Ottawa paramedics say 15 children have been taken to hospital as a precautionary measure after being exposed to an unknown substance on a school bus.

They say there were 40 children on the bus at the time when paramedics were called to the scene around 9:20 a.m.

Paramedic spokesman Marc-Antoine Deschamps says several students, aged 6 to 12, were vomiting, coughing and feeling nauseous.

Deschamps stresses the injuries were minor in nature and all children are now in stable condition.

He says some children reported a smell on the bus.

Deschamps says they may never know what the noxious substance was.