BREWER – One of the two women found unconscious earlier this week from breathing carbon monoxide gas that seeped into the apartment they were staying in has left the hospital and the other one is greatly improved, according to a relative of one of the women.

The CO gas leak, apparently caused by a separated furnace vent pipe, sickened several residents at the River House apartments on Penobscot Street, with one taken to St. Joseph Hospital on Sunday and seven others taken there Monday.

Lisa Ouellette, 43, whose apartment is located near where the separated pipe was releasing the poisonous gas, was found unresponsive by her boyfriend early Sunday evening.

Three of the seven people treated Monday were staying in Ouellette’s apartment. Brewer Fire Department crews evacuated the building around 11 a.m. Monday, after Ouellette’s niece Kristina MacKenzie, who is a nurse, recognized the signs of carbon monoxide poisoning and called 911.

Ouellette’s friend Joan Fisher, 55, was found unconscious by firefighters.

“My sister is coming along,” Sheila MacKenzie, Ouellette’s sister, said Thursday. “She’s awake and alert and she knows who everybody is.”

The last few days have been stressful, Sheila MacKenzie said, with her sister slow to respond to the treatment for the poisoning. Long-term exposure to carbon monoxide can rob the body of oxygen and can lead to brain damage and even death.

“It was a few hard days,” MacKenzie said. Until she started to come around on Wednesday, Lisa Ouellette was mostly drowsy and unresponsive, her sister said. “We were often afraid that was the way she was going to be. We’re thankful everybody is alive. Everyone seems to be doing better.”

Fisher was released from the hospital Thursday.

The Brewer Fire Department evacuated the entire 31-unit complex on Monday, but because some residents were at work, only 21 people were in the building at the time.

The property is owned by EWT LLC of Concord, N.H.

Concord Gas of Bow, N.H., installed the new propane furnace that is being blamed for the leak. Concord Gas did not obtain the required city permit to do the work, Brewer Code Enforcement Officer Dave Russell has said. State agencies also are investigating whether the New Hampshire technicians who did the work were licensed in Maine.

Three of Ouellette’s relatives and a family friend, who all live in Millinocket, drove down to the Bangor area after she grew sick Sunday. They joined Kristina MacKenzie at Ouellette’s apartment where they stayed that night.

All five women said they had headaches and felt dizzy Monday morning, but initially attributed the problem to being tired. It wasn’t until later in the morning, after two of the women had gone to the hospital to check on Ouellette’s condition, that Kristina MacKenzie recognized the symptoms she and the two women remaining at the apartment were experiencing as signs of CO poisoning.

The situation could have been avoided if the building had CO detectors, Sheila MacKenzie said.

“I would think everybody in that building would get one,” she said. “It would be good if they did.”