Air In Some National Parks As Polluted As That In Cities In Summer

That lung-straining hike to Mills Lake (~10,000 feet) in Rocky Mountain National Park could fill your lungs with unhealthy levels of ozone/Kurt Repanshek file

You're hiking to the top of Cadillac Mountain in Acadia National Park, and at one point you pause to both take in the scenery and take in big gulps of air. Fresh air, you believe. Unfortunately, in summer those gulps likely contain unhealthy amounts of ozone.

The National Park Service has been warning us of these conditions for some time, but now a study says ozone levels found in 33 national parks are equal to those found in many cities, and they are affecting visitation. The good news is that ozone levels in both cities and parks have decreased; in parks the decline started in 1999 with passage of the Regional Haze Rule adopted by the Environmental Protection Agency.

But improvements in ozone levels have been greater in cities than in parks included in the study by researchers at Cornell University and Iowa State University. 

"Average annual ozone concentrations in national parks are statistically indistinguishable from those in metropolitan areas for most of our sample," they wrote in the study, Air Pollution and Visitation at U.S. National Parks, released Wednesday in Science Advances. "Summertime ozone concentrations and the average number of unhealthy ozone days are nearly identical in national parks and metropolitan areas starting in the 2000s."

From 1990 until the early 2000s, the number of days in metropolitan areas where ozone levels exceeded EPA standards dropped from 53 per year to 18. In the national parks surveyed, those days dropped from 27 to 16.

"Sequoia National Park follows a similar trend in exceedance days as the metropolitan area with the highest ozone concentrations, Los Angeles," the researchers found. "Notably, exceedance days at Sequoia have surpassed those in Los Angeles in all but two years since 1996."

And while the number of days where daily 8-hour ozone levels have exceeded EPA guidelines of 70 ppb, a level that the agency considers "unhealthy for sensitive groups," such as the young, the elderly, and those with respiratory problems, have been declining, the researchers found that park visitation dips 1.5 percent (fall months) to 2 percent (summer months) when ozone levels exceed that 70 ppb ceiling by 1 ppb.

"Nearly 9 percent of visitor days (77 million visitor days) occurred at parks when ozone levels exceeded 70 ppb," the researchers found. "A large body of evidence finds that ozone exposure increases hospitalization rates, respiratory symptoms, and mortality. These adverse effects from exposure are greater during exercise. The number of national park visits suggests potentially large human health benefits to further air quality improvements."

National parks included in the study were Acadia, Arches, Badlands, Big Bend, Black Canyon of the Gunnison, Canyonlands, Carlsbad Caverns, Channel Islands, Congaree, Cuyahoga Valley, Death Valley, Denali, Everglades, Glacier, Grand Canyon, Grand Teton, Great Basin, Great Sand Dunes, Great Smoky Mountains, Guadalupe Mountains, Haleakala, Hawai'i Volcanoes, Isle Royale, Joshua Tree, Lassen Volcanic, Mammoth Cave, Mesa Verde, Mount Rainier, North Cascades, Olympic, Petrified Forest, Pinnacles, Redwood, Rocky Mountain, Saguaro, Sequoia, Shenandoah, Theodore Roosevelt, Virgin Islands, Voyageurs, Wind Cave, Yellowstone, Yosemite, and Zion.