This question is for Kim or anyone else who has an answer or a clue. I know that argon is a fairly viscous gas. Helium is used in place of argon for saturation deep diving because it is hard to get the argon in and out of the diver's lungs. However this should not be a problem if the pressure is less than sea level. Doing space walks is a problem for Shuttle astronauts because they have to pre-breath pure oxygen for a few hours to get the nitrogen out of their blood. This is to keep them from getting "bent" when their breathing pressure is reduced to the atmospheric partial pressure of oxygen prior to putting on their pressure suits. If they breathed oxygen diluted with an inert gas then its seems to me that they could significantly reduce their pre-breathing time. Helium is not an option because they turn into a flock of Donald Ducks and it's hard to understand them. However I don't think argon would cause that problem. Also argon is cheap (it's the third major component of air, viz. nitrogen, oxygen, argon). Any clues why argon isn't used as a dilutant?

Gary