Carbon monoxide can become increased by snowfall. Just how much it increases depends on how low to the ground your intake and exhaust are from your furnace. The lower to the ground they are, the greater the chance of snow blocking those vents.
In older furnaces, the exhaust went out through your chimney. This made the chance of snow getting in the way much less likely. However , now with these newer and more energy efficient furnaces, the vents are out towards the back or side of the house. Although guidelines say that there should be at least 12 inches between expected snowfall and the exhaust, that doesn't always mean that's what happens.
Greg Haley is a certified home inspector, he says, "The installers don't read the manuals, evidently, because they just stick them out the (side)... so consequently a lot of them are really close to the ground and potentially a carbon monoxide hazard.
For your safety, make sure to check that they are above the one foot threshold. Even in the event that they are properly installed, snow drifts can still sneak up and block the vents.
Haley went on to say, "If it re-circulates the exhaust fumes that should be going out into the air back into the intake it will cause the furnace to make a lot of carbon monoxide which could be potentially deadly."
In that extreme case, it is always good to be prepared ahead of time. Symptoms of early carbon monoxide poisoning can mirror those of the flu -- fatigue, nausea and headaches. The best action to take for someone with carbon monoxide poisoning is to take them to a local hospital for treatment. Typically, the earlier it's caught, the faster the recovery.