Strato, you are correct about CBRN/NBC conditions. The M1 had what was known as a "gas particulate system". What this was was just a big gaskmask filter in the tank that all the crewstations could plug into. A small pump circulated air through the system. The M1A1 and A2 have an "NBC Overpressure System". Basically a pump and condenser system located in the left track sponson pressurizes the air inside the vehicle to create a higher pressure inside than out. That allows in theory for a crew to fight buttoned up (hatches shut) without their masks on. Normally it is used in conjunction with masks. The Overpressure System does have a secondary ventilation function from a gunnery standpoint. When the gunner indexes "COAX" for the coaxial M-240 machine gun, the overpressure system kicks on to push the fumes from the breech end of the machine gun out of the vehicle. Truth be told, most crews disconnect a certain cable which disables this function. Frankly because it is noisy and is a pain in the butt and coax fumes do smell kinda cool!