Between the attacks on national monuments, relaxed safeguards against offshore oil drilling, weakened wildlife protections, reversed plastic bottle bans, looming budget cuts and increased user fees, America's national parks have had a rough go of it lately.

And adding to the National Park Service's ongoing bad news pile-on, a new study published in the journal Science Advances has found that the air quality in some of the most visited parks — Acadia, Yellowstone, Yosemite and Great Smoky Mountains among them — isn't all that much better (and in some cases worse) than in America's 20 largest metropolitan areas.

In fact, from 1994 to 2014, the average concentration of smog-forming ground-level ozone in certain national parks was found to be "statistically undistinguishable" from cities like Houston, Los Angeles, Chicago and Dallas-Fort Worth. So much for packing up the family and fleeing the stifling, smog-choked big city for the crisp and unsullied air of a national park.

Authored by researchers from Iowa State University and Cornell University, the sobering study focuses strictly on ozone, the most widely monitored pollutant in national parks. Ozone can be super-beneficial to humans, specifically when found higher several miles above the Earth's surface where it acts as a hole-y stratospheric helper to block harmful ultraviolet rays. But at ground level, ozone is undoubtedly "bad" — a health-compromising, smog-generating gas formed when two common pollutants, nitrogen oxides and volatile organic compounds (VOCs), react in sunlight.

As the study's authors note, the presence of ground-level ozone in national parks is correlated with damaged vegetation and decreased visibility along with the well-known respiratory health woes — chest pain, coughing, shortness of breath and on — that inhaling ozone can cause. Exposure to the lung-irritating gas is heightened when engaging in outdoor physical activity on hot days — the kind of days that Americans flock to national parks and other protected spaces to get their outdoor recreation on.

Traffic in Yellowstone National Park While vehicular traffic contributes to poor air quality within national parks, pollutants blown in from elsewhere are mostly to blame. (Photo: Yellowstone National Park/flickr)

"Even though the national parks are supposed to be icons of a pristine landscape, quite a lot of people are being exposed to ozone levels that could be detrimental to their health," study co-author Ivan Rudik tells USA Today.

In total, roughly 80 million people were exposed to potentially harmful levels of ozone while visiting national parks from 1990 to 2014. About 35 percent of all park visits occur on high-ozone days.

But are other park-goers heeding air quality warnings and staying home when national parks are at their smoggiest?

While the study finds a "robust, negative relationship" between park visitation numbers and ozone concentrations levels, there is some skepticism amongst other researchers who aren't entirely convinced that potential park-goers are abandoning their travel plans — coveted campsite reservations, included — due to less-than-ideal air quality reports.

"Correlation is not causation," Joel Burley, an air pollution scientist at St. Mary's College in California who was not involved with the study, argues to Scientific American. "How many visitors are actually changing their behavior after checking air quality?"

Burley goes on to call the study "fascinating" but points out that it doesn't truly measure the impact that air quality alerts have on visitor numbers at 33 of the largest and most beloved units of the National Park Service.

Acadia National Park While it may not look like it, Acadia National Park in Maine is the most polluted national park in the Northeast. (Photo: Peter Rintels/flickr)

Sequoia, Joshua Tree stand out (and not in a good way)

Whether or not national park visitors are indeed steering clear when things take a turn for the hazy, the study does establish a troubling trend. When measuring ozone pollution by annual trends in maximum daily eight-hour ozone concentrations and the number of "exceedance days" when the maximum daily concentrations reach levels deemed "unhealthy for sensitive groups" by the EPA, it becomes clear that while cities were once the worst offender, from 1990 onwards, national parks quickly caught up to the point where they were about the same. And in this specific instance, about the same means just as polluted.

Per the study:

Summertime ozone concentrations and the average number of unhealthy ozone days are nearly identical in national parks and metropolitan areas starting in the 2000s. Average summer ozone concentrations decreased by more than 13 percent from 1990 to 2014 in metropolitan areas. Meanwhile, summertime ozone levels increased in parks from 1990 to the early 2000s and decreased thereafter to 1990 levels by 2014. Over this same period, the average number of exceedance days in metropolitan areas fell from 53 to 18 days per year. National parks saw less progress, where average exceedance days decreased from 27 to 16 days per year.

The study's authors go on to note that California's Sequoia National Park has the highest average ozone concentrations of any national park. It has surpassed the metropolitan area with the highest average ozone concentrations, Los Angeles, in exceedance days nearly every year since 1996.

From 1993 to 2014, Los Angeles had 2,443 days in which smog levels surpassed federal safety standards. Sequoia National Park, along with adjacent Kings Canyon National Park, experienced 2,739 red-alert smog days over the same period.

That's right ... it's hard to fathom but Sequoia National Park, a 404,000-acre wonderland of towering trees and even more precipitous peaks perched high in the south Sierra Nevadas, experiences more polluted days than downtown Los Angeles.

Another NPS unit in California with exceedingly high ozone levels was Joshua Tree National Park, which racked up a total of 2,301 days in which the air quality was certifiably unhealthy due to ozone.

As CNN observes, this is roughly on par with America's largest metro area, New York City. From 1990 to 2000, Joshua Tree had an average of 105 unhealthy air days per year while the Big Apple had an annual average of 110. Both New York and Joshua Tree saw those annual averages drop from 2001 to 2014 although the average in New York plummeted more significantly to 78. Joshua Tree still hovers about 100.

This supports the conclusion that while the number of bad air days is on the decline in both cities and national parks, the drop is more dramatic in cities where anti-pollution efforts are several steps ahead of similar efforts in parks.

Joshua Tree National Park Renowned for its otherworldly beauty, Joshua Tree National Park experiences a surprising number of days with markedly poor air quality. (Photo: Joseph/flickr)

Park pollution: Blown in from somewhere else

So how in the world did jaw-droppingly gorgeous national parks like Sequoia and Joshua Tree end up more smoggy than America's two most sprawling, heavily populated metro areas?

As mentioned, a noxious bouquet of chemical pollutants forms ground-level ozone with sunlight acting as the catalyst. Swept up by the wind, these pollutants, originating from factories, refineries, power plants, agricultural operations, interstates and, yes, cities, are blown far and wide and eventually wind up in far-flung, otherwise pristine areas like national parks. So while some blame can be placed on the NOx emissions that come from heavy in-park automobile traffic, the components that cause ozone most often originate from somewhere else.

"Ozone takes time to form in the atmosphere — it's not emitted directly by cars or power plants," Dan Jaffe, an atmospheric scientist at the University of Washington, tells Scientific American. He notes that it's not surprising that national parks have tend to be hazy hotbeds of ozone. "We've known for years that ozone is higher outside of cities," he says.

So in the case of Sequoia and Joshua Tree national parks, where exactly are all the ozone-forming pollutants being blown in from?

At Sequoia/King's Canyon, the culprit is the farms and industries of California's Central Valley and its major population centers including Fresno and Bakersfield. While located further afield, the San Francisco Bay Area is also a contributor to ozone within these national parks, which are administered by the NPS as a single unit. Pollution in Joshua Tree, as one might suspect, is blown in straight from the Los Angeles Basin.

As Annie Esperanza, an air quality specialist for Sequoia and King's Canyon national parks, explains to LAist, ozone in remote areas tends to linger more than it does in cities due to a lack of vehicle emissions. In major cities, where cars tend to be on the road at all hours even if at a lesser volume at night, NOx emissions help to break down the same ozone that it helped create during the daylight hours. In effect, much of the damage inflicted during the day is reversed overnight. In national parks and other remote areas, however, a relative absence of after-dark traffic compared to cities means no nighttime NOx to help clean the air.

Sequoia National Park Sequoia National Park had more smog-heavy bad air days from 1993 through 2014 than the Los Angeles metro area. (Photo: H Matthew Howarth/flickr)

Smog-busting regulations at risk

California's national parks like Sequoia/King's Canyon, Joshua Tree and Yosemite are facing a potentially rough road ahead when it comes to boosting their annual number of smog-free days.

As reported by Bloomberg, the Trump administration plans to soon do away with the Obama-era Clean Air Act waiver that permitted California to more aggressively regulate automobile greenhouse gas emissions than the federal government and, in turn, better protect its citizens from air pollution. The decidedly "pro-smog" move, described as "the biggest regulatory rollback yet" by the Trump-era EPA if it indeed proves successful, would also put a hamper on the Golden State's push toward electric vehicle adaptation.

"California has done the math, and it's concluded that the only way to meet both its greenhouse gas goals and its ozone targets is to move away from fossil fuel-based transportation," Paul Cort, an attorney for California-based environmental charity Earthjustice, tells Bloomberg. "The law is very clear about California's authority to set these standards, and for the EPA to try to narrow it now means they have an uphill battle."

In May, California and 16 other states along with the District of Columbia sued the Trump administration in an effort to halt the dismantling of climate change-curbing emissions standards. The Regional Haze Rule, which was established in 1999 by the EPA as a means of improving visibility in smog-plagued national parks, is also on the chopping block.

Whatever transpires in California over the coming months in the fight to uphold stringent emissions standards, it's worth noting that there are national parks in other states where ozone is less of an issue. They exist! As study co-author Rudik tells Scientific American, there are "plenty" of parks where visitors — particularly city-escaping urbanites — can breath a bit easier knowing that they're not inhaling ozone. Two low-ozone wilderness areas mentioned by Rudik include Olympic National Park in Washington state and Montana's majestic Glacier National Park.

If you're curious about the air quality situation in a national park that you're planning to visit, 48 of them have handy-dandy Park Air Profiles compiled by the the National Park Service. Judging from a cursory look at the profiles, intrepid clean air-seekers might want to consider booking a trip to Alaska's massive (and massively remote) Denali National Park. For those looking to stick to the contiguous United States, Petrified Forest National Park (Arizona), Arches National Park (Utah), Grand Teton National Park (Wyoming) and Voyageurs National Park (Minnesota) are among the parks known to have "relatively" or "moderately" good air quality.

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.