A Montreal elementary school will reopen two days after 35 children and eight adults were hospitalized for suspected carbon monoxide poisoning.

The Marguerite-Bourgeoys School Board confirmed in a news release that Ecole des Decouvreurs in LaSallewill resume its regular schedule on Wednesday, just two days after emergency crews brought 43 people to hospital with suspected carbon monoxide poisoning.

Initially, between 15 and 20 people – including one adult – complained of symptoms of dizziness and nausea on Monday morning, prompting an evacuation at the school. Later, more children and teachers started to exhibit symptoms.

One young boy told CTV Montreal that he had a headache and chest pains before he fell to the floor and chipped his tooth.

Officials from McGill University Health Centre said 30 children and eight adults were transported in 11 ambulances and another five children were transported in a specially designated bus to different hospitals in the area.

As of Monday evening, the MUHC said, of the 16 patients it received, one child was undergoing oxygen therapy, two had been discharged, and three remained under observation in the emergency room. Ten patients had been sent to Sacre-Coeur Hospital for further treatment.

The hospital said one adult was put under observation at Royal Victoria Hospital. All 17 patients were in stable condition, according to MUHC.

As for what caused the illnesses, firefighters at the scene said there was a problem with the school’s heating system.

Ian Ritchie, an operations chief for the Montreal fire department, told CTV Montreal the suspected carbon monoxide leak could have been an ongoing issue in the school without anyone knowing.

“This could have been something going on for a couple of days, where these kids were exposed to it,” he said. 

Fellow Operations Chief Eric Martel told The Canadian Press firefighters discovered carbon monoxide levels in the school’s hallways that were five times the level that would typically initiate an evacuation.

Officials from Marguerite-Bourgeoys School Board said there were carbon monoxide detectors inside Ecole des Decouvreurs, but it’s unclear if they were operational at the time. The school board is now planning to check the devices in each of their schools.