As travelers begin to return to the country's airports, they'll notice the changes meant to prevent the spread of the coronavirus.

Wearing face masks will be customary, and even required by most domestic airlines. More airports may use machines to check passengers for fever. Security checkpoints may have plexiglass shields between passengers and screeners.

Only 119,629 travelers passed through airport security checkpoints Wednesday, still a fraction of the 2.25 million who did this time last year. But airports, airlines and federal agencies are taking steps to make it possible for air travel to return to normal levels.

American, United, Delta, Southwest, Alaska, Frontier and JetBlue have all announced passenger mask requirements. 

Beginning Monday, Delta, United and JetBlue will require all passengers to wear face coverings. Delta and JetBlue will make them mandatory from the time they check in through the time they deplane. Delta, which says customers can take them off for meals, will also require them in its Sky Club lounges.

"While we remain committed to our new standard of clean and to providing more space for our customers when they travel, we take seriously the CDC guidelines for adding this extra layer of protection," said Bill Lentsch, Delta's chief customer experience officer. "We believe this change will give customers and employees some additional comfort when traveling with us.''

United said it would provide the required face coverings to passengers starting Monday.

Delta and United flight attendants are already wearing masks; JetBlue's are now required to do so. United's new policy will also make masks compulsory for other front-line employees such as pilots, customer service agents and ramp workers. Employees using their flight benefits will also have to wear them.

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"Wearing a face covering isn’t about protecting yourself it’s about protecting those around you,” said Joanna Geraghty, JetBlue's president and operating chief, in a statement. “This is the new flying etiquette."

Starting May 11, American, Southwest and Alaska will also begin requiring their passengers to wear face masks.

"We will roll this out as quickly as possible and these kits will be widely available across our network in the coming weeks," said Kurt Stache, American's senior vice president of customer experience. "In the meantime, customers should bring their own masks or face coverings."

Masks will be mandatory for American's flight attendants beginning Friday. At that time, the airline will also begin distributing masks, sanitizing gel and disinfectant wipes to passengers.

Southwest said that it will provide masks if customers forget to bring their own.  The Dallas-based airline will mandate masks for employees beginning Sunday.

Alaska Airlines and Horizon Air, a regional carrier, expect passengers to bring masks themselves and keep them on while at the airport and during the flight. Masks will be available to those who forget.

Alaska will also provide disposable surgical and reusable fabric masks for employees, many of whom are required to wear masks beginning Monday, and has added social distancing floor decals at airports.

Starting May 8, Frontier Airlines will require its passengers to wear face masks that cover their noses and mouths "throughout their journey," the airline said in a statement, adding that its flight crews and airport workers are already doing so. "This will include all ticket counters, gate areas, and onboard our aircraft."

Southwest Airlines CEO Gary Kelly said Tuesday that the airline is encouraging travelers travelers to wear masks and is considering making it mandatory.

Legislators, unions call to make masks compulsory

Leading lawmakers and union leaders want all airlines to require passengers and crew members to wear face masks. On Wednesday, Rep. Peter DeFazio,  D-Ore., the chairman of the House Transportation and Infrastructure Committee, asked the Federal Aviation Administration to require them for both groups. 

Last week, Sara Nelson, the international president of the Association of Flight Attendants-CWA, asked the federal government to provide passengers with disposable paper or cloths masks.

"These should be provided free to all members of the public entering airport buildings with the stipulation that they be worn at all times on airport property and on airplanes," she said in a letter to Transportation Secretary Elaine Chao and Health and Human Services Secretary Alex Azar.

Meanwhile, the Transportation Security Administration is installing plexiglass shields at airport security checkpoints to protect its officers. More than 500 TSA employees have tested positive for the coronavirus, and five have died.

The TSA has also begun installing sneeze guards at the entrance to security checkpoints where officers check travelers' documents. They're already in place at New York's JFK International Airport and Boston's Logan International.

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More airports may require temperature checks. Paine Field, a small commercial airport in Everett, Washington, north of Seattle, said Wednesday that all passengers would be screened for fever by a thermal camera. Passengers whose temperature registers above 99.5 F will undergo a secondary screening and may be prevented from traveling.

Contributing: Dawn Gilbertson, David Oliver