Essentially, serious carbon-monoxide poisoning causes oxygen deprivation. Just why CO is so dangerous stems from the fact that it is so common. The most common sources are motor-vehicle exhausts. Internal-combustion engines generate a lot of carbon monoxide, and that's a problem if exhaust systems are not well maintained. Gasket leaks, holes in the muffler or holes in the pipes can be a real problem if a car is standing still, with its motor running and there is no wind.
Danger is especially high in the winter, when a vehicle gets stuck in snow.
If the driver runs the car to maintain heat and snow plugs up the exhaust pipe, carbon monoxide can find its way into the car. The most common sources of CO poisoning in the home are faulty heating and cooking appliances, according to the Wayne State University School of Medicine's Website. Since carbon monoxide is odorless and silent, it is hard to detect. At low levels of exposure, a headache is the most common symptom of CO poisoning. But as exposure levels increase, so do the symptoms' severity.