The health agency for Adams, Arapahoe and Douglas counties voted on Wednesday to institute a mandatory face covering order for residents, while providing localities the ability to opt out.

Tri-County Health Department’s Board of Health voted 5-4 to give the executive director authority to draft a mandate for wearing masks indoors in public spaces, as well as outside when 6 feet of distancing is impractical. The order would last for 90 days, The Denver Post reports.

“Wearing a mask: it’s the simplest thing that we can do to successfully battle against coronavirus in Colorado,” said Gov. Jared Polis in a video uploaded to his Facebook page on Tuesday afternoon. Polis, who has posted about the importance of mask-wearing 12 times in the past eight days, has not issued a statewide mandate, even as other governors have implemented such orders to slow infections.

As of Tuesday, Adams and Arapahoe counties trailed only Denver in the total number of identified COVID-19 infections. Adams County had nearly 4,500 and Arapahoe exceeded 5,400. Douglas County, by contrast, had 1,042.

Denver issued a mask order on May 14, which applies when people are inside or in line for a retail establishment, obtaining healthcare services and waiting on public transportation. Commercial vehicle drivers, including for ride-sharing apps, along with other workers are included.

Tri-County monitors public mask usage, and said that its observations noted around 80% of people in Arapahoe, 70% in Douglas, and 60% in Adams counties are covering their faces, The Post noted.

Abe Laydon, the District 1 commissioner in Douglas County, responded on Twitter that “While strong recommendations based on science are one thing, I will not support a one-size-fits-all mandate for the 370,000 people I represent. I will vote to opt DougCo out of this mask order.”

By contrast, Adams County Board of Commissioners Chair Emma Pinter celebrated the vote, writing, “Masks promote health and freedom of movement. This is an important step to protect ourselves and our neighbors.”

Even though many Americans have refused to wear masks, sparking viral confrontations in public spaces, evidence is mounting that such behavior is dangerous to general health.

Researchers have found that in countries with strong public mask-wearing norms, COVID-19 infections rose much slower than in countries without. And a simulation found that face coverings shortened the distance that droplets traveled from several feet to only a few inches.

Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, told a university reporter that wearing a mask, social distancing and washing hands were the three methods of preventing infection.

“But of the three, the most important thing is wearing a mask,” he said.