The Commission Scolaire de Montreal has confirmed that radon gas is still a problem in some of its schools.
Saint Marc and Notre Dame du Foyer in Rosemont-La Petite Patrie have radon levels above the norms set out by Health Canada.
Parents of students at the affected schools have been informed.
The school board is working on improving ventilation, and installing more sensors.
Quebec began testing schools throughout the province for radon in 2010
Radon is a radioactive, naturally occurring gas that can be released from solid rock, and is found in homes throughout Canada.
It's not a problem if the building is well-ventilated, but if left to accumulate it can build to levels that may cause cancer.
