The Biden administration said Tuesday it would appeal a judge's ruling that blocked a transportation mask requirement if federal health officials determine the mandate is still needed.

The Justice Department continued to defend the administration's policy and said in a statement that it would appeal the ruling if the Centers for Disease Control and Prevention determines the mandate is still necessary to protect public health. The department said it would defer to the CDC's assessment of health conditions before filing any appeal.

"The Department continues to believe that the order requiring masking in the transportation corridor is a valid exercise of the authority Congress has given CDC to protect the public health. That is an important authority the Department will continue to work to preserve," DOJ spokesman Anthony Coley said in a statement. "If CDC concludes that a mandatory order remains necessary for the public's health after that assessment, the Department of Justice will appeal the district court's decision."

The CDC didn't immediately respond to a request for comment Tuesday evening.

For large swaths of the country, the question of where and when people must wear masks in transportation settings was largely settled a day after U.S. District Judge Kathryn Kimball Mizelle upended the administration's plans to extend the mask mandate through May 3. The end of the mandate signaled another turning point two years into the pandemic, even as coronavirus cases rise and a new omicron subvariant becomes the predominant strain nationwide.

Many domestic airline passengers went maskless in the skies this week for the first time this week in nearly two years. However, airport officials in New York and Philadelphia, and the nation's largest subway system, on Tuesday cited local mandates in keeping a mask requirement in place.

Asked Tuesday whether Americans should be wearing masks, President Joe Biden replied it "was up to them."

"We've said from the start that our response should be guided by the science and data and by experts," White House press secretary Jen Psaki told reporters aboard Air Force One, adding: "Public health decisions shouldn't be made by the courts; they should be made by public health experts."

Before the Justice Department's announcement Tuesday evening, Carl Tobias, a professor at the University of Richmond School of Law, said the administration could face obstacles if it chooses to appeal Mizelle's decision.

"I think that some judges could be persuaded by the administration's perspective, but maybe not on the 11th Circuit where the appeal would go," Tobias said, referring to the federal court with appellate jurisdiction in Florida, Alabama and Georgia. "The 11th Circuit is ideologically quite conservative. It would be an uphill fight."

Around the country, most transportation officials dropped the mandate Tuesday, while a few others continued to require masks at the advice of public health experts.

Officials at the New York/New Jersey Port Authority, indicated that travelers flying through New York's John F. Kennedy International and LaGuardia airports must wear masks. However, citing guidance from local officials, they aren't required at New Jersey's Newark's Liberty International airport. In Philadelphia, where city officials recently reinstated an indoor mask mandate, masks are still required at the airport.

The New York subway system also continued to comply with state orders that required masks on public transportation. Meanwhile, the Bay Area Rapid Transit system in the San Francisco area said mask requirements would persist until the state provided more guidance.

Citing an Illinois executive order, the Chicago Department of Aviation had said Tuesday that masks would still be required on public transportation and in transportation hubs, including at O'Hare International and Midway airports. City transportation officials reversed that decision hours later.

Jurisdictions keeping the mandate were a marked contrast to announcements from airlines, Amtrak and ride-sharing companies Uber and Lyft, which announced that masks would be optional.

Public transit agencies in the Washington region, including Metro and commuter rail services VRE and MARC, also rescinded the requirement.

The patchwork mirrors an approach followed earlier in the pandemic, when the Trump administration took a largely hands-off stance to federal enforcement. At that time, it was left to local jurisdictions, airports and airlines to determine their own policies. That piecemeal approach led some to call for more federal intervention, although this time around, travel industries largely oppose the continuation of pandemic-related restrictions.

Conflicts over masks have sparked disputes on airplanes, where some flight attendants have been physically attacked and verbally abused for enforcing mask rules. Republican lawmakers launched several efforts to roll back the mandate. Last month, 21 mostly Republican-led states filed suit seeking to end the federal mask requirement.

The transportation mask mandate had been set to expire this week but the Biden administration - citing an uptick in infections fueled by the BA. 2 variant - extended it through May 3. In making the announcement, the CDC said it would use the time to assess public health conditions to determine whether the mandate should remain.

However, Mizelle's decision Monday in response to a lawsuit filed by a legal group known as Health Freedom Defense Fund and airline passengers abruptly altered that plan.

The abrupt end to the mandate left little time for airports, federal agencies - such as the Transportation Security Administration, which was charged with enforcing the mandate - and transit systems to prepare. At airports across the country, travelers offered a mix of reactions to the fallen mandate, ranging from elation to concern to confusion.

At Denver International Airport, announcements continued to play in terminals indicating that masks were required. Most people arriving at the airport early Tuesday wore masks, although several removed them upon hearing they were no longer mandated.

Airport officials said it would take time to change the recorded announcement and the hundreds of notices that dot the nation's largest airport, by physical size, that remind travelers to don face coverings.

"Our biggest challenge will be removing the hundreds of signs. Some are adhered, so it may take a bit," Stacey Stegman, senior vice president of communications for Denver International Airport, wrote in an email.

Scot Severson, 54, of St. Charles, Ill., was flying through O'Hare early Tuesday and didn't bother wearing a mask after hearing from a friend that the mandate was lifted.

Severson was scheduled to fly to Orlando for a convention for fellow franchise owners of a mold and water remediation company. It was his third flight of the pandemic and, despite his medical history - he had a rough bout with coronavirus and has Parkinson's disease - he said he's not concerned about possible health risks.

"If I didn't feel safe I wouldn't be taking the trip," he said.

Others, however, said they were taking no chances.

Ofelia Y. Domingo, 76, is fully vaccinated and tested negative for the coronavirus, which was required for her flight to her native Philippines, her first since the pandemic began. Still, she said she planned to wear a mask for the duration of her flight.

"It's up to the individual," she said. "I'll be wearing mine. I think a lot of people will be wearing masks still."

In Terminal C at Dallas-Fort Worth International Airport on Tuesday, news that masks would no longer be required was met with a mix of reactions. One woman going through security cheered, declaring, "We're free again!" Others greeted the news with wariness.

Suzanne and Don Janson stood outside waiting for a ride after their early-morning flight from Pittsburgh. They were still wearing N95 masks but said half the passengers on their flight were not masked.

"I wish they still would wear them," Suzanne Janson said. "I was surprised the stewardess wasn't."

But Katie Lankford, who was flying to Washington's Dulles International Airport with her 2-year-old son, Chance, was relieved. She said it had been difficult to persuade the toddler to wear a mask.

"We tried," she said about an earlier flight the pair had taken. "We did not succeed."

In Washington, Metro riders experienced their first day without the requirement in 700 days. The transit agency late Monday lifted its mask policy, citing Mizelle's ruling. On the system's trains and buses, most riders Tuesday seemed to continue wearing masks, many saying their own health concerns superseded any requirement.

In her decision, Mizelle, who was appointed by President Donald Trump and clerked for Supreme Court Justice Clarence Thomas, said the CDC had relied on a 1944 law, the Public Health Service Act, to impose the mandate. But the government's argument that it put the mask requirement in place for the purpose of "sanitation" falls short, Mizelle argued.

"Wearing a mask cleans nothing. At most, it traps virus droplets. But it neither 'sanitizes' the person wearing the mask nor 'sanitizes' the conveyance," Mizelle wrote.

Mizelle found for the plaintiffs on three key issues, ruling that the CDC had exceeded its legal authority, that it had improperly avoided notice and comment procedures, and that its mandate was "arbitrary and capricious." In her ruling, Mizelle argued that the mask mandate wrongly curtailed passengers' freedom of movement.

"Anyone who refuses to comply with the condition of mask-wearing is, in a sense, detained or partially quarantined by exclusion" from their means of transportation, she wrote.

Airlines began requiring customers to wear masks in mid-2020 as part of the effort to contain the spread of the virus. The Trump administration declined to put a mask mandate in place, but shortly after taking office, Biden issued an order that required mask-wearing in all transportation settings.