(narrator) Look to the horizon on a clear day in western North Carolina, and you'll see a soft, dreamy blue haze.

These mountains are so blue, the Cherokee tribes native to the region called these mountains "Shaconage," "the Place of Blue Smoke."

All this dense vegetation is breathing.

And while trees are inhaling pollutants like carbon dioxide, they're also exhaling compounds that could be dangerous to your health.

[exciting guitar music] But before we start clearcutting the forest, let's back up.

The Blue Ridge Mountains are home to at least 150 native tree species.

Compare that to Great Britain, which only has less than a quarter of that.

The reason this area is so diverse is because it was too far south to be covered by glaciers in the last two ice ages.

So these mountains became a refuge for plants and animals escaping the ice.

And what you can't see is that this rich assortment of trees are all emitting microscopic VOCs, or volatile organic compounds, the source of the Blue Ridge Mountains' signature haze.

[birds chirping] (speaker) So there are many multiple layers of leaves in the forest.

They're all taking in CO2, they're all giving off moisture, and they're also producing VOCs.

(narrator) Dr. Howard Neufeld is the Blue Ridge Mountains' tree expert.

(Neufeld) You want me to hug a tree?

[chuckles] (narrator) But his other love is air quality and the role that plant VOCs play in it.

VOCs are kind of like human pheromones.

It's thought that plants emit them to communicate distress and to repel or attract insects.

(Neufeld) And those may act as signals to nearby plants that, hey, something is munching on me.

Maybe you'd like to be protected.

(narrator) The smell of freshly-cut grass?

That's a VOC, and so is that classic Christmas tree smell.

Here in the mountains, fine mists of VOC particles scatter blue and violet wavelengths more than the other colors of the light spectrum.

And since the color receptors in our eyes are more sensitive to blue than violet, we perceive a blue haze bathing the mountains.

But VOCs do more than just scatter blue.

This is kind of an environmental paradox, but as trees soak up CO2, they also emit one of the key ingredients of ground-level ozone, a potentially dangerous pollutant.

Mix VOCs with the chemicals released from burning fossil fuels, zap 'em with sunlight, and you'll get ozone.

The world's plants emit 500 million tons of VOCs every year.

And that can make a lot of ozone.

Higher up in the atmosphere, ozone is a good thing, shielding the planet from ultraviolet radiation.

But ground-level ozone can cause breathing problems-- the kind you most often hear about in big cities, not national forests and parks.

By the mid '90s, ozone levels in parts of the Blue Ridge Mountains rivaled cities like New York and Atlanta.

But the trees alone weren't to blame.

It was the trees plus a whole lot of imported pollution from coal country.

Starting in the 1970s, scientists noticed that the Blue Ridge Mountains started looking less blue.

(Neufeld) I remember going to the Smokies in those years, and I couldn't see any of the mountains at all.

It was just a white wall almost.

Like, it looked like white sheet rock.

You wouldn't even know the Smokies were there.

(narrator) Here's some photos from the Great Smoky National Park during the mid '90s.

Not only is there very little blue, but you can barely make out a mile in the distance.

When the park first opened in the 1930s, visitors could make out landmarks 65 miles away on a clear day.

Here's the problem.

The chain of Blue Ridge Mountains is perfectly situated downwind of some of the biggest coal-producing areas in the country, like the Tennessee and Ohio River Valleys.

Winds carried about a third of those emissions over the Blue Ridge Mountains.

Back in the '90s, before modern regulations, power plants could belch out as much nitrogen oxides and sulfur dioxide as they wanted.

These same pollutants are released from car tailpipes.

So the problem was compounded by the 9 million tourists that flood mountain roads every year.

Not only were the views disappearing, the air was becoming dangerous to breathe.

The combination of jam-packed roads, tree-packed forests, and unfettered coal production made this mountain oasis an unlikely health hazard.

And tourists were often disappointed by the views.

Man-made compounds create a haze too, just not a pretty haze like plant VOCs do.

Instead of scattering hues of blue, they scatter hues of gray and yellowish-brown.

There are so few environmental stories with happy endings, but this is one of them.

As part of the Clean Air Act, states put limits on how much sulfur dioxide and nitrogen dioxide could be released from power plants and tail pipes.

(Neufeld) The Clean Air Act is an example of how we can come together as a nation.

We recognized that air pollution knows no political boundaries.

So you have to pass laws that look at the entire air space.

That's why the Clean Air Act is a national clean air act.

(narrator) And as natural gas and renewables get cheaper to produce, coal-fired plants are slowly powering down across the country.

Since 2015, only one new coal plant has been built in the U.S.

It's a huge departure from just a decade ago, when nearly half the electricity used in the U.S. came from coal.

Today, that number's more like 20%.

That change is made tangible by how many miles you can see in the distance on a clear day in the Blue Ridge, and, of course, in the blue on the horizon.

Accessibility provided by the U.S. Department of Education.

Hey, guys.

Joe here.

Thank you so much for watching the first episode of our new series, Overview.

Make sure you're subscribed here on PBS Terra so that you don't miss an episode.

We're going to keep brining you amazing stories of science and nature from a different perspective.

And I'll be here to guide you along the way.

See you next time, or "hear" you next time.

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