It took over two hours to clear out the air in the Brighton Building, 130 Court St., that was evacuated this morning after reports of high levels of carbon monoxide.
Watertown city fire officials and National Grid workers concluded that a malfunctioning space heater in the adjacent Velocity property, 122 Court St., was to blame.
Watertown City Fire Chief Dale C. Herman said nobody was taken to the hospital but between 20 and 25 occupants of the building were transported to the Burdick Building, 112 Franklin St., and many more went to work or to a friends home after reports of elevated levels of carbon monoxide at about 7 a.m. this morning.
All three properties are owned by Neighbors of Watertown Inc.
After ventilating the building through its windows with high-powered fans, fire officials deemed the building safe for tenants at about 9:30 a.m.
Officials said CO levels had greatly exceeded what would be deemed normal at 120 parts per million.
Mr. Herman said Occupational Safety and Health Administration regulations require firefighters to wear respirators in environments with CO levels higher than 35 parts per million.