The country's top infectious disease experts criticized airlines on Tuesday for planning full flights as the coronavirus continues to spread across the country.
Several US airlines have stated that they are limiting the capacity of the planes to between 60 and 67 percent.
But United Airlines has not promised to leave the seats empty, and American announced last week that it will stop taking those steps and begin filling its flights to the edge starting Wednesday as companies try to increase revenue.
"Obviously that is something that worries. I'm not sure what made that decision, "Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told a Senate panel.
"I think in the confines of an airplane that becomes even more troublesome," he added.
Dr. Robert Redfield, director of the US Centers for Disease Control and Prevention, joined Fauci to express "substantial disappointment" in the company's decision.
“I can say that this is under critical review by us at the CDC. We don't think it's the right message, "added Redfield.
The heads of the American and American airlines argued that it is impossible to maintain social distancing on an airplane, even with the seats half empty, and that other security measures are being taken, such as the use of masks, improved cleaning systems and filtration of air.
"It's less about social distancing and more about air and air quality on board the plane that makes people safe," said United Airlines CEO Scott Kirby.
American Airlines spokesman Ross Feinstein said the company has "multiple layers of protection for those flying with us, including required face covers, improved cleaning procedures, and a COVID-19 pre-flight symptom checklist."
American flyers have the option to change flights if they feel their flight is too full, Feinstein added.
After multiple reports that airlines were not enforcing their own mask mandates, United, American and Delta announced last week that they will ban passengers from refusing to cover their noses and mouths.
The policies occur when airlines seek to recoup huge financial losses due to the pandemic, which caused a 95 percent drop in passengers during April.
Passengers have begun to fly slowly again, but travel has still dropped 75 percent from normal.