The Edgartown, Oak Bluffs and Tisbury boards of health voted unanimously to institute a mask mandate for indoor public spaces during a rare emergency, joint meeting Tuesday morning, changing course again as cases continue to climb and a persistent pandemic refuses to relent.

The face-covering requirement comes more than three months after local officials rescinded all Covid-19 public health regulations at the start of summer, following guidance issued by the Centers for Disease Control and Gov. Charlie Baker as positivity rates declined and vaccination rates rose.

But August has seen cases climb exponentially, with health agents reporting 87 new positive Covid-19 tests in a weekly report Monday, the third most since the start of the pandemic.

Two patients remain hospitalized with the virus, and businesses across the down-Island towns have faced closures after employee case clusters, prompting officials to dust off their Covid-19 regulations — and Islanders to dust off their masks.

“We have been seeing a significant increase in the number of cases each week,” Tisbury health agent Maura Valley said in background remarks at the start of the meeting Tuesday. “With the increase in cases, we felt it was really time to look at an indoor mask mandate, to try to get more people to wear their masks indoors, where we do see most of the transmission.”

The mandate, which becomes effective at 12:01 a.m. on August 19, applies to all indoor public spaces, as well as indoor private spaces open to the public.

Restaurants, office spaces, bars, retail spaces, spas, gyms, barber shops and houses of worship are all included in the mandate.

Diners are only allowed to remove their mask when seated at tables or when eating, and bar patrons must wear a mask unless sitting at the bar. Masks are required on dance floors, in hotel and lodging hallways, and in indoor performance venues.

Employees of shared office spaces will not be required to wear masks while at work stations if there is a six-foot distance between occupants or a physical partition.

The order will be enforced by the boards of health, with daily fines ranging from $100 to $300 levied against establishments that do not make good faith efforts to supervise and enforce masking, according to health agents. Signs are required at all public entrances to establishments and businesses.

Facing questions from business owners about the challenges of enforcement, health agents said during the meeting that fines would fall on businesses because it would be too difficult to enforce the mandate on individuals. They added that there was wiggle room regarding fines if businesses were dealing with obstinate customers.

“The feeling was that we’re mandating it, we’re saying to the store, you need to make sure that your store is in compliance. And we’ll work with that as long as they’re trying to do that,” Ms. Valley said. “The fine . . . was more for establishments that were blatantly disregarding the mandate.”

Health agents also stopped short of suggesting capacity restrictions and social distancing guidelines that came to characterize the first year of the pandemic.

“We are viewing this as incremental, and hopefully we won’t need to go beyond step one,” Edgartown health agent Matt Poole said. “But if we can’t reverse our trends, we may find ourselves there.”

Although some members of the public pushed vocally and in the comments section for a continuation of the mask guidance, rather than a mandate, elected board of health members felt that the measure was necessary considering the rising case counts on the Island.

Oak Bluffs health agent Meegan Lancaster said the Island’s 14-day case rate was spiking, showing a graphic that signaled the Island could face a record virus peak by the end of the week.

“Based on our 14-day running, we are really up there in terms of case count,” Ms. Lancaster said. “We are making the best effort we can, to ensure people are wearing masks indoors in as many circumstances as possible.”

Other board of health members agreed, voting unanimously to institute the mandate before case counts rise further.

“I think this is going to get worse before it gets better,” Oak Bluffs board of health member James Butterick said.

Board members noted that the mandate also served as a message for Islanders and Island officials when considering their own personal behavior and event planning — an issue that arose near the meeting’s close when discussing the fate of Tivoli Day, a large street fair in Oak Bluffs.

“People will be looking to us for guidance, and I think if we do this right, the guidance will be well-received,” Tisbury board of health member Michael Loberg said. “I think we are doing something responsible.”