AIUI, there's two big schools of thought about He:
It's a fast gas, so your tissues fill up with it quicker and so it takes longer to offgas than N
It's a fast gas, so offgasses quicker and so takes less time than N
And then there's the intermediate of "It goes in quicker, comes out quicker, so it all depends on depths and mixes really"
You might want to put this one on the DecoWeenie mailing list - Phi could probably explain the rationales better than I can
