Emergency crews rushed to an elementary school in LaSalle on Monday when nine people fell unconscious and dozens more became ill after a carbon monoxide leak.

As many as 43 people -- 35 children and eight adults -- were taken to hospital to be evaluated after complaining of symptoms of nausea, dizziness, and vomiting at Ecole des Decouvreurs in LaSalle.

MUHC officials said 30 children and eight adults were transported in 11 ambulances and another five children were transported by a special STM bus.

As of Monday afternoon, one patient remained at the Montreal Children’s Hospital, and ten patients were sent to Sacre-Coeur Hospital for further treatment. Two others have been discharged.

Officials said Monday evening there were a few more patients still in the emergency room, but they could not give an exact number.

The school will be closed Tuesday as the heating system is tested, as is the air quality in the building.

Code orange dropped

Earlier in the day, the MUHC launched a code orange, which is used in an emergency with multiple victims. With it, the MUHC launched an appeal to the public not to bring anyone to the Montreal Children's Hospital ER.

“We ask that you not come to the Montreal Children’s Hospital Emergency Room for the rest of the day today,” Dr. Robert Barnes of the MUHC’s Glen campus. “We are focusing all our attention on the observation and recovery of these children.”

That is no longer the case, though, and operations are back to normal at the Children’s Hospital.

Likely carbon monoxide poisoning

Firefighters said the problem was caused by a heating system that released carbon monoxide into the school, and that when they arrived CO levels were five times the level that usually triggers an evacuation.

Officials from Marguerite-Bourgeoys School Board confirmed there were carbon monoxide detectors inside the school.

School officials were forced to evacuate the school and the healthy students were taken to Ecole Notre Dame des Rapides, which is about 400 metres away.

"Some kids starting throwing up, vomiting, throwing up, and not feeling well, about 30 kids and so they shut down the school," said the mother of a student.

"There are a couple of kids in the school's cabine, and the others they sent to the school over there."

Classes cancelled

School board officials cancelled classes for the 269 students at Ecole des Decouvreurs on Tuesday -- they will send a letter to parents and post the update on the school board's website.