AMNA NAWAZ: Welcome to the "NewsHour."

Hundreds of wildfires continue to burn across Canada, many of them out of control.

GEOFF BENNETT: The smoke has more than 55 million Americans under air quality alerts, prompting safety warnings to stay indoors and the cancellation of some major events, including professional baseball and basketball games.

Stephanie Sy has our report.

STEPHANIE SY: An ominous orange haze envelops the Statue of Liberty.

Wildfire smoke from Canada has billowed across the border, degrading air quality across the Northeast and Upper Midwest.

Pilots and passengers flew through thick clouds of black smoke, before the FAA temporarily grounded flights out of New York.

SAMUEL GARCIA ANGEL, New York Tourist: It's very unexpected.

Like, it really happened out of the blue.

And I just really hope that everyone stays safe, wear a mask outside, so the smoke doesn't go into their lungs.

STEPHANIE SY: Today, the Big Apple ranked worst in the world for its air quality.

New York City Mayor Eric Adams issued health advisories for all five boroughs.

ERIC ADAMS (D), Mayor of New York: This is an unprecedented event in our city, and New Yorkers must take precaution.

STEPHANIE SY: As wildfire smoke continues to blanket the U.S., 100 million Americans are under air quality alerts, the potential health threat posed by wildfire smoke spanning as far south as Georgia and Texas.

PETER DECARLO, Johns Hopkins University: The high levels of air pollution are going to be detrimental to people who are at risk.

But, really, everyone is impacted by this.

STEPHANIE SY: Peter DeCarlo is an associate professor of environmental health and engineering at Johns Hopkins University.

PETER DECARLO: Exposure to air pollutants isn't something you can undo.

These are particles that are about 100 times smaller than the width of our hair.

When we breathe those in, they get into our lungs.

They can basically travel to all parts of our body.

And so we usually think of the respiratory system as the most impacted.

But this also impacts our heart.

It can impact our brain.

STEPHANIE SY: In Canada, more than 20,000 Canadians can't go home, still under evacuation orders, as firefighters struggle to contain hundreds of wildfires.

BRENT JOHNSTON, Nova Scotia Evacuee: Fortunate enough to have a trailer right now that we could get out for the time being.

STEPHANIE SY: In British Columbia, this crew is fighting fire with fire, burning out fuel ahead of an approaching blaze.

Canada's minister of public safety says there are 414 active fires across the country; 239 are out of control.

BILL BLAIR, Canadian Minister of Public Safety: I want to assure Canadians that all orders of government are working closely together, including with our indigenous partners, to ensure a coordinated and effective response.

It's all hands on deck and it's around the clock.

STEPHANIE SY: Coast to coast, forest fires have scorched more than 9.1 million acres in Canada already this year, an area bigger than the state of Maryland.

And it's just the start of what is on track to be the most devastating fire season in Canada's history.

KRISTINA DAHL, Union of Concerned Scientists: The fact that we're seeing so many fires burning such large areas and burning this early in the season is definitely worrisome STEPHANIE SY: Climate scientist Kristina Dahl says several factors at play.

KRISTINA DAHL: We know that climate change isn't the only factor that's at play in driving worsening wildfires around the world.

We know that things like suppressing fires for many decades or even a century can cause a lot of undergrowth in our forests to build up that would have been naturally cleared by naturally occurring fires and low-severity fires.

But now that we have been suppressing those fires, we have a lot of overgrowth.

And that's causing fires to burn larger and more severely.

STEPHANIE SY: As firefighters try to contain the fires across Canada, officials in many U.S. cities warn, air quality is at Code Red.

It is not the highest alert level, but it does mean the air may be unhealthy for some and outdoor activity is discouraged.

For the "PBS NewsHour," I'm Stephanie Sy.