The Mjolnir is capable of up to fifteen minutes in vacuum without problems. However it has to draw air from it's surroundings when in an oxygenated environment, otherwise how would the Spartan breathe after fifteen minutes where there was oxygen? In other words, the Mjolnir has it's own oxygen system yes, but it also has to be able to take in air as well. To this end, it will likely have an inbuilt filtration system to ensure only unpolluted air gets through. So in a sense it would have a highly advanced gas mask, but it would just be considered part of the air filtration system instead of the gas mask.