An elderly man has died and his partner is in critical condition as a result of carbon monoxide poisoning in the Ottawa home they shared, city paramedics said Tuesday.

Spokesman Marc-Antoine Deschamps said the couple was found earlier in the day by a neighbour who went to check on their well-being, though he did not elaborate on what prompted the man's concern.

Upon arriving at the east-end home at 10:45 a.m., Deschamps said the neighbour found both the man and the woman unconscious and in “critical condition.”

He said paramedics responding to a summons for help didn't take long to determine the cause of the trouble.

“The first paramedic to get there, ... she quickly determined that it was possibly carbon monoxide poisoning,” he said in a telephone interview. “She notified the fire department that was coming on the scene, so everybody was evacuated out of the house.”

Deschamps said the man, believed to be in his 70s, was pronounced dead in the home. His partner was taken to hospital for treatment in a hyperbaric chamber and remains in life-threatening condition, Deschamps said, declining to release the couple's names.

The neighbour who found the pair was also briefly taken to hospital as a precaution, he added.

Ottawa Fire Services spokeswoman Jen McNeely said officers investigating the home were quickly able to confirm the paramedic's early assessment of carbon monoxide toxicity.

She said tests detected carbon monoxide levels of 600 parts per million in the home and 700 parts per million in the basement, adding such numbers are well above the threshold for lethal exposure.

“If you're exposed to 400 parts per million for two hours, you'll have a serious headache,” McNeely said. “If you're exposed for three hours, it's life-threatening.”

She said investigators are still trying to determine the cause of the high carbon monoxide levels, as well as the length of time the couple was exposed to the toxic gas.

This story by the Canadian Press was first published Nov. 19, 2019.