Chicago, IL – The Tribune – Nine pupils were taken to area hospitals Monday afternoon after carbon monoxide leaked at a Catholic grammar school on Chicago’s Northwest Side, authorities said.

The leak was reported about 2:30 p.m. at Our Lady of Grace School in the 2400 block of North Ridgeway Avenue, Police Officer John Mirabelli said.

Several pupils complained of headaches and nausea at the end of the school day, said Diane Dunagan, a spokeswoman for the Roman Catholic Archdiocese of Chicago.

Dunagan said it was unclear how many children became ill because some went home with their parents.

“It happened right at dismissal, so many of the parents were already there picking their children up,” Dunagan said.

The cause of the leak was under investigation Monday afternoon, Fire Department Cmdr. Will Knight said.