Re:A more down to earth answer... (Score 1)

He's an astronaut and an ISS science officer? Silly me! I forgot all about his special NASA olfactory training!!!

After all I, being only an earthbound armchair scientist, would never be able to sniff out the one single "outer-space" molecule swimming in each cubic centimeter of airlock air amongst the 2.7x10^19 other non-outer-space air molecules, but that's just me. And those welded compressed air storage tanks that contained the airlock air just before re-pressurizing?...well they DO they smell different than "outer space". A trained astronaut would know...