▶ Watch Video: Bison attacks family in Yellowstone National Park

Yellowstone National Park will now require visitors 2 years and older to wear face masks in its indoor facilities, citing a rise in COVID-19 cases. Several other parks have similar restrictions, and the National Park Service has implemented a mask mandate on public transit in all of its parks.

Of the 10 most-visited national parks of 2021, four now have indoor mask requirements: Yellowstone, the Grand Canyon, Yosemite and the Grand Tetons. All visitors must wear a mask, regardless of their vaccination status.

The National Park Service says masking requirements vary based on the park and its local COVID conditions. If an area is designated as having a high COVID community level by the Centers for Disease Control and Prevention, the park must enact restrictions. The community level is based on the number of hospital beds being used, hospital admissions and the total number of new cases, the CDC says.

According to the Mayo Clinic, Teton County, where the Grand Tetons and portions of Yellowstone are located, has the highest COVID rate in Wyoming. It is reporting an average of 16 cases a day, and there are 70 infections per 100,000 people. Yellowstone spans three states and five counties, three of which have a high COVID community level, according to the CDC.

Mariposa County, California, which is home to Yosemite, is seeing one of the state’s highest COVID rates, with 53.8 cases per 100,000 people. And Coconino County, where the Grand Canyon is located, is reporting 38.8 cases per 100,000 people, data from the Mayo Clinic shows.

Maine’s Acadia National Park says face masks are optional, and the Rocky Mountain National Park advised that visitors mask up while on its public transit and in indoor facilities, but it does not have a requirement. The Colorado park spans three counties, one of which has a high COVID community level.

Zion, the Great Smoky Mountains, Indiana Dunes and Glacier round out the top 10 most-visited national parks in the U.S. — and they do not, at this time, have any COVID restrictions in place. However, both Zion and Glacier span counties that the CDC says are at a medium COVID community level, suggesting that mask mandates could be enacted if COVID cases continue to rise there.

If you are planning to visit a national park this summer, the NPS suggests you visit the park’s website to learn more about possible COVID requirements.

Yellowstone’s announcement comes after a tumultuous start to the summer — and tourist — season. Last month, the park temporarily closed after historic flooding damaged roadways, homes and other structures within the park. Most of the park’s entrances have since reopened, but parts of the north loop are still closed. Also, three park visitors have been gored by bison since May, with the latest two incidents happening within three days of each other.

Yellowstone is celebrating its 150th anniversary this year.