The Associated Press

PORTSMOUTH, N.H. — A faulty vent in a hotel's basement boiler room leaked enough toxic gas on Monday to sicken 11 people, including nine guests hanging out near the pool, leading to an evacuation.

The sickened guests and two hotel workers in a nearby laundry room were hospitalized, but all but one of them had been treated and released by Monday afternoon. The worker who remained in the hospital had fallen unconscious and was taken there in serious condition but had been improving throughout the day, hotel spokesman Scott Tranchemontagne said.

The hotel, the Hilton Garden Inn in downtown Portsmouth, had to close at 10 a.m. Monday after an exhaust vent malfunctioned and caused a buildup on the basement level of odorless and colorless carbon monoxide, a potentially lethal gas created when gasoline, oil, wood or propane is burned.

Emergency crews had been called to the hotel minutes earlier to treat the female worker, who was unresponsive. Fire department Assistant Chief Steve Achilles said she was red-faced and had bloodshot eyes, symptoms of carbon monoxide poisoning, which causes about 300 deaths a year nationwide.

Firefighters who went floor to floor with carbon monoxide detectors found unacceptable levels throughout the 131-room hotel. They knocked on doors and told people to leave.

Achilles said 600 parts per million of carbon monoxide in the air were detected, far higher than the 35-45 parts per million considered normal. Authorities said the situation could have been worse if there had been high levels carbon monoxide, also known as CO, at night while people were sleeping.

Tranchemontagne said hotel workers had repaired the leaking vent by late Monday and planned to reopen the hotel on Tuesday.

The Hilton Garden Inn said it secured accommodations for its displaced guests at other hotels in the historic downtown area, which boasts many restaurants, shops, boutiques, theaters, galleries and waterfront gardens. It also said it installed carbon monoxide detectors throughout the building.