Skip to Content

Support MinnPost

UCare generously supports MinnPost’s Second Opinion coverage; learn why.

‘Bad air days’ in U.S. national parks are almost as frequent as they are in major cities, study finds

Voyageurs National Park
Researchers analyzed ozone-level data collected in Voyageurs National Park in northern Minnesota.

The ozone pollution in U.S. national parks is “statistically indistinguishable” from that in many of the country’s largest metropolitan areas, according to a study published Wednesday in the journal Science Advances.

The study also found that fewer people visit national parks when ozone levels climb — a sign that health concerns may be keeping people away.   

“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,” write the study’s authors, Iowa State University economists David Keiser and Gabriel Lade and Cornell University economist Ivan Rudik, in an article for The Conversation. 

“We also found that park visits fall on high ozone days — especially during summer and fall, when peak ozone levels typically occur,” they add. 

These findings have important health implications. Even modestly elevated levels of ozone can be a health risk. Ozone can make it more difficult to breathe, particularly for children, the elderly and people with chronic lung diseases, such as asthma, emphysema and bronchitis. It also raises the risk of respiratory infections. 

Inhaling elevated amounts of ozone — even for a short period of time — can affect the heart as well as the lungs. One study, for example, linked just one to two days of exposure to high levels of ozone with an increased risk of heart attack among middle-aged adults.

The harmful effects of elevated ozone levels have also been found to be greater during exercise.

Study details

For the current study, the three researchers analyzed ozone-level data collected during the years 1990-2014 by the National Park Service and the Environmental Protection Agency (EPA) in 33 of the largest national parks in the U.S., including several in the Midwest, such as the Badlands in South Dakota, Isle Royale in Michigan and Voyageurs in northern Minnesota.

They also analyzed similar data for the 20 largest U.S. metropolitan areas. (The Minneapolis-St. Paul area was not included.)

Keiser, Lade and Rudik say were surprised by what the data revealed.

“In 1990, cities had far more bad ozone days on average than national parks,” they write in their article for The Conversation. “But through the decade, summertime ozone and unhealthy ozone days worsened in national parks. By the year 2000, ozone levels in national parks were, on average, very similar to those in metropolitan areas.”


Keiser et al., Science Advances eaat1613, 18 July 2018, CC BY-NC
Trends in ozone concentrations and unhealthy ozone days. (A) Average annual maximum daily 8-hour ozone concentrations. (B) Average summertime maximum daily 8-hour ozone concentrations. (C) Average days per year with maximum daily 8-hour ozone concentrations exceeding 70 ppb. (D) Unhealthy ozone days at Sequoia National Park and the Los Angeles metro area.

Since the early 2000s, ozone levels in both national parks and metropolitan areas have improved — aided, as the researchers point out, by several federal efforts, particularly the EPA’s 1999 Regional Haze Rule, which required states to take steps to improve visibility and air quality in parks and wilderness areas.  

“But bad air days still occur,” the researchers add. “On average, among the locations we studied, metro areas currently have 18 unhealthy ozone days per year, while parks have 16.”

Impact on visitor numbers

Keiser, Lade and Runik also took a look at the National Park Service’s monthly visitor data to see if people were using the parks less during periods of ozone pollution.

They found that a 1 percent increase in ozone concentrations was associated, on average, with about a 1 percent decrease in park visits, even though the air quality tended to be at its worse during the parks’ popular summer and autumn months.

Why are fewer people going to national parks during those “bad air days”?

“We see two possibilities,” write Keiser, Lade and Redik.  “First, visitors may worry about adverse impact on their health. [Parks issue air quality warnings.] Second, visibility is typically poor when ozone levels are high because ozone participates in chemical reactions in the air that can form haze.”

More improvement needed

Although air pollution may be keeping some people away from national parks, it hasn’t deterred millions of others. More than 300 million people visit U.S. national parks each year — and about a third of them go on days when ozone levels are considered unhealthy. 

“This suggests that improving air quality in U.S. national parks could produce significant human health benefits,” write Keiser, Lade and Redik. “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.”

In January, the Trump administration announced it was considering rolling back updates to the Regional Haze Rule — updates that were developed during the Obama administration to reduce air pollution in national parks.

FMI:  You can read the new study in full on the website for Science Advances, which is a publication of the America Association for the Advancement of Science. The article written by the study’s authors for The Conversation can also be read online.

Get MinnPost's top stories in your inbox

Related Tags:

About the Author:

Comments (1)

Cleaning the air in our national parklands

The US Department of Energy Clean Cities program has worked with the National Park Service through its Clean Cities National Parks Initiative to support transportation projects that educate park visitors on the benefits of cutting petroleum use and vehicle emissions. Since 2010, Clean Cities and NPS have partnered on 35 projects to deploy alternative fuel and fuel-efficient vehicles to the road, reduce vehicle idling, and improve vehicle efficiency. These actions support and coincide with the park service efforts to address climate change impacts.

Locally, the Twin Cities Clean Cities coalition worked with the Mississippi National River and Recreation Area to add electric vehicle charging stations to the area.