MONTREAL — Thirty-three people have died in Quebec from one of the worst heat waves to hit eastern and central Canada in decades, the authorities said on Thursday, underscoring the vulnerability of elderly, ill and poor people during the hot summer months.

Dr. David Kaiser, the senior physician at Montreal’s public health agency, said that most of those who had died from the heat in Montreal were 65 or older, had histories of health problems or mental illness and had been living without air-conditioning.

He said that disadvantaged people who had chronic illnesses or were living alone were particularly vulnerable. “While we are an advanced industrialized economy, these deaths reflect that there are serious social inequalities,” he said, adding that many wealthy countries the world over had been hit by heat waves associated with climate change.

Public health authorities in Quebec have made a concerted effort to improve the reporting of heat-related deaths. They have set up a special liaison program with hospitals and emergency rooms to monitor heat-related illnesses and mortality, Dr. Kaiser said, which may explain why the number of deaths seems high compared to elsewhere in the nation.