Since there would only be oxygen in the system and whatever nitorgen or inert gas is contained in the loop, I doubt decompressoin would be much of a problem. It can't be a pure oxygen system since oxygen is toxic to humans when the partial pressure hits 200%. A pure oxygen system becomes toxic at 2 atmospheres, or about 30 feet down. This is one problem with traditional re-breathers which use pure oxygen (the other big problem is that the scrubbing agents give off chlorine gas if exposed to water).

It would be a simple matter to allow gasses to exchange across a semi-permiable membrane, but how many square meters would it take to support a diver?

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