The Centers for Disease Control and Prevention (CDC) asked the Justice Department on Wednesday to appeal a ruling by a Florida judge that struck down a mask mandate on airplanes and other forms of public transportation.

The CDC made its decision following a ruling by US District Judge Kathryn Kimball Mizelle in Tampa, Florida, who threw out the federal government's transportation mask mandate on Monday, which meant that people no longer had to wear masks on planes, trains and other mass transit.

It was unclear if the Justice Department would ask an appeals court to grant an emergency stay of the judge's ruling and reimpose the mask mandate, which would cause an about face by travelers, most airlines and airports and many public transit systems to reimpose mask wearing after dropping it hours after the judge's decision.

In a bid to challenge the ruling, the CDC said in a statement: "It is [the] CDC's continuing assessment that at this time an order requiring masking in the indoor transportation corridor remains necessary for the public health.

"CDC will continue to monitor public health conditions to determine whether such an order remains necessary. CDC believes this is a lawful order, well within CDC's legal authority to protect public health."

Following the ruling, President Joe Biden said on Tuesday that it was up to individuals to decide whether to wear a mask on an airplane.

White House press secretary Jen Psaki said on Wednesday that she recommended people follow the CDC's mask guidelines and "everyone wear masks on a plane".

She added: "People are not legally bound to wear masks. So, it is a point in time where it is up to people, it is their choice."

After the ruling by the Florida judge, the Justice Department announced that it would appeal it if the CDC said that the mask mandate was necessary to protect public health.

Shortly before the judge's ruling, the CDC extended its mask mandate until May 3 for planes, trains and other mass transit.

Mizelle struck down the mandate, saying that it exceeded the statutory authority of the CDC. She also questioned a law used to prop up the mandate.

Biden had supported the use of the mandate on mass transit and airplanes by using the 1944 Public Health Service Act.

The act states that if the government wants to stop the spread of communicable diseases, it can "provide for such inspection, fumigation, disinfection, sanitation, pest extermination, destruction of animals or articles found to be so infected or contaminated as to be sources of dangerous infection to human beings, and other measures, as in his judgment may be necessary".

The Biden administration had argued that masks could be defined as "sanitation" under the law.

Mizelle said that since the word "sanitation" wasn't defined in the statute, the meaning of the word in law relates to the time that the law was passed.

She used a dictionary definition to suggest that "sanitation" could have been defined as meaning either actively cleaning something or measuring to keep something clean, but ultimately settled on the former definition.

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

She added that other words mentioned in the law like fumigation also refer to cleaning something or trying to wipe out a disease.

Mizelle's ruling was seen as one that would have an impact on the CDC's ability to choose what to do in public health emergencies in the future.

The ruling came as the highly transmissible omicron BA.2 subvariant pushes new COVID-19 cases higher across the US.

After her announcement, several airlines, including Delta, United and Southwest Airlines said that masks were optional.

Amtrak and Washington Metropolitan Area Transit Authority, which runs DC's Metro subway system, also announced masks were optional.

New York Governor Kathy Hochul announced Wednesday that New York City would keep its mask mandate for local airports and public transit. COVID-19 hospitalizations were up 60 percent in the past month in the state.

Philadelphia officials also chose to keep its mask mandate for indoor activities.

Mizelle was appointed to the federal court by President Donald Trump. She was opposed by Democrats. She was a former clerk to Supreme Court Justice Clarence Thomas and was 33 at the time of her confirmation.

The American Bar Association said she was "not qualified" due to "the short time she has actually practiced law and her lack of meaningful trial experience."