(BSS/AFP) - The Manhattan skyline was obscured by thick haze and Chicago closed its beaches Thursday as out-of-control Canadian wildfires raged, sending smoke spewing into the United States and exposing millions of people to dangerously unhealthy air.

US states near the Canadian border including Minnesota, Wisconsin, Michigan and Illinois were particularly choked, while the Northeast including New York was also experiencing deteriorating air quality.

On Thursday evening, tracker IQAir said Chicago and Detroit were the top polluted cities in the world, with New York not far behind.

The most populous US city was shrouded by the smoke that state authorities warned contained fine particulate matter deemed unhealthy for everyone across the New York metro area and Long Island.

Even worse air enveloped the central and western regions of the state.

Authorities encouraged New Yorkers to spend as little time outside as possible, with Mayor Zohran Mamdani warning of the "serious threat" of heavy heat and unsafe air.

Libraries and train stations were offering free masks, while hundreds of cooling centers were open across the city for those without access to air conditioning.

"This could become the most significant smoke event in New York City since 2023, and conditions will be closely monitored for any deterioration," the city`s Department of Emergency Management said.

That year, the city`s skies turned an eerie orange, with the air quality index reaching a dangerous 465.

`Our chests feel tight` -

In the Midwest, current index levels on Thursday had soared beyond that figure, according to the US Environmental Protection Agency tracker.

Chicago along with Toledo, Ohio, and areas in Minnesota relatively close to the fires were well into the 700s on the air quality index during the afternoon, with much of the rest of the upper Midwest also experiencing AQI levels in the worst category of "hazardous."

A Major League Soccer match in Chicago was postponed as municipalities across the region postponed outdoor events.

Erin Lucey, 38, works on her family`s organic vegetable farm in south-central Wisconsin and spent the morning harvesting zucchini, beans and cucumbers, among other tasks, with laborers wearing masks in muggy heat to filter out the hazy smoke blanketing the area.

"Our chests feel tight," she told AFP, saying the smoke combined with recent searing heat waves and parched fields left conditions feeling "eerie."

"We are all thinking of the delicate balance of growing food in this type of future, and remarking how we can`t imagine what it will be like here in 100 years," she said.

"If people were outside like us working and seeing how everything looks, if they didn`t have the convenience of air conditioning, we would be making much faster changes to address the climate crisis," she added.

The Michigan Department of Environment, Great Lakes, and Energy said it expected the dangerous conditions to continue through Friday, and that "our best model for smoke only predicts out to 48 hours."

"That model is currently showing improvements on Saturday, but it is likely that smoke will linger and recirculate for a while," the agency said.

Evacuations in Canada -

Toronto was also grappling with the dangerous air quality, as the latest data in Canada showed more than 130 active fires in northwestern Ontario province, with at least 60 out of control.

Authorities there have formally requested additional assistance from the federal government, in particular seeking air support to evacuate remote communities.

The Ontario provincial police said 15 communities and their surrounding areas have been evacuated so far.

"More than 150 fire crews and nearly 50 firefighting aircraft are working around the clock to protect communities from the fires in northern Ontario," Ontario Premier Doug Ford said on X.

So far, Canadian wildfires have scorched at least 1.9 million hectares (4.7 million acres) this year, an area nearly the size of Slovenia.

That damage remains far off the pace of 2023, Canada`s worst wildfire season on record, when nearly 18 million hectares burned in the country.