Chicago and Detroit have the worst air quality in the world due to Canadian wildfires. Here's how to stay safe.

Weeks after wildfire smoke from fires burning in Canada blanketed much of the East Coast with hazy, orange skies and unsafe air quality, parts of the Midwest were startled Tuesday as Chicago and Detroit saw the worst air quality in the world.

Thick smoke clogged the sky in Chicago early Tuesday, with a gray haze limiting visibility, and a faint burning smell filled the air as residents commuted to work. In downtown Detroit, smoke could be seen lingering over the skyline and the National Weather Service issued an air quality advisory for the city until Wednesday night.

Chicago and Detroit held the top two rankings for worst air quality in the world, according to IQAir’s Air Quality Index. The two Midwest metropolitans alternated between the "unhealthy" and "very unhealthy" categories through Tuesday.

Unhealthy levels of pollutants from the smoke spread across parts of the Great Lakes Region surrounding Chicago, including most of Wisconsin and parts of Michigan, Illinois, Indiana, and Ohio, according to tracker AirNow.gov.

Meanwhile, more smoky air is headed back New Yorkers' way. Gov. Kathy Hochul warned air quality is expected to reach unhealthy levels in parts of western and central New York and eastern Lake Ontario on Wednesday and Thursday.

That means anyone in a sensitive group – including young children, older adults or those with heart and lung disease – should stay indoors for any physical activity, and everyone else should limit the amount of time they are active outdoors.

Here's what to know about the air quality:

Where is the air quality bad on Tuesday?

Major cities in the Midwest had air deemed unhealthy or unhealthy for sensitive groups Tuesday, including Chicago, Milwaukee, Indianapolis, Cincinnati, Detroit and Minneapolis.

• Three U.S. cities ranked in the top 10 cities with the worst air quality in the world Tuesday morning, according to IQAir: Chicago, Minneapolis and Detroit. Other cities in the top 10 were Dubai; Jakarta, Indonesia; and Delhi, India.

• Wisconsin's Department of Natural Resources issued an advisory for much of the state that's expected to last through Thursday afternoon, with air quality expected to be most severe on Tuesday and Wednesday. The sky in Milwaukee was hazy and smelled like smoke starting Monday.

• Tuesday was declared a statewide day of action by the Michigan Department of Environment, Great Lakes and Energy because of unhealthy and hazardous air quality.

• On Sunday and Monday, a smoky haze had settled over parts of northwestern Vermont and was carried out Monday afternoon by southerly winds.

Why is the air quality so bad in Chicago and Detroit?

Air quality is unhealthy due to the level of particulates in the air from Canadian wildfires, which have drifted into the U.S. at various points in the last several weeks. Zachary Yack, a meteorologist with the National Weather Service in Chicago, said wind patterns were largely driving where the smoky air goes.

The smoke is affecting a vast area of the Upper Peninsula of Michigan and will blow downward into the Lower Peninsula's southern region. The dense smoke has also been moving into other southern regions of the state and sensitive groups were advised to avoid outdoor activities on Tuesday and Wednesday.

Low pressure over the eastern Great Lakes is also causing the smoke to drift through northern Michigan, and across southern Wisconsin and Chicago, said Bryan Jackson, a meteorologist with the National Weather Service.

Jackson said a north wind will push the smoke further south into Indiana and Kentucky later Tuesday and overnight.

US air quality map

How can you stay safe?

Experts say it's best to avoid strenuous activity when air quality levels are unsafe outside and to stay indoors as much as possible. Use air purifiers to help the air quality inside, and wear an N95 mask if you must go outside.

Hochul said on Tuesday masks would be available for free to New Yorkers ahead of expected unhealthy air later this week. She announced earlier this month the state would have 1 million free N95 masks available amid hazardous air quality.

When will Chicago air quality get better?

The smoky air and haze is expected to linger in the region Tuesday and overnight, but should be slowly pushed south and west throughout Tuesday, and gradually diminish in coverage on Wednesday, Yack said.

Yack said wind off Lake Michigan will to help push the smoke in a southwest direction later in the day.

"Some spots may still see some white concentration of the smoke, hint of the smoke smell maybe into tomorrow morning, but most of it should be a thinning trend as we go forward in time," Yack said.

HOW TO KEEP YOUR PETS SAFE: Bad air quality from Canada wildfire smoke harms your pets, too.

US wildfire map

What are the wildfires like in Canada now?

There were 492 active wildfires burning throughout Canada as of Monday, according to the Canadian Interagency Forest Fire Centre. Of those, 259 are considered out of control.

It's one of the worst wildfire seasons on record for the country. Smoke from wildland fires burning in Quebec has even reached parts of southwestern Europe, NASA's Earth Observatory reported on Monday. Soot particles reached across 2,000 miles of the Atlantic Ocean, causing hazy skies in Europe. Air quality there is mostly fair because the particulates are higher in the atmosphere, NASA said.

"Vivid sunrises and sunsets" are expected in western Europe, the UK's meteorological agency said.

SEE HOW THE AIR AFFECTS HEALTH: How bad is breathing in wildfire smoke? Graphics show how toxic air affects your health

Contributing: Grace Hauck, USA TODAY; Detroit Free Press; Associated Press

This article originally appeared on USA TODAY: Canadian wildfires: Chicago has the worst air quality in the world