GEOFF BENNETT: Raging wildfires are burning in parts of the Western us and Canada, forcing thousands of people to flee their homes and forcing Canada to call up the military to help.

In addition to the physical devastation the flames are causing, researchers are getting a clearer picture of just how dangerous all that toxic smoke is to human health.

Here's William Brangham.

MAN: Hey, watch your back.

We're getting flanked again!

WILLIAM BRANGHAM: It's an apocalyptic scene in Northern California, as the massive Park Fire outside the city of Chico exploded in size overnight, burning 4,000 acres an hour.

MAN: I'm going to end up getting caught.

WILLIAM BRANGHAM: Firefighters faced triple-digit temperatures as neighborhoods went up in flames and gas tanks detonated in the heat.

Yesterday, wildfire cameras caught the formation of a so-called firenado, a massive vortex of flame and smoke.

The Park Fire was only 3 percent contained by this morning, forcing thousands of people to flee.

PAUL MALLORY, Fire Evacuee: We set a prayer over our house, and we just prayed to God.

WILLIAM BRANGHAM: Paul and Janet Mallory (ph) were forced to evacuate, but their home was spared.

PAUL MALLORY: We're happy that we were out and we just prayed over it, and I believe that our house is standing today because of a miracle.

WILLIAM BRANGHAM: There are now more than 60 wildfires burning across Oregon and Washington.

The country's largest, Oregon's Durkee Fire, has consumed nearly 300,000 acres and is less than a quarter contained.

MAN: There's mom and dad's house.

WILLIAM BRANGHAM: In Canada, this is what's left of Jasper, Alberta.

Alberta's premier, Danielle Smith, echoed the feelings of so many.

DANIELLE SMITH, Premier of Alberta, Canada: We share the sense of loss with all of those who live in the town, who care for it, and who have helped build it.

WILLIAM BRANGHAM: The devastation is all too familiar this fire season, as extreme heat and dry conditions, supercharged by climate change, fuel catastrophic fires across the American and Canadian West.

Already this year, seven million acres have burned in North America.

These fires also generate massive plumes of smoke, which carry tiny dangerous particles.

And that smoke spreads across a much larger region, covering huge swathes of North America all the way to the East Coast.

LOREN WOLD, Ohio State College of Medicine: We think that living in a smaller city, or living particularly in farm country, where I grew up, that you're protected from the effects of high particle levels.

But, unfortunately, with the wildfires, everyone is affected, either large city or small city.

WILLIAM BRANGHAM: Loren Wold runs a lab at The Ohio State University that studies how those tiny airborne particles, they're known as PM2.5, when breathed in, can harm human health.

LOREN WOLD: They can actually pass through the lining, get into your bloodstream, and then affect your heart, your lungs, your brain, any organ system.

On days when PM levels are high, particularly during the wildfires, for example, that there is a significant increase in the amount of patients who are going to emergency rooms for things like sudden cardiac events.

WILLIAM BRANGHAM: Wold's research indicates that not only do these negative health impacts last long after the air is cleared, but in studies with pregnant mice exposed to contaminated air, their offspring were also seriously harmed by the smoky air.

So mice that are in utero, that they themselves are not breathing contaminated air, their mothers are, that passes to them and then lasts for those baby mice for a good chunk of their own lifetime?

LOREN WOLD: Yes, correct.

Once it's in the bloodstream, it can get into the maternal and fetal circulation and have effects on the offspring.

And not only does it have cardiac effects, but it also has -- we have shown that it predisposes animals to development of a neurocognitive phenotype similar to an Alzheimer's disease animal.

WILLIAM BRANGHAM: One recent study showed that exposure to smoke filled air can also shorten a person's life.

It estimated that between 2008 and 2018, wildfire smoke in California was responsible for over 50,000 premature deaths, equating to an economic impact of over $400 billion.

Anthony Wexler directs the air quality research center at the University of California, Davis.

He wasn't involved in this study, but says this is further evidence of how climate change is impacting human health.

ANTHONY WEXLER, Air Quality Research Center Director, University of California, Davis: Climate change is changing everything about the weather patterns that we're used to.

Wildfire smoke emits a lot of PM2.5.

And so just extending those correlations, the higher the concentration, the more people will die from heart attack.

And the wildfire smoke makes a lot of this PM2.5.

And so we're going to see excess deaths.

WILLIAM BRANGHAM: So how do you know if the air you're breathing today is risky?

Researchers measure that using what's called the AQI, or Air Quality Index.

The higher the number, the more dangerous the air.

LOREN WOLD: Fifty percent or so of the U.S. is under unsafe levels.

And the scary part is that those with sensitive conditions, as well as the elderly or the very young, the levels are actually different.

So 100 is not considered safe for those individuals.

WILLIAM BRANGHAM: That's a huge number of Americans today breathing this dangerous air.

LOREN WOLD: It is.

It's very scary.

WILLIAM BRANGHAM: Researchers say limiting the time spent outdoors, wearing a mask when you can't, and making sure air conditioning and heating systems have high-quality filters can all help.

But until these massive fires are brought under control, the air that many Americans are currently breathing will continue to be a threat.

For the PBS "News Hour," I'm William Brangham.