Paradoxical helium and sulfur hexafluoride single-breath washouts in short-term vs. sustained microgravity. J. Appl. Physiol. 82(3): 859-865, 1997.-During single-breath washouts in normal gravity (1 G), the phase III slope of sulfur hexafluoride (SF 6 ) is steeper than that of helium (He). Two mechanisms can account for this: 1) the higher diffusivity of He enhances its homogeneous distribution; and 2) the lower diffusivity of SF 6 results in a more peripheral location of the diffusion front,

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... ere airway asymmetry is larger. These mechanisms were thought to be gravity independent. However, we showed during the Spacelab Life Sciences-2 spaceflight that in sustained microgravity (µG) the SF 6 -to-He slope difference is abolished. We repeated the protocol during short periods (27 s) of µG (parabolic flights). The subjects performed a vital-capacity inspiration and expiration of a gas containing 5% He-1.