Four Montrealers nearly succumbed to carbon-monoxide poisoning while sitting in their vehicles as their engines warmed up, buried in snow, reports CTV News.

The nearly-fatal incidents occurred mid-March after a very heavy snowfall buried vehicles parked outdoors in a thick blanket of snow.

Internal-combustion engines produce increased amounts of carbon-monoxide, or CO, when they are running inefficiently—namely on startup, when cold. CO is a by-product of incomplete combustion of any fossil fuel, including diesel and gasoline.

Public health officials say that CO can build up to fatal levels in vehicles in less than five minutes, rendering it dangerous to idle any vehicle in an enclosed space—including a car-shaped igloo of snow.

While CO is difficult to detect, being odourless and colourless, those exposed to the gas are likely to experience the onset of symptoms including headaches, dizziness, nausea, and fatigue before falling unconscious—it’s important to evacuate from any area suspected of CO gas at the earliest signs of exposure.

All four Montreal motorists were treated for CO poisoning in hospital after being found unconscious in their vehicles; despite their near-death encounter with CO, they were revived and recovered to full health.

(CTV News)