Air pollution in national parks such as Yellowstone, Yosemite and Acadia is as bad as some of the nation's largest metropolitan areas, and it may be causing tourists to cut visits short or avoid going all together, according to a new study.
"Visits are lower on high-ozone days," said study co-author Ivan Rudik of Cornell University. "This doesn't completely prove that ozone causes declines in visitation, but it is highly suggestive."
Researchers from Iowa State and Cornell universities say 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 were more of a worry than poor visibility.
“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,” Rudik said.
Ground-level ozone, also known as smog, forms on warm, sunny days and is made worse from chemicals that come from car and truck tailpipes and from power plant and industrial smokestacks.
This ozone is harmful to human health because it can exacerbate asthma attacks and cause difficulty breathing. It's different than the "good" ozone up in the stratosphere, which protects life on Earth from the sun's harmful ultraviolet rays.
Some of the smog might be produced in the parks by cars or it might blow in from urban areas, Rudik said.
Researchers studied ozone levels in 33 of the largest national parks in the U.S. They found from 1990 to 2014, average ozone concentrations in national parks were statistically indistinguishable from those of the 20 largest U.S. metropolitan areas, such as New York, 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 all but two years since 1996.
Although pollution levels have declined nationwide since air quality regulations went into effect in the 1990s, air quality in many national parks remains unhealthy for sensitive groups for up to three weeks a year on average, the study said.
The National Park Service has raised concerns over high levels of ozone and poor visibility in national parks, which receive more than 300 million visitors each year.
The study was published in Science Advances, a peer-reviewed journal of the American Associated for the Advancement of Science.