Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

CAROLINA, Puerto Rico — Inside the steamy San Juan airport, mothers sleep on the floor with their children. Travelers, many of whom have been there for days, fan themselves as they wait for a flight out.

The lines are long and there is no air conditioning, but after Hurricane Maria, Puerto Ricans are eager to just get out — and the island's governor fears many will not return.

Related: Lawmakers Press White House to Step Up Aid to Puerto Rico

"My expectation is to rebuild stronger than ever," Gov. Ricardo Rossello told NBC News. "But clearly if this is not taken seriously ... Puerto Rico is going to collapse into a humanitarian crisis."

Saddled with a ballooning debt crisis, Puerto Rico has already seen a historic migration of about half a million people from the island in the past 10 years. Now, following the most powerful hurricane to hit the U.S. territory in decades, the outflow is sure to hasten.

"People were leaving in search of better economic opportunities. Now people are leaving because of a humanitarian crisis," said Teresita Levy, a professor of Latin American and Latino studies at Lehman College in the Bronx. "They’re trying to get out because they are worried about not having enough food and supplies, services not being restored. What happens if you have a medical emergency and the hospital doesn’t have a generator?"

"If anybody was thinking of coming [to the mainland], they are probably now coming anyway."

For many Puerto Ricans, there is no alternative but to leave for the mainland: Gas is scarce, there's no running water, and only about 5 percent of the island has had power restored.

"If anybody was thinking of coming [to the mainland United States], they are probably now coming anyway," Levy said.

Related: Puerto Ricans Plead for More Federal Aid to Devastated Island

The Morning Rundown Get a head start on the morning's top stories. This site is protected by recaptcha

Rossello has pleaded for more aid, and warns of a mass exodus if his island doesn't get help from the federal government, which is also still contending with the aftermath of Hurricanes Harvey and Irma in the Southeast.

"There needs to be unprecedented relief for Puerto Rico so that we can start the immediate effort right now with the deployment of resources, but also the mid- to long-run recovery," he said. "If we have that, we can avoid a humanitarian crisis in the United States. But if we don’t have that, you will see thousands, if not millions, of Puerto Ricans flocking to the United States."

Experts say major hurricanes often cause a mass migration, particularly among those in a higher socioeconomic status.

Related: Trump to Visit Puerto Rico Next Week

Tatyana Deryugina, a professor of finance at the University of Illinois who studies the economic impact of disasters, tracked Hurricane Katrina evacuees for years after the Category 5 storm ravaged New Orleans.

She said her research indicates that about a third more people left New Orleans than would have otherwise, and only half of them came back in the eight years after the storm.

"Obviously a big difference with Puerto Rico is it's an island," she said. "As far as I can tell, there's no place within Puerto Rico that’s really better, so if you want to leave, probably you're going to just leave the island entirely."

And the type of people who leave will affect Puerto Rico's recovery efforts and its essential services, she added.

"I think we're going to see more advantaged people leaving, the people who have an easier time buying airfare or a boat ticket — which could be, unfortunately, some of the wealthier individuals, like doctors," she said.

Brock Long, the head of the Federal Emergency Management Agency, said he had yet to see evidence of a mass migration in Puerto Rico, and said FEMA was treating Puerto Rico "just like any other state."

"We need you to know that we are working our butts off to help the people of Puerto Rico," Long said on MSNBC.

For some, though, that's not enough.

Luis Romero is one of tens of thousands of Puerto Ricans who have already fled since the hurricane hit. He and his wife left their San Juan home for refuge near Sarasota, Florida, and they're considering making the move permanent.

"People are not going to wait for long," he said. "They are going to start flying over here in droves."

If that happens, Rossello says, "it would devastate our revenues. It would be a brain drain, most likely."

People wait in line to get a flight at the Luis Munoz Marin International Airport on Sept. 25 in San Juan, Puerto Rico. Joe Raedle / Getty Images

The experts say there are ways to negate that. Congress could allocate money specifically for rebuilding efforts in Puerto Rico, which would attract construction companies and developers, said Deryugina, the finance professor.

Lawmakers could also allow a suspension of Puerto Rico's Promesa debt payments, Levy suggested, referring to the federal law that restructured the island's debt.

"It’s not people leaving that will be the issue, but it’s really, do you have the resources you need to fix what was damaged by the hurricane?" she said. "A year-long or longer suspension of the Promesa debt payments would mean there is money to invest in the infrastructure, schools, to make sure the hospitals are maintained."

Gabe Gutierrez reported from Carolina, Puerto Rico. Elizabeth Chuck reported from New York.