Air traffic controllers play a critical role in the US aviation industry. Every day, as over 45,000 flights take off across the country, they are responsible for ensuring the safety of flights through airspace oversight to help connect passengers to their final destinations.

United Airlines suggested Wednesday that an air traffic controller shortage is to blame for “significant delays” at its Newark Liberty International Airport (EWR) hub, as people in the U.S. began their holiday travel.

The airline is citing delays at its Newark hub saying more than 348,000 passengers have been disrupted by delays since the start of November due to the air traffic controller shortage.