Brattleboro, NH- A carbon monoxide leak from a furnace in a three-story building in the Galanes Block on Main Street led to the evacuation of approximately 25 people Thursday morning, according to a fire official.

One woman exhibited symptoms of carbon monoxide poisoning and was taken to Brattleboro Memorial Hospital as a precaution; a man who also had symptoms was checked out on site but refused further medical treatment, Brattleboro Fire Chief Michael Bucossi said.

Firefighters were called to the building at 116 Main St. around 10:15 a.m., when a carbon monoxide detector on the third floor began sounding.

There is retail space on the building’s first floor, offices on the second and apartments on the third. The building also has a full basement, Bucossi said.

“Our crews could smell oil smoke and we were getting (carbon monoxide) readings on all three floors and the basement,” Bucossi said.

After evacuating the building, firefighters shut off the furnace and ventilated the building by opening doors and windows and bringing in fans. The building’s occupants were allowed back in just before 12:30 p.m.

Incomplete combustion in the furnace may have caused the leak, which remains under investigation, Bucossi said.