GEOFF BENNETT: Between now and Saturday, more than 115 million people may be exposed to dangerous smoke and unhealthy air quality levels.

That's because wildfire smoke is spreading and triggering health advisories in at least 17 states from the Midwest to the mid-Atlantic.

Thousands have evacuated northern Minnesota.

In other states, it's led to flight delays, the closure of pools and beaches, and the cancellation of many other outdoor activities.

Stephanie Sy has our report.

STEPHANIE SY: People in Duluth, Minnesota, woke up to some of the world's worst air quality this morning.

Same for the residents of Detroit and Chicago.

CAITLIN SMITH, Minnesota Resident: I felt pretty sick just from... STEPHANIE SY: In Minneapolis, Caitlin Smith was struggling just to breathe.

CAITLIN SMITH: This is just crazy.

I have never experienced something like this in my life before.

I was just exposed to the smoke last night for like five minutes on my bike ride home from work, and I felt really sick.

STEPHANIE SY: It's becoming an all-too-familiar summer site in North America, orange-tinted, smoke-filled skies, like the one over Toronto's famous CN Tower.

PAULA ORESKOVICH, Toronto Resident: As soon as I woke up in the morning, I went outside on my terrace, and you could definitely smell the smoke in the air, and it's very hazy out.

STEPHANIE SY: It's the result of hundreds of wildfires burning in Canada and the Northern U.S.

A train crew near Armstrong, Ontario, capturing the intensity of the flames.

MAN: Oh, yes, look over there.

STEPHANIE SY: Mike Flannigan is a professor of wildland fire at Thompson Rivers University in British Columbia.

MIKE FLANNIGAN, Thompson Rivers University: Most of the fires were started by lightning.

There were some human-caused fires, but the majority are lightning-caused.

STEPHANIE SY: In Northern Minnesota, the fires and smoke have forced the evacuation of the Boundary Waters Canoe Area.

Adding to the fire danger is the oppressive heat enveloping a large section of the country.

MIKE FLANNIGAN: It has been hot, dry, and windy in Minnesota and Northwestern Ontario.

Temperatures exceeded 100 degrees Fahrenheit in Ontario and Minnesota.

It was windy.

You get lightning strike.

Fire starts.

It grows rapidly.

Those temperatures are record-breaking, if not record-smashing, for many parts of Ontario and Minnesota.

And it's part of that heat dome that affected much of Western and Central United States in the last few days.

STEPHANIE SY: A weather system in Canada is steering smoke into the U.S., across the Great Lakes and into the Northeast and mid-Atlantic region.

The cluster of darker dots show where fine smoke particles in the air were measured at dangerous levels today.

MIKE FLANNIGAN: What is typically happening with a high-pressure system -- and these heat zones are high-pressure systems -- as the smoke gets injected high into the atmosphere, the winds carry it, but this ridge of air sinks, warms and dry.

And, as it sinks, it takes the smoke with it.

SEAN SCOVILLE, Minnesota Resident: Pretty Armageddon-like out here.

It is that I think I'm more affected by the way it looks than the way it smells.

STEPHANIE SY: That smoke is more than unsightly.

It's unhealthy.

LUKE MONTROSE, Colorado State University: From a health standpoint, there is no such thing as good smoke.

STEPHANIE SY: Luke Montrose is an environmental toxicologist at Colorado State University.

LUKE MONTROSE: We know it gets into the systemic circulation and likely has impacts on other organs.

And we're starting to see some instances where researchers are seeing effects on brain health, mental health and even the reproductive system.

STEPHANIE SY: And Montrose says, when the air is as bad as it is in the Upper Midwest, there's no sure way to escape it.

LUKE MONTROSE: This is catastrophic smoke.

There are very few spaces that you can go to get away from that kind of smoke.

It is going to infiltrate those spaces.

Even people who have great immune systems and are really healthy will start to have respiratory effects from these levels of smoke, particularly if it lasts an entire day.

STEPHANIE SY: Experts say children, the elderly, and people with chronic illnesses are the most vulnerable.

Symptoms may include sore throats, excessive phlegm, coughing, headaches, and brain fog.

Staying indoors, turning on air conditioners and air filters, and wearing N95 masks can help mitigate the risks.

Forecasters are predicting that the smoke conditions will likely ease by Saturday, but with fire weather getting more intense, this won't be the last of such events.

MIKE FLANNIGAN: In Canada and the United States, we're getting warmer because of human-caused climate change.

And the increases in area burn, fire severity are largely, not solely, due to human-caused climate change.

I can't be any clearer than that.

STEPHANIE SY: As for the fires currently burning, experts say they may continue through autumn and maybe even until the first snowfall.

For the "PBS News Hour," I'm Stephanie Sy.