Ozone levels in national parks are now similar to cities, why does it matter

Most Americans associate U.S. national parks with pristine environments that represent the very best of nature. In the 1916 law that established the National Park Service, Congress directed the new agency to “conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations.”

But over the past century it has become increasingly hard to protect the parks from impacts of human activities outside their boundaries. In 2015 the National Parks Conservation Association, a national advocacy group, released a blistering report giving many popular parks poor grades for unhealthy air, haze and impacts from climate change.


In a study just published in Science Advances, we analyzed levels of ozone, the most widely monitored pollutant in parks, and their impact on visits to 33 national parks from 1990 to 2014. The sites we studied included popular parks such as Acadia, the Grand Canyon, Great Smoky Mountains, Joshua Tree, Sequoia and Kings Canyon and Yosemite. We found that while cities once had more “bad air days” with unhealthy ozone levels than national parks, today parks and metro areas have virtually the same number of unhealthy ozone days per year on average. We also found that park visits fall on high ozone days – especially during summer and fall, when peak ozone levels typically occur.

Major sources of park air pollution include power plants, automobiles and industrial facilities. Unlike other pollutants emitted directly from these sources, like sulfur dioxide or lead, ozone is a secondary pollutant. It forms in the atmosphere through chemical reactions between nitrogen oxides, volatile organic compounds and sunlight. Nitrogen oxides originate from the usual urban pollution sources, but biogenic sources like trees are actually the largest source of volatile organic compounds, above industrial sources and cars.

To see whether visitors responded to changing ozone levels in the parks, we matched monthly visitation data from the National Park Service with various measures of monthly average ozone levels. We found that a one percent increase in ozone concentrations was associated with approximately a one percent decrease in park visitation on average. This response was most pronounced during summer and fall, when both visitation and average ozone levels are highest.

Across the United States, ozone levels declined by 31 percent between 1980 and 2016. But city residents and tourists in national parks still experience unhealthy ozone levels for two to three weeks per year. Exposure to high ozone levels may be particularly harmful in national parks, since health effects from ozone are greater during exercise, such as hiking, backpacking or rock climbing.

Although we found that some people decrease their visits during unhealthy days, we still observed that since 1990, nearly 80 million visitor days have occurred during high ozone periods. This suggests that improving air quality in U.S. national parks could produce significant human health benefits. We hope that state and federal policy makers will weigh these benefits of improved air quality along with their costs as discussions move forward on air pollution regulations.