CHARLOTTE, N.C. -- A dozen people were treated for carbon monoxide poisoning after dangerous levels of the gas were found in uptown Charlotte’s Harris Teeter grocery store.

Officials said a contracted crew with Crescent Construction was working in the store at the intersection of Sixth and Pine streets around 5 a.m. Wednesday when a contractor pressure washing the drains in the floor passed out. The carbon monoxide fumes from the propane-powered machine overcame him, and his condition was initially considered life-threatening.

That worker was rushed to the hospital, where he is expected to fully recover.

Five other workers, four firefighters and two emergency medical services workers were also taken to the hospital. Officials at the scene said the emergency responders were in the store for about 10 minutes before they discovered the carbon monoxide levels were dangerously high. They were all transported as a precaution and have now been released from the hospital.

Capt. Rob Brisley with the Charlotte Fire Department said crews didn’t know what they were walking into because the call came in as one person sick.

“As firefighters respond to many calls, sometimes there is uncertainty. But we’re grateful to have the equipment, the monitoring -- the meters on every fire truck were valuable this morning because in a matter of minutes upon arrival we realized we had a dangerous situation,” he said.

Fire crews used fans to clear the air and allowed employees back into the store just after 7:30 a.m. The store opened to the public around 10 a.m.

The North Carolina Department of Labor and the Occupational Safety and Health Administration are investigating and will determine whether any fines should be levied.

Officials said residents from townhouses in the same building above the store did not have to be evacuated and were not endangered by the gas.