Water Vapour Pressure in Expired Air

The water vapour pressure in the expired air of normal subjects during rest and exercise was measured by a mass spectrometer. The mean vapour pressure was found to be 37.9 mm Hg during tidal breathing and 39.7 mm Hg after a vital capacity manoeuvre. The pressure was reduced to 36.1 mm Hg during exercise. This decrease was not significantly different from breathing at rest. When offering warm wet air with a water vapour pressure of about 75 mm Hg, the end-tidal pressure was found to be 47 mm Hg. The evidence for complete saturation of expired gas is discussed.