As social media images of fuller flights spark health concerns amid the COVID-19 pandemic -- Democratic lawmakers and unions are calling for federal action to mandate social distancing policies on planes.

Why do some flights have a higher number of passengers if air travel is down?

The number of U.S. travelers screened by the Transportation Security Administration (TSA) is down over 90% compared to this time last year, but the number of available flights has also been slashed considerably.

Domestic commercial flights are down by almost 70%, compared to last year, the Bureau of Transportation Statistics reported.

On average, domestic U.S. flights are now carrying 31 people, according to Airlines for America (A4A), which represents major U.S. passenger airlines. But as the number of travelers slowly ticks up, flights on certain routes have reached load factors of up to 85%.

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What have airlines done to encourage social distancing?

A photo a doctor took on a United flight that the airline said was 85% full went viral over the weekend.

"I guess @united is relaxing their social distancing policy these days?" Ethan Weiss tweeted, a cardiologist and assistant professor at the University of California, San Francisco (UCSF) School of Medicine.

Weiss was traveling home to San Francisco after volunteering to treat COVID-19 patients in New York.

Starting next week, United says it will contact customers 24 hours before departure if their flight ends up close to full capacity "so they can decide whether to adjust their plans before they arrive at the airport." If a traveler gets to the gate and more than 70% of customers on that flight have checked in, the airline says the customer will be able to adjust their plans there.

Previously, United said it was limiting advanced seat selections, but could not guarantee each passenger would be next to an unoccupied seat.

Other major U.S. airlines are implementing similar policies aimed at encouraging social distancing.

American Airlines announced it is blocking half of all middle seats.

Delta Airlines is currently booking only half of its first-class seats, and 60% of the main cabin.

As the global airline industry stands to take a hit of more than 300 billion amid the pandemic -- President and CEO of A4A Nick Calio said capping seats isn't sustainable for airlines.

"In the long-term, that's a business model that cannot be sustained," Calio said at a Senate hearing last week. "Because if we can't -- if it costs more to fly people from point A to point B -- it's a total money-losing proposition which then means it's a job-loss proposition."

Dr. Matthew Heinz, a hospitalist in Tucson, Arizona, told ABC News he recommends that if people must travel, that they should both wear a mask and try to social distance as much as possible.

"It's the length of time as well," Heinz said. "If a passenger is on a four-and-a-half-hour flight that's a long time to be breathing in the respiratory secretions of someone else."

Should the government step in?

All major U.S. airlines have made masks mandatory for both passengers and crew, but prior to the airlines' announcements both Sens. Ed Markey, D-Mass., and Richard Blumenthal, D-Conn., sent a letter to Department of Transportation (DOT) 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."

At the time, the DOT directed ABC News to the Federal Aviation Administration (FAA), which responded in a statement that it's "not a public health agency," but that it had been lending its aviation safety expertise to federal public health authorities.

Both lawmakers are now calling on the DOT again to set a uniform standard regarding social distancing on aircraft.

"There should absolutely be a standard guideline," Markey said. "There should be a rule that all airlines have to abide by."

Blumenthal is not ruling out congressional intervention.

"If the Department of Transportation fails to do its job, Congress should do it for them," Blumenthal told ABC News. "Because the lives of passengers are depending on protections against contaminated air, against lack of proper precaution."

The DOT has not responded repeatedly to ABC News' request for information about this issue.