Workers also repaired an air-handling unit serving the J5 space on Oct. 3. After that, CO2 levels began dropping and have been consistently below the ASHRAE standard since, peaking on most days below 800 parts per million, according to data provided by StratCom.

“I can tell there’s a difference. Something’s been changed,” Duncan said.

“I feel it is (better). The air’s cleaner, fresher,” said Al Gay, a civilian who has worked at StratCom since 2003.

Cotton said the problem may simply be the structure’s age. It was built in 1957 to house the headquarters of what was then known as the Strategic Air Command.

“When you have an old building, you’re going to have issues,” Cotton said.

That’s especially true in buildings with basements, he added. Much of StratCom’s work is done underground, in a facility built to serve as a command bunker.

“Our basements have basements,” Karbler said.

With no definite cause, it’s not clear what else StratCom may do, though officials are planning a deep clean of the space in January.