Air traffic controller staffing shortages worsened over the weekend as the nation’s government shutdown hit its fourth week, leading to delays and anxiety, and experts say it won’t get better until air traffic controllers get paid.

More than 50 staffing shortages have been reported since Friday morning, causing delays from Los Angeles to Washington, DC, according to an operations update. Controllers are considered essential workers, so they must work during the shutdown, but are not being paid.

Flights for Los Angeles International Airport were temporarily halted Sunday because of a staffing shortage at the Southern California TRACON, which handles flights arriving or departing. At Reagan National Airport in Arlington, Virginia, staffing issues caused a ground delay Sunday that was expected to last until midnight.

Since October 1, there have been at least 264 instances of staffing problems reported at FAA facilities. That’s more than four times the 60 that reported problems on the same dates last year.

Transportation Secretary Sean Duffy said Sunday controllers are “wearing thin” and calling in sick as they continue to go without pay for what is difficult, complex work.

The shutdown that started October 1 after lawmakers failed to reach a spending agreement has left thousands of other federal employees unpaid or furloughed and led to a possible break in food assistance that could start next month for tens of millions of Americans.

“Just yesterday … we had 22 staffing triggers,” he said on the Fox News program “Sunday Morning Futures.” “That’s one of the highest that we have seen in the system since the shutdown began.”

Chad Mourning, an assistant professor of computer science at Ohio University who specializes in aviation safety, told CNN he expects the shortage to continue and get worse as the shutdown goes on, “because people can only work so much over time before they burn out.”

During a past shutdown, it was air traffic controllers and TSA agents not arriving for work that triggered a breaking point.

The government shut down for 35 days from December 2018 to January 2019. The shutdown ended after 10 air traffic controllers stayed home, snarling air traffic – though TSA staffing shortages at some of the nation’s largest airports were also a contributing factor.