"Extremely high" levels of carbon monoxide sent about 20 children attending Sunday school in Waltham, Mass., to the hospital, authorities said.
Waltham fire officials said they discovered the gas leak in St. Mary's Religious Education Center after they were summoned about 9 a.m. EST when several teachers and students felt dizzy and light-headed, Deputy Fire Chief Michael Quinn told The Boston Globe.
He said the children, ages 8-12, had non-life-threatening injuries and were expected to be released soon.
"A lot of it was precautionary," he said.
The children taken to the hospital were among about 50 children and a handful of staff in the building at the time of the leak, Quinn said.
The building was evacuated and its heating system was shut off until it is determined how the leak happened.
"I know they were going to check the boilers, the chimney and the hot water heater," Quinn said.