Some 85 percent of U.S. national parks have levels of air pollution that are hazardous to human health, according to a new report from the National Parks Conservation Association. Some of the country’s most popular parks — such as Sequoia, Kings Canyon, and Joshua Tree — experienced up to two months of dangerously high ozone levels, The Guardian reported.

In 88 percent of parks, air pollution is having a direct impact on sensitive species or habitat, such as stifling tree growth, damaging leaves, and changing soil and water chemistry. And nearly 90 percent suffer from haze pollution, including every single national park in the Lower 48 states. The report estimates that on average, each visitor misses out on 50 miles of scenery due to haze. An estimated 330 million people visit national parks in the United States each year.

Much of the air pollution comes from nearby oil and gas drilling sites, coal mines, power plants, vehicles, and agricultural activities.

“The poor air quality in our national parks is both disturbing and unacceptable,” Theresa Pierno, president and CEO of the National Parks Conservation Association, said in a statement. “Nearly every single one of our more than 400 national parks is plagued by air pollution. If we don’t take immediate action to combat this, the results will be devastating and irreversible.”

The report reinforces the findings of a study published last year in the journal Science Advances, which found that over the last two decades, air pollution in U.S. national parks has rivaled pollution levels in the country’s 20 largest metropolitan areas, including Los Angeles and Houston.

“This is a tremendous amount of exposure to the U.S. population, and one of the things I don’t think too many people think about,” David Keiser, an environmental economist at Iowa State University and the lead author of last year’s study, told The Guardian.