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Anyone traveling to a national park in the hope of enjoying some fresh air may be disappointed.

A study published in Science Advances looked at air pollution levels in Yellowstone, Grand Canyon, Yosemite, Glacier and 29 other parks, and found that they differed little from those in the country’s 20 largest metropolitan areas.

The study focused on ozone levels, which are used in national parks to notify visitors of air quality conditions. Using data gathered from 1990 to 2014, they estimated annual trends in eight-hour ozone concentrations in each park, the same standard used by the Environmental Protection Agency. They also counted the number of days in a year when ozone concentrations reached 70 parts per billion, a level the E.P.A. considers unhealthy for children and older people, and for anyone with lung disease.

Summertime ozone levels and number of unhealthy days in parks and cities were almost identical. In Sequoia National Park, the most polluted in the country, unhealthy days surpassed those in Los Angeles in all but two years since 1996.

“Since 1990, 35 percent of all visits to parks occurred when ozone exceeded 55 parts per billion, the ‘moderate’ level of air pollution,” said the senior author, Ivan Rudik, an assistant professor of economics at Cornell. “We’re trying to document some facts, contribute some numbers to the discussion.”