Boston urged people to start wearing masks Thursday and the Biden administration weighed its next legal step in what is shaping up to be a high-stakes court fight over the abrupt end of the national mask mandate on airplanes and mass transit.

The Boston Public Health Commission noted a rise in hospitalizations, as well as a 65% increase in cases and an even larger spike in COVID-19 levels in local wastewater samples. It also stressed that the guidance was merely a recommendation, not an order.

The country is wrestling with how to deal with the next phase of the pandemic and find the right balance in enacting health measures at a time when many Americans are ready to move on after two exhausting years.

A federal judge in Florida this week threw out a national mask mandate on mass transportation, and airlines and airports responded swiftly Monday by repealing their requirements that passengers wear face coverings. That put the Biden administration in the position of trying to navigate an appeal that could have sweeping ramifications over the power that the Centers for Disease Control and Prevention has in regulating future health emergencies.

Philadelphia last week became the first big city to bring back a mask mandate, responding to a rise and infections and hospitalizations there, and other cities in the Northeast have been closely watching the trend lines and a new color-coded map from the CDC to decide next steps.

The map that the CDC switched to in late February is less focused on positive test results and more on what’s happening at hospitals to give community leaders clearer guidelines on when to urge masking. Nearly 95% of U.S. counties still have low transmission based on the map, but more places have shifted to medium and high transmission in recent weeks, including many places in upstate New York.

Hospitalizations nationally have ticked up in recent weeks but are nowhere near the peak reached at the height of the omicron surge.

“These precautions are how we protect the progress we’ve made in our community,” said Dr. Bisola Ojikutu, the Boston commission’s executive director.

The Boston recommendation came two days after the city’s transit system lifted mask requirements in response to the national transportation ruling, reflecting the mishmash of reactions following the court decision by an appointee of former President Donald Trump.

As the Biden administration figures out an appeal, Lawrence Gostin, a public health law expert at Georgetown University, said a “monumental battle” was shaping up, with the future of the CDC at stake. The agency continues to recommend that people wear masks in all indoor public transportation settings.

“The question the courts are going to have to decide, and the public will have to decide, is when the next health crisis hits — and it will — will we have a strong public health agency to protect the population?” he said. “Or will the CDC simply have its hands tied behind its back? I think it’s a very really possibility we’re going to see the CDC handcuffed.”

While the Supreme Court did strike down the agency’s eviction moratorium for housing, that was more at the edge of the agency’s authority. Setting rules for mask wearing on public transit is a basic, core tenant of the CDC’s power, Gostin said.

“If someone gets on a flight from New York to LA, there’s no state stopping them. The only thing preventing that transmission is the CDC,” Gostin said.

An appeal would go to the 11th Circuit Court of Appeals, which is considered a right-leaning court, and conservative justices have a majority on the U.S. Supreme Court. A ruling could take away the CDC’s power to issue mask orders and cast any future orders under a “legal cloud,” he said.

Temple Law’s Craig Green said the federal government’s strategy is “really almost brilliant” because it could win in two ways with its appeal. If COVID-19 cases numbers continue to fall, Justice Department attorneys could argue that the issue is moot and ask to have the case thrown out.

“No one will have reason to cite it ever in the future as a precedent,” he said.

But he said that if cases rise, the federal government would be better positioned to reimpose a mask mandate.

“I think the arguments about what a government can do, what the federal level can do under conditions of emergency were very difficult and problematic,” he said. “I can understand why the Department of Justice and the United States government really did not want to see that kind of limit on their authority in the future, even if COVID ends up being more controlled in the future.”

Amid the court battle, American, United and Delta have all indicated that they will lift the bans they imposed on passengers who refused to wear masks now that masks are optional on flights.

“We have talked to them individually,” United CEO Scott Kirby told NBC on Thursday. “Many of them assure us that now that the mask mandate is off, everything is going to be fine, and I trust that the vast majority of them will.”

Many passengers were shrugging off the changes. When Jon Schaudies flies from Chicago to San Antonio next week, he’ll wear a mask, but won’t worry if the passenger next to him doesn’t do the same.

Schaudies, who travels frequently as vice president of a small manufacturing company, feels that he has enough protection from the COVID-19 vaccine and booster to avoid becoming seriously ill if he does contract it.

“I feel like people are at such extremes, but I’m sort of right down the middle,” said Schaudies, 51, who plans to get a second booster shot.

He understands the worries of parents traveling with children who are too young to be vaccinated, but says “they have decision to make” about whether to fly. “But for business travelers, we can’t stop.”

“The world has to go on at some point.”

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Hollingsworth reported from Mission, Kansas, and Whitehurst from Salt Lake City.

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