Washington (CNN) — Every day of the federal government shutdown air traffic controllers calling out sick have caused flight delays, Transportation Secretary Sean Duffy said Friday.

“It’s different every day,” he told reporters at a news conference at Philadelphia International Airport. “The average… is about 5% of our delays come from staffing shortages with air traffic controllers. We’ve gone up as high as 53% of the delays… because of staffing shortages.”

On Friday, 11 air traffic control facilities were short-staffed, including the control towers in Dallas Fort Worth, Newark and Phoenix, TRACONs which handle flights approaching and departing Houston, Newark, Southern California as well as centers, which handle high altitude flights around Albuquerque, Atlanta, Denver and New York.

There have been 221 staffing shortages reported since the start of the shutdown, more than four times the number reported on the same dates last year. Air traffic controllers, like Transportation Security Administration screeners, are required to work during the government shutdown but are not being paid.

“Their paycheck is going to be a big fat zero,” Duffy said. “There’s great frustration. There’s anxiety because as any one of you, you look at the expectation that a paycheck comes in and then you plan for that.”

Controllers, approximately 10,800 federal workers, will receive the first $0 paycheck on October 28. On October 14, they received a partial paycheck, about 90% of the normal total, for hours worked before the shutdown started.

The lapse in government funding makes air travel “less safe,” according to Nick Daniels, president of the National Air Traffic Controllers Association, the union which represents controllers.

“As this shutdown continues and air traffic controllers are not paid for the vital work that they do… that leads to an unnecessary distraction, and they cannot be 100% focused on their jobs, which makes this system less safe,” he said. “Every day that this shutdown continues, tomorrow, we’ll be less safe than today.”

Duffy noted the FAA will slow planes down or cancel flights if they cannot be operated safely.

“If we don’t have enough controllers, if we have controllers that are more stressed and less able to do the job… we will reduce the capacity of airplanes taking off and landing, or we will cancel flights,” Duffy said. “I’m less concerned about you getting there on time. I want you to be safe.”