More than a dozen children on their way to school became ill due to an "unknown substance" aboard their school bus Thursday morning, emergency officials said.

Emergency crews were called to the bus on Kaladar Avenue in south Ottawa around 9:40 a.m.

Fifteen of the 40 children aboard the bus were taken to hospital as a precaution, paramedics said. All the patients were between six and 12 years old.

The bus was on its way to nearby école Lamoureux, according to the Conseil des écoles catholiques du Centre-Est (CECCE), eastern Ontario's French Catholic school board.

The board said parents of all the children have been contacted.

Marc-Antoine Deschamps of the Ottawa Paramedic Service says emergency crews responded to a bus on Kaladar Ave. this morning. Children on the bus were coughing, vomiting and feeling nauseous. 0:28

Coughing, vomiting

When paramedics arrived, some of the children were coughing, vomiting and feeling nauseous.

The Ottawa Fire Services hazmat team was also on the scene, but didn't find anything noxious or dangerous, said spokesperson Danielle Cardinal.

She said the driver did everything right by getting the children off the bus safely and calmly when some of them became ill.

The bus was taken to a garage to be examined. The school board later said there might be a problem with the vehicle's brakes.

Police closed Kaladar Avenue until about 12:50 p.m..