Randy, both diving bottles and SCBAs are designed to be filled with grade D breathing air; may sound weird, but that's it's name. It's dew point is at -60*C so the air inside is DRY and wont rust out the bottle or cause bacterial/microbial growth. Some dive shops fill with grade C, which has a dew point of -30*C and is allowed higher impurity levels. The positive air pressure systems that fire/rescues use generally have a 1psi overpressure in the mask to keep out foreign particulates/atmosphere, similar to your M1A1s biological system.
I've used the same Draeger system with the NOVA Panoramic mask, but we don't use the Sentinel system since we all have I/S radios and don't have to have rapid extraction like you guys do. We actually use the Scott SKA-Pak more often when doing entries