Delta, American, United and Frontier joined JetBlue on Thursday in announcing that they'll require passengers to wear face coverings.
It remains up to individual airlines whether to require masks, but Democratic lawmakers are calling for federal action to mandate coverings for all air travelers.
On Wednesday, Sens. Ed Markey, D-Mass., and Richard Blumenthal, D-Conn., sent a letter to Department of Transportation Secretary Elaine Chao and Health and Human Services Secretary Alex Azar urging them to "immediately issue a rule requiring face masks for all individuals engaged in air travel."
"In the absence of federal action," the lawmakers wrote, "different airlines and airports have adopted conflicting policies that will undermine overall public health if they are not unified around a single, strong standard."
The largest flight attendant union in the U.S., the Association of Flight Attendants-CWA, AFL-CIO, sent a similar letter last week to the DOT and HHS.
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"From the airport door to the airplane door, on the airplane, and then back out through the airport, we want people wearing face coverings in all those areas," Sara Nelson, a current flight attendant and the president of the Association of Flight Attendants-CWA, AFL-CIO, told ABC News. "That is what is recommended by the CDC to the general public when they are out in public, and that is exactly what should be happening in our airports and on our airplanes to help contain the spread of the virus."
HHS did not respond to a request for comment, and the DOT directed ABC News to the Federal Aviation Administration. The FAA on Monday said in a statement that it's "not a public health agency," but that it's been lending aviation safety expertise to federal public health authorities.
House Transportation and Infrastructure Committee Chair Rep. Peter DeFazio, D-Ore., believes a mask requirement is "well within the FAA's jurisdiction" and spoke with FAA Administrator Steve Dickson Wednesday to voice his concerns.
"The administrator appreciated the opportunity to speak with Chairman DeFazio and shares his concerns about the importance of protecting the health and safety of passengers and crews," the FAA said in a statement. "The FAA is working with air carriers to ensure they have processes in place for addressing public health risks for their crews and passengers."
President Donald Trump said it sounded "like a good idea" on Tuesday when asked if masks were something we would consider "rolling out for all flights."
The Centers for Disease Control and Prevention recommends everyone wear a cloth face covering over one's nose and mouth when around other people, "including during travel if they must travel."