Northborough, MA- A carbon monoxide leak yesterday morning led to the evacuation of the Marion E. Zeh School and sent a janitor to the hospital.

A carbon monoxide detector in the cafeteria’s kitchen prep area set off an alarm after detecting the odorless, colorless gas around 9:25 a.m., said Fire Chief David Durgin.

The school was evacuated, though no students were in the cafeteria at the time of the leak.

“This was a very isolated part of the building,” Durgin said. “There were no children in the area.”

Carbon monoxide detectors were installed in all Northborough schools several years ago, said Durgin.

“I give the school a lot of credit for having the foresight to do it,” he said.

The detectors sound an alarm if a reading of 30 parts per million of the gas is measured. Normal levels of carbon monoxide are one to 10 parts per million, said the chief.

Fire officials used meters in the boiler room, and isolated the area after getting a carbon monoxide reading of 100 parts per million.

A mechanical failure of the boiler was blamed for the leak, Durgin said.

The room was ventilated using power fans, a ventilation machine and opening doors and windows. The burner and boiler were shut off, as was the natural gas supply to the building, Durgin said.

Classrooms near the cafeteria were tested for the gas, and normal levels were found, Durgin said.

A janitor who inspected the boiler after the alarm had sounded complained of headaches and was treated by fire officials outside the school. He went to the hospital on his own accord as a precaution, said Durgin.

Fire Capt. Robert Theve stressed the importance of having carbon monoxide detectors in homes and schools.

“This is a case-in-point of that system working perfectly,” he said.

About 350 students and staff members played games and waited on school grounds until fire officials gave the OK for classes to resume around 11:30 a.m.

Superintendent Charles Gobron praised the staff and fire and police departments.

Parents were informed of the incident through Connect-Ed phone and e-mail messages sent automatically by school officials.

“We informed them that everything was fine, everyone was safe and the day is continuing as normal,” said Gobron.