Montreal's public health agency is warning drivers that leaving the motor running while clearing snow from a vehicle is a bad, and potentially fatal, idea.

If the car is started before the tailpipe is cleared, it can take as little as a minute for enough carbon monoxide to build up that it reaches dangerous levels, said Dr. David Kaiser, who works for Montreal public health.

Carbon monoxide is a gas produced by combustion, and it's odourless and colourless. The more encased a car is, the more quickly the gas builds up, he explained.

In March 2017, four Montrealers died while clearing their cars after a big snowstorm. Kaiser said they may have gotten inside the cars to take a break from clearing snow, lost consciousness due to gas buildup and died.

“A child shouldn’t be in the car until snow is cleared off.” Carbon monoxide poisoning can happen in minutes. Dr. David Kaiser joins <a href="https://twitter.com/DebraArbecCBC?ref_src=twsrc%5Etfw">@DebraArbecCBC</a> to explain. <a href="https://t.co/Q3jvuzkmEs">https://t.co/Q3jvuzkmEs</a> <a href="https://t.co/MqPIkeol4F">pic.twitter.com/MqPIkeol4F</a> —@CBCMontreal

Parents should also be aware of the dangers, Kaiser said — often, they will put their kids in the car, then brush off the snow.

"We saw that, without having deaths, in that same snowstorm we had several cases of kids with carbon monoxide poisoning," he said, adding that while they may recover in the short-term, there could be long-term effects on memory and attention.

And people who keep their cars in tempos should heed the same advice — even though they are outside, the gas can still build up quickly, he said.

Public health usually sees about 12 cases of CO poisoning in the winter. Not a high number, Kaiser acknowledged, but "one death is too many for something that is so easily preventable if you just do the right thing — don't start your car until it's cleared off."

How do you know if you are poisoned?

Carbon monoxide poisoning is tricky, Kaiser said, because the symptoms — headache, fatigue, dizziness, nausea — are not specific, and can be hard to identify in the winter when many people are already sick.

But those symptoms can quickly escalate to loss of consciousness and death, he said.

What to do if you feel sick?

"What's really important is, in a situation where you've been clearing your car, you get in and you start to feel unwell, that may be a stomach flu, but you should get out of the car, air it out and make sure that you're not putting yourself in a dangerous situation."

He said the same thing applies for those with gas heating or wood fireplaces in their homes — those symptoms can be a sign of danger.

Dr. David Kaiser said everyone needs to be aware of the dangers of carbon monoxide and its effects. (CBC)

Smokers, who already have high levels of carbon monoxide in their blood, and children are more susceptible.

For those with more severe symptoms the treatment is oxygen, so the victim should be taken to an emergency room as soon as possible.

Kaiser also said anyone who has a possible source of carbon monoxide in their home should have a detector, even though they are not required by law.