One man is dead after the leak of a so-far unknown toxic substance in a Montreal apartment.

A building was evacuated, a street was closed and nine people were temporarily quarantined after a toxic spill in downtown Montreal.

The event happened at 10 a.m. when someone inside a four-storey greystone apartment building on De Maisonneuve Blvd. and Towers called 9-1-1 for help and Montreal firefighters rushed to the scene.

They found a 28-year-old man in cardiac arrest and they were not able to save his life.

"The body was evacuated to the outside of the building. The death was witnessed and stated on scene by Urgences Sante paramedics," said Marc-Andre Gagnon.

Those paramedics also smelled something very strange in the building and so brought in the Hazardous Materials team, prompting emergency officials to close De Maisonneuve between St. Marc Ave. and Fort St. for two and a half hours.

Firefighters evacuated the building while the HazMat team cordoned off the area and put nine people in isolation.

Two people were taken to hospital because they were having difficulty breathing and were vomiting.

So far nobody is certain exactly what type of hazardous material is involved.

At noon "it has yet to be determined by the fire department of Montreal" what the substance is, said Gagnon.

One of the firefighters collapsed from heat stroke and authorities say it was not due to exposure to the dangerous substance.

Tenant Lyndsey Shapansky had just returned home from a doctor's appointment for respiratory issues when she suddenly found herself under quarantine.

“I came back and I was like, 'ok hold on. Is it possible that the reason I was having trouble breathing is because there's some sort of chemical in my building?'” said Lindsey Shapansky. She was soon reassured that there was no connection. “I feel fine, I’m confident that’s not the issue.”

The nine tenants of the building at 1947 de Maisonneuve W. were soon after all cleared and told that they were not in danger and would not be required to go to hospital.

The individual who died appears to have intentionally ingested the toxic concoction but investigators are continuing to investigate.