A group of Democratic lawmakers on Wednesday called on airlines to offer cash refunds — not just travel vouchers — to customers who canceled their flights due the coronavirus pandemic. This comes as carriers are struggling to balance social distancing guidelines with their financial survival.

There is fear and frustration from flyers over what they see as overly crowded flights.

One flight from San Juan to Miami left Puerto Rico on Wednesday morning with 145 passengers; the airline said it blocked 27 seats.

Packed flights are an exception: the vast majority of domestic flights average about 31 passengers, according to Airlines for America.

Can people safely social distance on an airplane?

“That’s why we’re asking all of our customers to wear masks, because you know when you’re in an airline seat even when you’re keeping the middle seat free, you’re not going to be 6-feet away from someone,” JetBlue Airways Corp. CEO Robin Hayes told CBS News.

Hayes said JetBlue is blocking up to 40% of its seats. All U.S. carriers are requiring masks. “We’ve really had no issues with getting our customers to comply,” Hayes said. “It’s really the new flying etiquette.”

Hayes said any customers refusing to wear a mask will not be able to board. He also explained what would happen if a customer during the flight decides against wearing a mask.

“If a customer does not wear a mask … Then it’s someone that we will not permit to fly on JetBlue again,” he said.

While the number of travelers has doubled since the lowest point last month, it’s still down 92% from 2019. U.S. airlines are losing up to $7 billion a month, Airlines for America said.

“We have a massive threat to our industry,” Hayes said. “I mean we are now entering the third month of effectively zero revenue. We have a new challenge ahead of us which is in the era of COVID-19, how do we get people comfortable that flying is safe?”

There is no federal standard for air travel. Each airlines policies are a little different. Meanwhile, all airliners are waiving change fees, but as people choose to fly, planes will get more crowded. Even with all the middle seats blocked, a 737 could still hold more than 100 passengers.