The U.S. National Park Service (NPS) announced Monday that it was reinstating its nationwide face mask mandate.

In a release, the agency cited recent science and U.S. Centers for Disease Control and Prevention (CDC) guidance.

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"Following the latest science and guidance from the Centers for Disease Control and Prevention, the National Park Service is immediately requiring visitors, employees and contractors to wear a mask inside all NPS buildings and in crowded outdoor spaces, regardless of vaccination status or community transmission levels," NPS wrote

The order will be in effect "until further notice" and applies to all NPS buildings and public transportation systems in addition to outdoor spaces where physical distancing cannot be maintained. 

"Visitors to national parks are coming from locations across the country, if not across the world. Because of this, and recognizing that the majority of the United States is currently in substantial or high transmission categories, we are implementing a service-wide mask requirement to ensure our staff and visitors’ safety," NPS Deputy Director Shawn Benge said in the release.  

"Being vaccinated is the most effective way to protect yourself and your loved ones from the dangers of the coronavirus. Masking in addition to being vaccinated will help prevent the spread of new variants and protect those who are more at risk of severe disease. This simple act of kindness allows us to be safe while we continue to enjoy the benefits of our national parks," Capt. Maria Said, NPS Office of Public Health epidemiologist and a U.S. Public Health Service Commissioned Corps member, noted. 

Visitors to the 423 national parks are advised to monitor NPS websites and social media channels for further details on operations prior to their visit.

This requirement comes on the heels of a previous February 2021 mandate linked to a January presidential executive order.

The CDC's counsel on masking has changed, as the coronavirus – the delta variant in particular – continues to spread across the country and while more Americans are getting vaccinated. 

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In July, the agency reversed course on some guidelines, recommending that even vaccinated people return to wearing masks indoors in parts of the U.S. where the delta variant is rapidly spreading.

The CDC notes that outdoor activities are still the safer choice.