For air travellers, the most visible manifestation of the ongoing shutdown of the US government may be longer lines at security checkpoints.

Behind the scenes, however, airlines are counting costs that increase the longer the political stalemate goes on.

Business Insider reports that passengers at Atlanta’s Hartsfield-Jackson International Airport were reporting waits of more than an hour at undermanned Transportation Security Administration checkpoints, with some missing flights as a result.

Airport officials advised travellers to arrive three hours ahead of flight departure times in anticipation of delays in clearing security.

Ed Bastian, CEO of Delta Air Lines, estimated that the shutdown would cost the airline $25 million in revenue in January alone, due in part to fewer government employees and contractors flying, CNBC reports.

Delta has also been unable to roll out its new Airbus A220 jets due to the absence of FAA safety inspectors.

Meanwhile Southwest Airlines has been planning to launch new service between the mainland US and Hawaii, but due to the shutdown has not been able to obtain needed certifications from the US Federal Aviation Administration (FAA), USA Today reports.,