Photo: CTV News

Carbon monoxide levels in the hallways of a Montreal elementary school where 43 people fell ill Monday and required hospital treatment were as much as five times the level that usually triggers an evacuation, a fire official said.

"At 35 (parts per million) of carbon monoxide in the air, we evacuate a building," Eric Martel, an operations chief with the Montreal fire department, told The Canadian Press. Inside the Ecole des Decouvreurs in the city's LaSalle neighbourhood, firefighters detected up to 175 parts per million in the hallways — and 900 in the furnace room

Martel said a defective heating system was to blame for the gas, which left dozens of students and staff feeling nausea and dizziness. Some were vomiting, and an emergency room physician said nine children lost consciousness at the school.

On Monday afternoon, the Montreal Children's Hospital reported that 10 patients who received the most serious exposure — including the nine who lost consciousness — had been transferred to another area hospital to receive hyperbaric oxygen therapy. The treatment involves breathing pure oxygen in a pressurized room.

"The good news is all of these patients are in stable condition .... These are precautionary measures, and we expect that they will be released healthy very shortly," Dr. Robert Barnes, the hospital's associate director of professional services, said.

In addition to the 10 transferred for hyperbaric therapy, one child was being kept overnight at the Montreal Children's Hospital for oxygen therapy and observation.

The hospital had declared a code orange, which is used for an emergency situation with multiple victims, at around 12:30 p.m. Between 15 and 20 people, including one adult, were initially reported to have been affected, but then a second wave reported symptoms.

Carbon monoxide poisoning can result in coma or death, but emergency room physician Dominic Chalut said there are no signs of acute danger for the students treated at the hospital.

"The reason we use the hyperbaric chamber is to avoid long-term neurological effects," he told reporters. "Certain studies have shown that sometimes memory and concentration can be effected in the long term .... We are not taking any chances."

In total, 35 children and eight adults were transported to hospitals from the school. Ste-Justine hospital, the city's other pediatric facility, reported that it received 22 students, none of whom was in serious condition.

Hajer Hadj Taieb, who had a seven-year-old at the evacuated school, said she was sent an email from the administration saying some children had fallen sick.

"I was afraid because in the email, it said a few kids had been taken to hospital, and I didn't know if my son was sick or not," she said. "Everything is OK, thank God. This is the first time something like that has happened. There are security measures in the school, but accidents happen. They reassured me inside the school that everything is fine."

After the Ecole des Decouvreurs students began feeling dizzy and collapsing, other students were quickly moved to safety in a neighbouring school. Ecaterina Melnic, whose daughter attends the neighbouring school, said the incident has shaken her trust in the building's safety.