The 1916 Organic Act establishing the National Park Service obliges the government to “leave [the parks] unimpaired for the enjoyment of future generations.” Over the course of the last century, however, there has been one pretty big impairment slowly infiltrating almost all national parks: haze, which not only obscures the views from iconic vistas in the Grand Canyon and Great Smoky Mountains, but can also cause respiratory and other health issues. To combat the problem, the EPA established the Regional Haze Rule in 1999, designed to restore views at 156 national parks and wilderness areas by cutting the emissions from factories and coal-fired power plants that lead to haze.  

The EPA under Donald Trump and individual states are in the process of challenging those rules, which have been updated several times, including a major 2016 revision. Now a new study released today on air pollution and visitation to national parks in the journal Science Advances could inform that debate: Its key findings show that between 1990 and 2014, average ozone levels in the parks were indistinguishable from levels in the United States’ 20 largest metro areas. 

In the study, Ivan Rudik, an environmental economist at Cornell’s Dyson School of Applied Economics and Management, and his colleagues looked at historical ozone readings in 33 parks including Acadia, Yellowstone, and Yosemite. Ozone is a colorless, odorless gas, formed in a complex process by other pollutants, like nitrous oxide and volatile organic compounds, and is one of the major components of haze and smog. Exposure to ozone causes irritation of the throat and lungs and sets off serious medical problems for people with respiratory disease. 

While over the 24-year study period ozone levels in the cities dropped by 13 percent, in the parks, they actually increased from 1990 to the early 2000s. It wasn’t until the implementation of the Regional Haze Rule that the ozone levels began to drop in the parks as well, returning to 1990 levels around 2014.

The team then wanted to see if that ozone pollution impacted whether people chose to visit the parks. Examining monthly visitor records, which the NPS began compiling in 1990, researchers compared visitor numbers to ozone levels, finding that for every one part per billion increase in ozone pollution above the maximum average, there was a 1.6 percent drop in visitation to the parks. Many of those people, they hypothesize, decided to cancel visits based on air-quality warnings issued by the parks. It’s also possible that visitors cut their time in the parks short when exposed to the irritating ozone and increased haze.

But not everyone stayed home during those bad air days; the study also shows that during the 24-year data period, 284 million visitor days, or 35 percent of all trips to parks, took place when the air-quality index was at the moderate risk level. About 9 percent of visits took place when ozone levels were especially high.   

Rudik says that he began the study hoping to publish it in time for the 100th anniversary of the National Park Service in 2016. The study missed that deadline, but publishing it now might be even timelier. Despite several updates and amendments during the 2000s, critics of the original regional haze rule argued that the regulations were too soft, and that only 30 national parks would return to normal visibility by 2064 following the rules’ original regulations. By some estimates it would take hundreds of years for other parks to improve. That’s why, in 2016, the Obama administration put forth a final rule pushing the Haze Rule into a second stage that would require scrubbers and other technologies to reduce emissions from coal plants and other major sources of air pollution. Those rules are now being challenged in court, and the Trump EPA is trying to roll back those regulations and create or protect loopholes in those rules.      

According to Rudik, this study adds new dimensions to the debate. “We don’t go so far as to put dollar values on anything that would feed directly into whether this regulation provides net benefits to society, but we’re showing that people do care about ozone pollution in parks through their observed behavior,” he says. “Some of the arguments that people are making against the Regional Haze Rule are that the benefits are basically zero, that these visibility rules don’t matter that much or maybe the health improvements are overstated. But if you look at what people actually do, they clearly do care.” 

Rudik points out that there is a lot of good evidence showing that improvements in air quality have big impacts on health in general, which could also extend to national park visits. “The dollar value of these health benefits could be very large, especially if you aggregate them over these millions of visits over the years,” Rudik says.

There is other evidence that the Haze Rule is also having the intended impact of improving visibility, though there is still a long way to go. For instance, over the last decade average visibility at Great Smoky Mountains has jumped from 20 miles to 46 miles, a good start, though full visibility is considered 112 miles. Other parks have also seen improvements, gains that, hopefully, won’t disappear into the haze any time soon.