Colorado is the latest state in the Mountain West to issue a mask requirement, joining Nevada and New Mexico.
According to the Association of State and Territorial Health Officials, about half of U.S. states have such mandates.
Colorado Gov. Jared Polis issued the order Thursday, saying everyone over the age of 10 has to wear masks in indoor public spaces, and while "using or waiting to use" transportation including ride shares, taxis or mass transit.
"It's clear that at this point in time, this is the least bad of the options that we have at our disposal," Polis said during a press conference. "It will increase mask wearing, it will decrease the spread of the virus, it'll provide clarity across borders, and it's the right thing to do. And I want to give you an example of how effective masks are."
He went on to cite a study in Missouri, where two hair stylists went to work while experiencing symptoms of the virus. The stylists, wearing primarily cloth masks, served 139 clients, also wearing masks. No clients appear to have contracted the virus. (One of the stylists appears to have passed along the virus to family members and close contacts outside of work, where she may not have been wearing a mask).
Still, in much of the region and the country, masks continue to be a polarizing issue. That's not new. During the 1918 flu pandemic, the Anti-Mask League of San Francisco held rallies denouncing masks and questioning whether that virus was any more serious than the common cold.
Earlier this week Robert Redfield, director of the Centers for Disease Control and Prevention, said if everyone would wear a mask, "we could bring this epidemic under control" in a matter of weeks.
Colorado's governor initially opposed a mask mandate, saying it was unenforceable, but said he changed his mind after seeing encouraging data about their effectiveness. He also pointed to the state's recent rise in hospitalizations.
"We have a choice in Colorado," Polis said. "Either more mask wearing and attention to social distancing, or more damage to our economy and loss of life. I mean, that's an easy decision to make."
The order, which lasts at least one month, does make a number of exceptions, including for people eating at a restaurant, exercising alone, getting a facial or beard trimming. It also exempts "individuals who are actively engaged in a public safety role, such as law enforcement, firefighters, or emergency medical personnel."
Nevada and New Mexico already have mandates requiring masks in public spaces. Other states are a patchwork.
Montana requires masks, but only if a county has a certain number of active cases. Utah requires masks in state facilities and in schools, school buses and liquor stores. Idaho and Wyoming do not have statewide mask mandates.
A study in the journal Health Affairs found that states with mask mandates saw slower growth in COVID-19 cases than those without mandates. Polis cited similar findings in Colorado counties that issued their own mask requirements.
During the Thursday press conference, Polis likened walking into a store with a bare face to walking into a store completely naked, and said that violators could be charged with trespassing.
This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Boise State Public Radio in Idaho, KUNR in Nevada, the O'Connor Center for the Rocky Mountain West in Montana, KUNC in Colorado, KUNM in New Mexico, with support from affiliate stations across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting.