At least four Montrealers were rushed to hospital Wednesday morning after being found unconscious in their cars.

In each case, the people were sitting inside a car partially buried in snow with the engine running.

All were treated for carbon monoxide poisoning and came close to dying.

The Montreal Children's Hospital said that it has since seen several cases of carbon monoxide poisoning in children.

Carbon monoxide is a colourless, odourless gas that is produced when fuel -- such as charcoal, gasoline, or other flammable substances -- does not burn completely.

It can be trapped in a car if the exhaust is blocked and the engine is running.

Mild exposure can cause headaches, dizziness, nausea, and fatigue.

Public health officials say it can take less than five minutes for carbon monoxide to build up to lethal levels.

Two people died near Quebec City during the storm after they were trapped in their cars, and carbon monoxide poisoning is being examined as a possible cause.

Montreal's Public Health department has more information on carbon monoxide and how to protect yourself.