Dense clouds of smoke from wildfires, moving south from Canada, triggered new air quality warnings in the United States on Friday, raising concerns about the World Cup final scheduled for Sunday near New York.

Air quality indices in Detroit and Chicago entered the 'hazardous' range, and tracking site IQAir reported they were the most polluted cities in the world.

The southward wind shift also severely affected the capital Washington, which recorded the second highest level on the index within the 'very unhealthy' category, a stage where authorities urge residents to avoid unnecessary outdoor activities.

In New York and neighboring New Jersey, where the final will be held Sunday at an open stadium, conditions became 'unhealthy,' after smoky haze made the Manhattan skyline barely visible on Thursday.

Andrew Giuliani, executive director of the White House World Cup Task Force, said in a press briefing that tournament organizers are 'monitoring the situation closely.'

Experts at the National Weather Service warned that smoke could become denser overnight and into Saturday morning.

Expert Peter Mullinax from the agency told AFP: 'What we'll start to see is a return of northwesterly winds over the Great Lakes region, and these winds will carry some of that smoke and push it toward the Northeast.'

But he clarified that forecasts do not indicate that air quality in the northeastern US on Sunday will be as poor as what the region experienced during the week.

He added that during the match between Spain and Argentina, 'we don't expect to see the intense smoke density we are seeing now,' even if hazy conditions persist.

He continued: 'I don't think this has the same impact that playing a match right now would have.'

Mark Parrington, an expert at the Copernicus Atmosphere Monitoring Service, noted that rainfall over the weekend may also help reduce smoke intensity.

He told AFP: 'We just have to see if those fires continue at the same scale.'

In cities across the Midwest and Northeast, people wore masks outdoors to filter polluted and hazardous air, with libraries and train stations distributing them for free in New York.

The impact is more severe in the Upper Midwest, closer to the fire locations, with parts of Michigan, Minnesota, and Wisconsin recording air quality in the 'hazardous' range.

The weather service extended its air quality warning for Chicago until Friday, noting that 'wildfire smoke may return tomorrow evening and continue until Sunday.'

Role of climate change

Experts stressed the link between repeated episodes of wildfire smoke and climate change.

Paul Mathewson, director of scientific programs at Clean Wisconsin, one of the states that has seen a sharp increase in smoky days in recent years, said: 'The increasing prevalence of smoke in the air highlights the importance of a rapid transition to clean energy, instead of building more polluting fossil fuel infrastructure that worsens climate change.'

Mark Parrington emphasized in a statement to AFP that climate change is creating conditions for a longer fire season, accompanied by higher surface air temperatures and lower soil moisture.

He explained: 'When ignition occurs, we see large-scale and sustained burning, so these fires can keep burning for weeks throughout the summer.'

Fire activity intensified on Friday in Canada, where more than 200 fires were out of control, particularly in Ontario province, according to authorities.

Damage remains far from the levels of 2023, which saw the worst wildfire season in Canada's history, when nearly 18 million hectares burned.

However, fire intensity has increased rapidly over the past week, with nearly 2.8 million hectares burned since the start of the year, according to the latest government figures, noting that this number was around 1.6 million hectares until last Friday.

The fires in Ontario have not caused any casualties, and many residents of remote areas have been evacuated.