How U.S. airlines are changing their mask rules.

After a federal judge struck down a nationwide mask requirement on airplanes, trains, buses and other public transportation on Monday, the country's largest airlines said they would stop requiring masks.

After a Biden administration official said the Transportation Security Administration would no longer enforce the mask requirement while the White House reviewed the decision, the airlines weighed in.

The airlines said they would no longer require masks at airports and on flights within the United States, but several said they would still require them when flying into cities and countries where requirements were still in place. Some airports may still require masks.

Each of the major national airlines had to say what they had to say.

American supported federal measures to slow the spread of the coronaviruses and prioritized the health and safety of its employees and customers. The airline will no longer require masks at airports and on flights within the United States, but customers and employees can choose to wear them at their own discretion.

We are grateful to our team members for their enforcement of the mandate, and will share more information about this transition in the coming days.

Southwest encouraged employees and customers to make the best decision for their personal well-being by wearing a mask. The airline said safety would remain a priority, pointing to the high-end air purification on the planes that it and other carriers use.

The airline asked for patience as the shift in policy was rolled out, while Delta announced that it would stop enforcing a mask requirement.

Customers, airline employees and federal agency employees may be receiving this information at different times. This may take a short period of time, but communications to customers and in-airport signs will be updated to say that masking is optional.

United said that it would stop requiring masks on domestic flights, but that they would still be required on flights into countries where a mandate was still in place.

While this means that our employees are no longer required to wear a mask, and no longer have to enforce a mask requirement for most of the flying public, they will be able to wear masks if they choose to do so.

In a statement to customers, Alaska noted the significance of the moment and said it welcomed the opportunity to see your smiling faces, while acknowledging that some may still feel conflicted about the shift in policy.

The airline's vice president of safety and security said that it had been a long 24 months with nearly constant change. We are thankful for our guests who were patient and kind throughout the entire process.

Customers and flight crews are still welcome to wear masks on the airline, but it will now be optional.

The airline said late Monday that masks are now optional.

Some guests may want to continue wearing face coverings on flights, and that's perfectly fine under our optional policy.

Frontier said it would stop requiring masks on its planes, but warned that they might still be required at some airports.

Regardless of whether a mask mandate is in effect, customers and team members should abide by mask rules.