Another unsafe characteristic of the Corvair is the design of the heater system in all but 1960 models, which had an optional direct gasoline-burning heater. Vehicles with water cooled engines use a heat exchanger known as a heater core and blow air across it either with a vent or a blower motor (usually with a squirrelcage fan) in order to heat the cab. Vehicles which are air-cooled do not have this luxury. To provide heat the corvair had heating ducts which wrapped around the exhaust manifolds, the hottest part of the car (if you discount the exhaust valves themselves.) Over time the exhaust manifolds can (and sometimes do) develop leaks, either through rusting, cracking, or gasket failure, and the exhaust leaks lead directly into the heating system. From there, they naturally lead directly into the seating area of the car. Carbon monoxide is generally nothing you want to be breathing as it is extremely toxic and carbon monoxide poisoning is responsible for thousands of accidental deaths worldwide each year - in fact, in the US, there are more deaths yearly from carbon monoxide poisoning than from any other kind of poisoning - about six hundred of them.