The U.S. government shutdown is affecting the livelihood of thousands of air traffic controllers, as they’re expected to keep working without a paycheck. It’s led to major staffing shortages and employees calling out sick, delaying flights and packing airports with travelers anxious about safety risks.

Airports all around the country—from Newark and Phoenix to Denver and Los Angeles—are all experiencing staffing issues. In California, Hollywood Burbank Airport had no air traffic controllers at its tower from 4:15 p.m. to 10 p.m. on Monday as it grappled with the issue. About 13,000 air traffic controllers and 50,000 Transportation Security Administration (TSA) officers were required to show up to work, despite not being paid. Transportation Secretary Sean Duffy said air traffic staffing has been cut by 50% in some areas since the government shutdown started last week.

The shutdown is only exacerbating an air traffic controller shortage that America has been grappling with for years. As of May 7, only two of America’s 313 airports met staffing targets set out by the Federal Aviation Administration (FAA). This problem rides on the heels of the American Airlines tragedy that happened in February, when a passenger jet collided with an Army helicopter right outside Reagan Washington National Airport, killing all 67 people involved.