“Historically, there’s about 5% of delays attributed to staffing issues in our towers,” Duffy said Wednesday on Fox News’ The Will Cain Show. “The last couple of days, it’s been 53%.” 

Many of the nation’s largest airports have experienced delays this week. That includes the following list, as well as airports in Houston, Las Vegas, Nashville, Philadelphia, and Phoenix:

• Reagan Washington National (D.C.)
• Newark Liberty International (New Jersey)
• Orlando International (Florida)
• Boston Logan (Massachusetts)
• Burbank (California)
• O’Hare (Chicago)
• Denver International (Colorado)

The bulk of delays stem from air traffic controllers working without pay due to the shutdown, which began at midnight on Oct. 1 after the government failed to reach an agreement on funding operations. Without a promise of pay, many air traffic controllers have called in sick or just not shown up for work. 

“My message to the air traffic controllers who work for DOT is show up for work,” Duffy said. “You have a job to do. By the way, those who have shown up, I’m grateful for them and their service.”

There are about 13,000 air traffic controllers nationwide, but thousands have failed to show up for work. While the controllers’ union and federal officials haven’t released an exact public figure for total no-shows, its evident towers are understaffed in the number of flight delays. About 10,000 flights were delayed Monday and Tuesday, and roughly 3,200 were delayed Wednesday, CNBC reported. 

Air traffic controller shortage and an antiquated system

To make matters worse, there was already a shortage of air traffic controllers. As of May, only two of America’s 313 airports met staffing targets set by the Federal Aviation Administration. 

There are several reasons for the air traffic controller shortage, including costly and time-consuming training, Fortune’s Emma Burleigh previously reported. Training a new air traffic controller can take anywhere from 16 months to several years—and it’s very expensive.

“It takes a long time to train an air traffic controller,” Mary Schiavo, the former inspector general of the Department of Transportation, told CNN earlier this year. “It’s very expensive. And about a third of them wash out because it’s very rigorous.” 

But for those who hold out in the profession, they can make about $145,000 per year, more than double the average annual salary in the U.S. But pay doesn’t always make up for the high-stakes nature of the job.

“The bottom line is, these controllers are stressed out, and they’re rebelling on this shutdown because they may not get paid,” Duffy said. “They’re working six days a week. They’re keeping America operational, and they’re not guaranteed a paycheck, and they’re frustrated by it, and so not coming into work.”

The delays and air traffic controller shortages have also raised eyebrows about air travel safety. For one, the Hollywood Burbank Airport air traffic control tower was left unmanned for hours this week due to the shutdown. And earlier this year, the tower responding to an American Airlines flight and a U.S. Army Black Hawk helicopter was manned by just one air traffic controller. Ultimately, the planes crashed, resulting in the deaths of 67 people.

Plus, Delta CEO Ed Bastian has said air traffic control systems are so outdated some commercial flight routes were faster in the 1950s than they are today. 

“That’s the air traffic control system. It’s very slow. It’s congested,” Bastian told TODAY in May. “If you modernize the skies, you can kind of bring greater efficiency.”

“The screens look like something out of the 1960s and ‘70s,” he continued. “Our controllers and our operators need the best in order to keep us safe and make sure it maintains its safety record.”

Duffy insists, however, the Transportation Department is making sure flights remain safe during the shutdown. 

“When you talk about safety, we make sure safety comes first,” he said. “And so if we don’t feel like it’s safe, we will slow down traffic. We will stop traffic. That is the first priority.”