Eleven people were taken to the hospital Monday after being sickened by a carbon monoxide leak at a New Hampshire hotel.

The problem was traced to a leaking vent in the basement boiler room, said Scott Tranchemontagne, a spokesman for the Hilton Garden Inn in Portsmouth.

Two hotel workers were in a nearby laundry room and nine guests were in the pool area on the same level. They all were brought to the hospital, and all but one had been treated and released by Monday afternoon, Tranchemontagne said.

One of the two workers remained hospitalized but had been improving throughout the day, Tranchemontagne said. The woman, whose identity has not been released, had gone unconscious and was brought to the hospital in serious condition in the morning, he said.

Emergency crews were called to the hotel called at 9:40 a.m. after receiving a report of an unresponsive person. Assistant Fire Chief Steve Achilles said the unconscious woman was red-faced and had bloodshot eyes, symptoms of carbon monoxide poisoning.

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

Achilles said 600 parts per million of carbon monoxide 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 at night while people were sleeping.

Tranchemontagne said the hotel remained evacuated late Monday afternoon as workers repaired the leaking vent.