About three dozen Boston students have been sickened by natural gas fumes after a maintenance worker inadvertently opened a natural gas line next to their school's ventilation system.

Boston Fire Department spokesman Steve MacDonald says the fumes were blasted into the Excel Academy Charter School building in East Boston on Monday morning. The school was evacuated while firefighters and utility workers sought the source of the gas.

A spokeswoman for Boston Emergency Medical Services says 34 students between 9 and 12 years old were taken to four hospitals. They were complaining of nausea, dizziness and scratchy throats. Some were given oxygen at the scene. All have been released.

Excel has about 200 students in grades 5 through 8.