Dense wildfire smoke billowing down from Canada set off unhealthy air quality alerts across the United States again on Friday, prompting concern over this weekend’s FIFA World Cup final outside New York.

Detroit and Chicago posted air quality index readings in the “hazardous” range, with tracker IQAir saying they were the top polluted cities in the world.

Winds drifting south meant the capital Washington was also hard-hit, coming in at the second-highest “very unhealthy” ranking on the index, when authorities urge all people to avoid unnecessary outdoor activity.

In New York and neighbouring New Jersey, where the final will be played on Sunday in an open stadium, the metro area was experiencing “unhealthy” air conditions after smog on Thursday made the Manhattan skyline barely visible.

Forecasters at the National Weather Service warned “the smoke may thicken again in the overnight into Saturday morning.”

“What we should start to see is a return of north-westerly winds over the Great Lakes, and that’s going to pick up some of that smoke and push it over the Northeast,” Peter Mullinax, a meteorologist for NWS, told AFP.

But he said forecasts are not anticipating air quality on Sunday in the Northeast US to be as bad as experienced during the week.

“The extent of the really thick smoke we’re seeing, we shouldn’t be seeing necessarily that kind of smoke” during the match, he said, even if hazy skies persist.

“I don’t believe that this should be as impactful as if you might be playing a game today.” Weekend rain could also help alleviate the worst of the smoke, said Mark Parrington, a scientist at the Copernicus Atmosphere Monitoring Service.

“We just have to see whether those fires continue in the same scale,” he told AFP.

The World Cup final between Spain and Argentina will be held at Met Life Stadium in New Jersey at 3pm local time (5am Monday morning AEST).

USA women’s soccer star Trinity Rodman said “air quality was rough” during her team’s NSWSL game against Sam Kerr’s Gotham FC at Citi Field in Queens on Wednesday, where there were hydration breaks every 15 minutes.

“Not to make excuses at all, but I think on both sides we were all like, ‘another break, another break, another break’,” the Washington Spirit star said.

Star Poland striker Robert Lewandowski’s Chicago Fire debut was delayed as their MLS match against Vancouver Whitecaps was postponed due to poor air quality.

In cities across the Midwest and Northeast, people wore masks outdoors to filter out the dangerous air. In New York, libraries and train stations were handing them out for free.

The upper Midwest that’s closer to the fires was especially affected, with parts of Michigan, Minnesota and Wisconsin all recording air quality readings deep into the “hazardous” range for days.

The NWS extended its air quality alert in Chicago through Friday, adding “wildfire smoke may return tomorrow evening and continue into Sunday.”

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

But the intensity has rapidly escalated over the past week, with nearly 2.8 million hectares burned since the start of the year, per the latest government figures.

As of last Friday, that figure had stood at nearly 1.6 million hectares. Fires in Ontario have not caused any casualties, and several remote communities have been evacuated

According to the BBC, there are over 800 wildfires burning in Canada and smoke has spread across the border into the USA.