The consequences of America’s swelling wildfire problem are traveling well beyond blackened, ashy forests. They're now tainting the air in cities and towns over vast regions of the western U.S.

Since the mid-1980s, fine bits of air pollution that have been repeatedly linked to heart and lung diseases have diminished in a good portion of the United States — except in an expansive zone of the western part of the country.

In this region, which extends north from Utah to Montana and west from Oregon to Wyoming, the most polluted days — when the air quality is at its most harmful — are getting even worse.

The culprit, say atmospheric scientists in a Proceedings of the National Academy of Sciences study published Monday, is expanding wildfires, which are burning considerably more land than they were three decades ago.

“It’s not just a rural thing anymore,” Anthony Wexler, director of the UC Davis Air Quality Research Center who took no part in the research, said of the bad air.

The red area shows the zone in the U.S. where the worst days of particulate matter pollution (PM 2.5) are getting worse. Image: Image adapted from McClure C.D. and Jaffe D.A/PNAS

Wexler, speaking from smoky Aspen, Colorado, near where blazes had recently charred the Rocky Mountains, noted these tiny particles — which at their largest are still 30 times thinner than a human hair — don’t just transform blue skies into unsightly hazes.

“It’s also a killer,” said Wexler.

A recent 10-year-long Environmental Protection Agency study observed some 6,000 people and found exposure to this particulate matter, known formally as PM 2.5 (for particulate matter less than 2.5 microns across), accelerated the build-up of plaque inside the walls of blood vessels, which leads to heart attacks, strokes, and even death.

A size comparison of Particulate Matter 2.5 (PM 2.5) pollution versus human hair and sand particles. Image: epa

“Lots of people have been predicting that this [harmful air pollution] would be happening in the next decades — but we’re starting to see it now,” Dan Jaffe, a study coauthor and University of Washington atmospheric researcher, said in an interview.

The worst polluted days, when this tiny pollution is at its highest, comes out to around seven extremely unhealthy days a year.

“At high levels, it affects everybody,” said Jaffe, even people in fine health.

“In short, it’s harder to breathe.”

And wildfires aren’t expected to diminish anytime soon. They’re a result of a hotter climate, which parches the land and provides fires with more fuel. The recent return of large blazes to California’s wine country, for example, were largely stoked by hot temperatures, turning abundant vegetation to tinder.

“We’re right in the middle of climate change,” Gabriele Pfister, deputy director of the National Center of Atmospheric Research’s atmospheric chemistry lab who had no involvement in the study, said in an interview.

A wildfire burns in southern Colorado on June 28. Image: Uncredited/AP/REX/Shutterstock

“There’s yet another record, and yet another record, and yet another record,” said Pfister, referring to the increased frequency of record-breaking heat as average temperatures continue to climb. “Climate change is definitely not getting better.”

All three atmospheric scientists, however, pointed out that that climate change certainly isn’t the only culprit in increased wildfires over the last few decades.

The U.S. government’s historic mismanagement of forests, specifically by not allowing fire-prone underbrush to burn away in typical fires, has given large fires more fuel to burn, and an upper hand. But higher temperatures are simply the elephant in the room.

“It’s hotter than it used to be,” said Wexler.

And this heat, unfortunately, helps drive a vicious cycle of more heat.

As more forests burn, they don’t just release bounties of tiny bits of matter into the air; they also fill the atmosphere with more carbon dioxide, a potent greenhouse gas that contributes “substantially to the global greenhouse effect,” according to the U.S. Forest Service. And more greenhouse gases mean more trapped heat in Earth’s atmosphere.

“It’s a circle,” said Pfister.

A plane dumping fire retardant near Idaho's 2016 Pioneer Fire. Image: U.s. Forest Service/Kari Greer

As the planet continues its accelerated warming trend, this means government air quality warnings in communities long distances from burning forests will likely become more frequent, she said.

“Take it seriously. Stay indoors,” said Pfister.

“Sometimes it’s pretty healthy to be a couch potato."

Bad air may make for a grimmer future, but there is the ever-conspicuous solution: Slashing the greenhouse gases emitted into the atmosphere – which requires the transformation of how nations generate electricity and fuel transportation.

“We can do something about it by addressing climate change,” added Pfister.

Though in the short term, the West will burn, like it is this summer.

“It’s going to be a tough season,” said Pfister.