More than 15,000 flights across the country have experienced delays since Monday, according to the Federal Aviation Administration — one of the many tangible impacts of the ongoing government shutdown.

Continued disruptions to the nation’s air travel network are among the factors that “build the pressure” on Washington lawmakers to reach a compromise, Sen. Shelley Moore Capito, R-W.Va., said Friday

“They had a ground stop at DCA (Ronald Reagan Washington National Airport) yesterday; Nashville has had one; Burbank, California, had to close their tower because they didn’t have an air traffic controller,” Capito said. “These are the kind of pressure points that are very difficult.”

U.S. Transportation Secretary Sean Duffy acknowledged Thursday on Fox Business that a “small subset” of air traffic controllers has begun calling in sick.

“If we have some on our staff that aren’t dedicated like we need, we’re going to let them go,” Duffy said.

The majority of air traffic controllers are still showing up for work despite uncertainty over when they will receive a paycheck. But when even a small fraction of controllers calls off, it can cause major disruptions, he said.

The National Air Traffic Controllers Association, an AFL-CIO-affiliated union, has encouraged its members to continue reporting to work.

“NATCA does not endorse, support or condone any federal employees participating in or endorsing a coordinated activity that negatively affects the capacity of the NAS (National Airspace System), or any other activities that undermine the professional image and reputation of the people we represent,” the association said on its website. “Air traffic controllers and other aviation safety professionals take their responsibility to protect the safety of the flying public very seriously.”

Members of Congress are aware the shutdown’s impact extends beyond air travel, Capito said.

“There’s a whole host of other things out here,” she said, citing funding for WIC, national parks, the military, border agents, the Coast Guard and other federal services.