Emergency crews were initially called to the scene for a 28-year-old man in cardiac arrest at around 10 a.m. When they arrived, they noticed an unusual smell in the unit, and they evacuated the building.
The 28-year-old man was later pronounced dead in hospital.
"I can confirm that it seems to be someone who committed suicide," said Const. Simon Delorme, a spokesman for the Montreal police service, which took over the investigation.
Nine people were isolated at the scene as a precaution as officials worked to identify the substance.
Two were taken to hospital with unspecified injuries. A firefighter was also treated in hospital for heat stroke.
Fire officials said they have identified the nature of the substance but did not elaborate further. They said it was not hazardous and posed no risk to the public.
The perimeter around the apartment building was lifted just after 12 p.m.