- [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 call 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.
[wind howls] [upbeat music] But before we start clear cutting 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 VOC or volatile organic compounds, the source of the Blue Ridge Mountains' signature haze.
- 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 taking, and they're also producing VOC.
- [Narrator] Dr. Howard Neufeld is the Blue Ridge Mountains tree expert.
- Want me to hug a tree.
- [Narrator] But his other love is air quality, and the role that plant VOCs is play in it.
VOCs are like human pheromones.
It's thought that plants emit them to communicate distress and to repel or attract insects.
- [Dr Howard] 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 VOC is 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 VOC with the chemicals released from burning fossil fuels, zap them with sunlight and you'll get ozone.
The world's plants emit 500 million tons of VOC 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.
[car hisses] By the mid nineties 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.
- [Dr. Howard] I remember going to the Smokies then those years, and I couldn't see any of the mountains at all.
It was just a white wall, almost like look like white sheet rock.
You wouldn't even know the Smokies were there.
- [Narrator] Here are some photos from the Great Smoky Mountains National Park during the mid nineties.
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 nineties 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.
Manmade 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.
[water drizzles] 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 tailpipes.
- The Clean Air Act is an example of how we can come together as a nation.
We recognize that air pollution knows no political boundaries.
So you have to pass laws that look at the entire airspace.
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 new coal plant has been built in the US.
It's a huge departure from just a decade ago when nearly half the electricity used in the US came from coal.
Today that number is 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.