DENVER ‒ They sit inside darkened facilities or lofty towers, surrounded by aging computers, speaking in clipped and cryptic phrases. And the slips of paper they pass between each other represent the lives of millions of Americans who travel safely every day with their help.

For the nation's air traffic controllers, the ongoing federal government shutdown is the latest insult in their high-stress, high-stakes jobs where mandatory retirement comes at age 56, access to cell phones during the workday is strictly limited, they get as little as 10 hours off between shifts and they are subjected to random alcohol and drug testing multiple times a year.

Before the shutdown, the nation's air traffic control system included just under 11,000 fully certified controllers - 30% fewer than the government says we need. That meant controllers were already consistently working six days a week, up to 10 hours a day on mandatory overtime, with vacations strictly limited.

Now they're not even being paid.

"This situation creates substantial distractions for individuals who are already engaged in extremely stressful work," Nick Daniels, the president of the National Air Traffic Controllers Association union, said in a statement. "The financial and mental strain increases risks within the National Airspace System, making it less safe with each passing day of the shutdown."