(AP) – The strain of a 34-day partial government shutdown is weighing on the nation’s air-travel system, both the federal workers who make it go and the airlines that depend on them.

Air traffic controllers and airport security agents continue to work without pay – they will miss a second biweekly paycheck on Friday – but high absentee rates raise the threat of long airport lines, or worse.

Unions that represent air traffic controllers, flight attendants and pilots are growing concerned about safety with the shutdown well into its fifth week.

Airline executives say they are worried that long airport lines could scare off passengers. The economic damage, while small, is starting to show up in their financial reports.

Federal workers say going without pay is grinding them down, and they’re not sure how much longer they can take it.

“The stress is getting to everyone,” said Al Zamborsky, a radar specialist at Reagan National Airport outside Washington.

Zamborsky said co-workers are preoccupied with the shutdown and their personal finances. Safety isn’t compromised, he said, but efficiency has suffered.

The presidents of unions representing air traffic controllers, pilots and flight attendants outlined their security concerns in a prepared statement.

“We have a growing concern for the safety and security of our members, our airlines, and the traveling public due to the government shutdown,” they said. “In our risk averse industry, we cannot even calculate the level of risk currently at play, nor predict the point at which the entire system will break.”

The Transportation Safety Administration said 7.5 percent of its airport security officers scheduled to work on Wednesday did not show up. That is down from Sunday’s 10 percent absent rate but more than double the 3 percent rate of the comparable Wednesday in 2018.

TSA has resorted to sending backup officers to beef up staffing at more than 30 airports and at times, closing a couple checkpoints at major airports.

Air traffic controllers are hired by the Federal Aviation Administration.

“We have seen no unusual increased absenteeism and there are no operational disruptions due to staffing,” said FAA spokesman Greg Martin said. He declined to provide figures.

The impact to airlines’ business has been small so far. Southwest Airlines said Thursday that the shutdown has cost it $10 million to $15 million in lost revenue. The airline had $5.7 billion revenue in the fourth quarter. Delta Air Lines, which had $10.7 billion in fourth-quarter revenue, expects to lose $25 million in sales this month.

However, there are other effects. The shutdown is holding up Southwest’s plans to begin flights from California to Hawaii – the FAA regulators who must approve the long, over-water flights are not working during the shutdown. Delta’s service using new planes is also likely to be pushed back because FAA officials are off the job.

Airlines are walking a fine line: They fear more financial harm if the shutdown continues, but they do not want to alarm passengers.

Southwest CEO Gary Kelly called the shutdown “maddening.”

“This shutdown could harm the economy and it could harm air travel,” Kelly said. “This is crazy, it just absolutely needs to end.” He said the airline would do its best “to work through this slop and contain the damage.”

Kelly and Doug Parker, the CEO of American Airlines, said the shutdown has not affected safety. However, if the shutdown leads to fewer TSA agents or air traffic controllers on the job, Parker said, “you get long lines at TSA but still the same level of screening, you get larger separation (in the air) of aircraft, so you have delayed airspace – things that are really concerning to us. That is what we fear may happen.”

Parker urged government officials to resolve the shutdown so that customers don’t have to fear “delays at the airport because air traffic controllers that can’t afford to show up to work.”

The toll is greatest, of course, on the federal workers who are going without paychecks.

“I have an 8-year-old son who asked me, ‘Hey Dad, what are we going to do to pay our mortgage? Are we going to lose our house?'” said Mike Christine, an air traffic controller in Leesburg, Virginia. “That is not something an 8-year-old should be saying.”

Christine’s wife is a federal worker who is also about to miss her second paycheck.