Carbon monoxide levels inside the Montreal elementary school where 35 children and eight adults were hospitalized reached as high as five times the safety limit, according to a fire department official.

Eric Martel, an operations chief with the Montreal fire department, told The Canadian Press that inside the Ecole des Decouvreurs, firefighters found the hallways had 175 parts per million of carbon monoxide and the levels increased to 900 parts per million in the furnace room. Martel added 35 parts per million should trigger an evacuation.

Students and staff at Ecole des Decouvreurs in LaSalle, a school of about 270 pupils, suddenly started falling ill about 11:30 a.m. An ambulance service spokesman said those hospitalized complained of symptoms including dizziness and nausea.

Between 15 and 20 people, including one adult, were initially affected, but after the school was evacuated and children were moved to a neighbouring school, another wave of children and teachers complained of symptoms. One student at the school told CTV Montreal he felt a headache and chest pains before collapsing to the floor and chipping his tooth.

An emergency room physician at Montreal Children's Hospital said nine children lost consciousness, but are now in stable condition and under observation. These nine patients and one other have been transferred to another part of the hospital for hyperbaric oxygen therapy.

Another child will be kept in the hospital overnight for oxygen therapy and observation.

The problem stems from the school’s heating system, which released deadly carbon monoxide into the building, said Francis Leduc, chief of operations for the Montreal fire service, from the scene. He said about 300 people were evacuated from the building and fire crews would ventilate the structure before workers repair the heating system.

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

The cause of the illnesses has not been confirmed, said Stephane Smith of ambulance service Urgences-Sante. In a letter to parents published online, the school administration raised the possibility of a problem "related to the heating system."

One aunt who came to get a little girl, who was complaining of a headache, said school officials were organized and handling the situation smoothly.

The school board said classes will be cancelled Tuesday to ensure the building is safe before for students to return.

With a report from The Canadian Press and CTV Montreal