BREWER, Maine&nbsp- A separated vent pipe from a new water heater is believed to be the cause of carbon monoxide poisoning that sent several residents of an apartment complex to the hospital over a two-day period.

One woman, 43-year-old Lisa Ouellette, whose apartment reportedly was located right above the separated vent pipe, was found unconscious by her boyfriend Sunday afternoon. She was taken to St. Joseph Hospital in Bangor, but it wasn’ t until Monday when others in the apartment complex got sick from the accumulating odorless gas that the Fire Department was called.

Brewer firefighters arrived at the River House apartments on Penobscot Street at about 11 a.m. to find several people complaining of headaches, blurred vision, weakness and upset stomachs. One woman was unconscious, Lt. Robbie Wildes of the Brewer Fire Department, said Monday afternoon.

“The key here was three people in one apartment” got sick, indicating there was probably a gas leak, he said.

Firefighters immediately evacuated the 31 apartments, and ambulance crews transferred five people to St. Joseph Hospital, where hyperbaric chambers can provide a supersaturated oxygen environment. Other residents were taken to the hospital by car. Because some residents were at work, only 21 people were evacuated, Wildes said. It was unclear how many residents were actually treated at the hospital for carbon monoxide poisoning.

A nursing supervisor at St. Joseph Hospital declined Monday evening to release the conditions of any of the patients.

Ouellette’ s mother, Joyce Ouellette of Millinocket, said Monday afternoon that carbon monoxide poisoning is believed to have starved her daughter’ s body of oxygen.

“She’ s not responding” to treatment, Ouellette said.

After finding out her daughter was sick, Ouellette rushed to Bangor Sunday evening with her other daughters, Sheila MacKenzie, 49, and Mary Ouellette, 48, and family friend Joan Fisher, 55. Her granddaughter Kristina MacKenzie, 24, was staying with Lisa Ouellette in the Brewer apartment, where the four visiting women also stayed Sunday night.

“None of us felt good this morning,” Ouellette said. “We all had headaches and were all dizzy. We thought it was because we were tired. We didn’ t know.”

She and her daughter Sheila MacKenzie left early Monday to go to the hospital, where doctors still were trying to determine what was wrong with Lisa Ouellette. “They couldn’ t figure out what was going on,” Joyce Ouellette said.

But back at the River House apartment where Mary Ouellette, Joan Fisher and Kristina MacKenzie had remained, MacKenzie, who is a nurse, began to recognize the symptoms of carbon monoxide poisoning that she and the other women were exhibiting and she called 911.

“We were the lucky ones,” said Joyce Ouellette, who was still at the hospital when the other women and residents arrived. “The next thing you know, they’ re all there by ambulance. They’ re all in the hyperbaric chambers,” she said referring to her family.

St. Joseph Hospital “was full of people with oxygen masks,” Ouellette said.

Now that doctors know the cause of Lisa Ouellette’ s illness, she is being treated with oxygen, but her diagnosis is still up in the air, her sister said.

“The damage might be done,” Sheila MacKenzie said. “We just don’ t know.”

By Monday evening, Mary Ouellette and Kristina MacKenzie both had finished their hyperbaric chamber treatments and were released from the hospital, Sheila MacKenzie said, adding that Fisher was staying overnight in the hospital.

“My sister Lisa still has not changed,” she said.

Carbon monoxide poisoning has a cumulative effect and can cause brain damage and death with lengthy exposure, according to the national Centers for Disease Control and Prevention Web site.

“Each year, more than 500 Americans die from unintentional CO poisoning,” the site states.

A call Monday afternoon to Keystone Management Co. Inc. of Orono, which is believed to be the company that manages the River House apartment complex, was not immediately returned.

Wildes of the Brewer Fire Department said carbon monoxide gas is invisible and tasteless, and often people think they are sick with the flu and therefore overlook early warning signs. Sometimes they just don’ t wake up.

Wildes said he worries more about CO poisoning than about fire and suggested that all homes be equipped with a carbon monoxide detector.

“It’ s pretty bad,” he said of Monday’ s evacuation. “We evacuated the building, opened up all the windows, put in some fans and disconnected the water heater.”

The water heater was installed recently and apparently a portion of the vent pipe separated, allowing CO to escape into the building, he said. “We monitored the air and let people back in when it was at a safe level.”

The MacKenzies and Ouellettes, however, packed up their clothes and headed to a hotel.