If you leave your car behind and join a ranger-led hike in Southwest Colorado’s Mesa Verde National Park, you’ll find yourself at a spot where the scrubby pinyon-juniper forest drops off into a sandstone chasm, revealing a maze of 800-year-old stone dwellings wedged beneath an overhang in the canyon wall. They’re so well preserved that it’s easy to imagine you’ve stepped back in time; that nothing has changed in this high desert landscape since the Ancestral Puebloans built these chambers in the 12th century.

But there’s a modern problem plaguing Mesa Verde and dozens of other national parks: air pollution. Mesa Verde lies downwind of several coal-fired power plants, which release nitrogen, mercury and sulfur into the air. Huge natural gas fields lurk to the south, belching methane. And as nearby towns and cities grow, everyday activities like driving increase levels of harmful ozone. Hundreds of years ago, Ancestral Puebloans would have been able to look out from Mesa Verde and see views that stretched 170 miles. Today, haze reduces those views to just 66 miles on the worst days.

“Air pollution knows no boundaries,” says Ulla Reeves, Clean Air Campaign Manager with the National Parks Conservation Association, a nonprofit that advocates on behalf of parks. “It reaches many, many miles away from the source.” In Mesa Verde, one of the sources of pollution is Las Vegas, 500 miles away.

In an analysis last year, the NPCA found that even parks with the most protection under the Clear Air Act—icons like Mesa Verde, Everglades, Yosemite, Acadia and Sequoia—continue to experience pollution that can affect wildlife and human health, as well as the climate. According to the National Park Service’s own data, ozone levels on the peaks of the Great Smoky Mountains, for example, are nearly twice those in nearby cities like Atlanta. Up to 90 percent of black cherry trees in the park (depending on location) have sickly yellow leaves and other signs of ozone damage, and visitors with asthma can have trouble breathing. In California, Sequoia and Kings Canyon national parks regularly have ozone pollution that exceeds the 70 parts per billion standard set by the Environmental Protection Agency.

The federal government has long recognized that air pollution doesn’t stop at park borders. In 1999, the EPA created a regulation called the Regional Haze Rule, designed to return visibility in 156 national parks and wilderness areas back to “natural” conditions by cutting emissions from polluters like coal-fired power plants. Though the rule only tackles visibility, “the pollutants that affect visibility can also affect ecosystems and human health,” says John Vimont, chief of the research and monitoring branch of the National Park Service’s Air Resources Division.

The rule has played an important role in getting some facilities to adopt cleaner technologies—over the last 10 years, average visibility in Great Smoky Mountains National Park has risen from 20 miles to 46 miles, says Reeves. But there’s still a long way to go. Visibility in Great Smoky Mountains should be 112 miles on the best days. Part of the reason for the slow progress is because the rule is largely interpreted and carried out at the state level, rather than by federal agencies, and many states have struggled to muster resources and meet deadlines.

That’s why the EPA is currently working on a series of changes meant to strengthen the Regional Haze Rule. The changes will force states to keep more robust data on their progress and submit regular plans to ensure they’re meeting legal requirements and cutting emissions. At the same time, the changes allow states even more time to implement their next round of plans.

Even if the Regional Haze Rule is strengthened, though, it’ll still take a long time for the air in national parks to return to pre-industrial quality. Under standards imposed a decade ago, the NPCA estimates that the soonest that goal could be achieved is the year 2064; 30 out of 157 national parks are predicted to return to natural conditions by that year. Others, like Arizona’s Saguaro National Park, might take much longer—750 years. Again, these dates don’t take into account the latest changes, which could speed up recovery time. But they’re still a sobering reminder that even in some of the most protected landscapes on the planet, the effects of human activity can linger well beyond our own lifetimes.

In Mesa Verde, natural resource manager George San Miguel is keenly aware of the effect that air pollution has on the park’s visitors. Airborne nitrogen and sulfur is deposited into the soil, which leads to more invasive weeds and fewer native grasses. Methane hovering overhead accelerates climate change. And then, of course, there are the views.

“One of the things we try to instill in visitors is a sense of going back in time,” San Miguel says. “We want visitors to immerse themselves in the past; to put themselves in the sandals of the Native Americans that lived here, so to speak.” To do that, he explains, you need to be able to see a long distance, because the Ancestral Puebloans likely used distant desert towers as navigation aids. Until Mesa Verde’s natural visibility is restored, visitors remain solidly planted in the 21st century.