Houston, TX – Nine people were evacuated from a Houston home after suffering severe carbon monoxide poisoning Monday night in a case firefighters said could easily have been fatal.

Houston Fire Chief Christian Hartley said one resident at the home on Corn Street called emergency crews shortly before 10 p.m. Monday, reporting that one of the victims was unconscious. The home had a carbon monoxide detector, but Hartley said it wasn’t working.

“One of the guys realized, ‘I’ve got a headache, everyone’s sick, the dog is puking and we’ve got this person who can’t wake up,'” Hartley said. “He was conscious of the fact that he wasn’t completely alert, so he called a friend for help – he helped everyone get out of there.”

The friend also opened windows and doors to help ventilate the structure before crews arrived.

Roughly 30 responders converged on the home. One of the victims was flown to Anchorage via air ambulance, and five ambulances took the other victims to area hospitals. Hartley said all nine were in serious condition when they were evacuated; two were nearly critical.

When firefighters entered the home, Hartley said, they found CO levels above 1,000 parts per million – near the level federal authorities say poses an immediate threat to human health. The odorless and colorless gas can initially cause symptoms like headaches and nausea, and can be fatal in prolonged or intense doses.

Firefighters found similar levels in a South Anchorage home where Trevor Noble, 18, died of CO poisoning last month. Seven of his family members were sickened by the gas, which investigators said came from a disconnected exhaust pipe on a heating boiler in the garage.