A new study out Wednesday says U.S. national parks have air quality similar to major U.S. cities.
The study published in the journal Science Advances sampled 33 national parks and 20 cities.
It found that "from 1990 to 2014, average ozone concentrations in national parks were statistically indistinguishable from the 20 largest U.S. metropolitan areas."
The study links higher visitor population to higher pollution. Sequoia National Park in California has the highest average ozone concentrations.