flow, and pushed out into the states,” Kines says. “That’s basically what’s been happening.”

Is smoke worse at higher or lower elevations?

Kines says that smoke levels are generally greater in high-elevation areas than low ones, but not always. “Usually when you get smoke it’s higher up in the atmosphere and it comes across as haze,” he says. “But in this particular instance, there’s so much smoke, it’s close to the ground. And not only is it reducing visibility, it’s causing air quality issues, and also you can smell the smoke.”

Smog had already been plaguing the northeastern U.S. from late May fires in Nova Scotia when plumes came again in June. Although the current fires are much stronger, the compounding smoke influx contributed to straining air quality throughout both countries.

How long will the smoke last?

Meteorologists predict that smoke around the Great Lakes will slowly dissipate today and tomorrow, leaving the skies mostly clear by late Wednesday.

Read more: Even As Smoke Engulfs Us, We Can’t Wrap Our Heads Around Climate Change

While there are signs of hope, the weather could still change, experts note. “Sometime next week, the winds become more northerly [blowing from north to south] again. If the fires are still burning, we could have more smoke issues again,” Kines says.

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Government officials in affected states and counties are urging people to stay indoors as much as possible, especially children, senior citizens, and people with asthma or other lung and heart conditions.

“The City of Chicago is carefully monitoring and taking precautions,” Chicago Mayor Brandon Johnson said in a statement on Tuesday. “As these unsafe conditions continue, the city will continue to provide updates and take swift action to ensure that vulnerable individuals have the resources they need to protect themselves and their families.”