Air pollution in national parks such as Yellowstone, Yosemite and Acadia is as bad as some of America's largest cities, and the foul air may be causing tourists to cut visits short or avoid going at all, according to a study released Wednesday.
“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,” said study co-author Ivan Rudik of Cornell University.
Researchers from Iowa State and Cornell said visitor numbers dropped almost 2 percent when ozone levels went up even slightly and at least 8 percent during months with bad air quality. Health concerns for visitors were more of a worry than poor visibility.
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"Visits are lower on high-ozone days," Rudik said. "This doesn't completely prove that ozone causes declines in visitation, but it is highly suggestive."
Ground-level ozone, also known as smog, forms on warm, sunny days and is made worse from chemicals from car and truck tailpipes and from power plant and industrial smokestacks.
This ozone can exacerbate asthma attacks and cause difficulty breathing. It differs from the "good" ozone in the stratosphere, which protects life on Earth from the sun's harmful ultraviolet rays.
Some of the smog in the parks might come from cars or blows in from urban areas, Rudik said.
Researchers studied ozone levels in 33 of the largest national parks. From 1990 to 2014, average ozone concentrations in national parks were nearly the same as in the 20 largest U.S. metropolitan areas, including New York City, Los Angeles and Chicago.
At Sequoia National Park, about 200 miles north of Los Angeles, the study found more bad ozone days in the park than in the city in every year since 1996, except for two years.
Although pollution levels have declined nationwide since air quality regulations went into effect in the 1990s, air quality in many national parks is still considered unhealthy for up to three weeks a year, especially for people sensitive to pollution.
The National Park Service has raised concerns over high levels of ozone and poor visibility in its parks, which attract more than 300 million visitors each year.
The study was published in Science Advances, a peer-reviewed journal of the American Association for the Advancement of Science.