The air in US national parks contains just as much ozone pollution as the air in many of the country’s largest cities, according to a study ¹ published on 18 July in Science Advances .

The findings raise important health questions for people who visit the parks, says Ivan Rudik, an environmental economist at Cornell University in Ithaca, New York, and a study co-author. That’s because exposure to ozone pollution can irritate the nose and throat, lead to chest pain or exacerbate conditions such as asthma ² .

Ozone starts to form when nitrogen oxide gases — often emitted by cars — or particulates from coal-fired power plants combine with organic compounds given off by vegetation such as trees, Rudik explains. Sunlight reacts with this mixture to then produce ozone.

Rudik and his colleagues compared ozone levels in 33 national parks and in 20 of the largest US cities between 1990 and 2014. After controlling for weather and the season, the team found that pollution levels in the parks and cities were similar. In fact, before 2000, summer ozone levels in the parks increased before they started to drop. And even then, the decrease was modest when compared to the cities. The researchers aren't sure what caused this pattern.

“That’s something I didn’t expect at all,” Rudik says. This is because an amendment to the Clean Air Act in 1970 generally resulted in cleaner air across the US, he explains.

Although the US National Park Service has tracked ozone levels over national parks since 2008 , they didn't comment on the study's findings.