At least two children were overcome by carbon monoxide at their daycare in St-Eustache, north of Montreal.
The city says on its website that 72 children and about 10 adults were taken to hospital as a preventive measure.
The incident occurred at Les petits explorateurs daycare centre earlier today.
The city says that all parents were advised and the leak is contained within the daycare building.
The St-Eustache fire department is at the scene.
Carbon monoxide is a colourless, odourless, and tasteless gas that is toxic to humans in high concentrations.