Montreal health authorities now say 66 people died in the city during a heat wave between June 30 and July 8 last year.
An investigation has shown two-thirds of the victims were over the age of 65 and most were suffering from chronic illnesses.
A spokeswoman for the Montreal-area health authority said that social isolation, having a low income and living in a so-called heat island also significantly increased a person's risk of death.
The health authority is recommending better strategies to identify vulnerable people through partnerships between local officials, health services and community organizations.
It is also suggesting that some neighbourhoods immediately plant more trees and other greenery to combat the heat island effect.
Previous estimates had suggested up to 53 died in the city when maximum daily temperatures soared to between 31.9 and 35.5 degrees at the end of June and early July.