By Kathy McCormack
The Associated Press

CONCORD, N.H. — More than 20 students and staff members showed symptoms of carbon monoxide exposure at a school Wednesday, but officials were stumped when air tests turned up none of the toxic gas.

Officials had not ruled out carbon monoxide as the cause of the situation at St. Thomas Aquinas School in Derry, they said.

"We have not been able to isolate a source yet," Derry fire Capt. Scott Haggart said. "So what we do now is we make all of the equipment work like it was working when the incident happened, but harder. We turn up the heat as high it will go, we make the furnace work hard, and we'll be returning throughout the night to re-meter the building."

The readings were taken minutes after firefighters responded to the school at 10:50 a.m., along with 10 ambulances, Haggart said. The 22 people showing symptoms were not seriously ill.

Most of the affected people were students and were on the third floor of the building, Haggart said. They reported feeling faint and experiencing headaches and nausea and were blood-tested for exposure to carbon monoxide, an odorless and colorless gas created when gasoline, oil, wood or propane is burned.

"We did find several patients with very low levels," Haggart said. "The trouble is the levels were so low they fall within the range that could be found normally in the public, but certainly it does pique our curiosity."

The school has more than 220 students in preschool through eighth grade; they were taken to a nearby church. They were not allowed back in the building as maintenance and fire crews tested.

Most patients were taken to Parkland Medical Center in Derry, where hospital spokeswoman Pat Collopy said most had been discharged by Wednesday afternoon and none would be admitted.

"They all had oxygen and were evaluated," she said.

Earlier this week, a New Hampshire hotel was evacuated after a faulty boiler room vent leaked enough carbon monoxide to sicken 11 people.

The Hilton Garden Inn, in downtown Portsmouth, was closed Monday morning after two workers and nine guests hanging out near the pool were overcome by the potentially lethal gas, also known as CO.

Most of the sickened people were treated at a hospital and released by Monday afternoon. The last was released Tuesday, the day the hotel reopened.

The hotel installed detectors to help prevent carbon monoxide poisoning, which causes about 300 deaths a year nationwide.