Discussion: oxygen and humidity

Does anybody know if the water content in air has any bearing on the relative oxygen content? i.e. does the humidity saturating the air displace, by volume, the oxygen, thus reducing your efficiency? Or is the extra pain entirely caused by the work required for cooling?

For the same pressure/temperature, a mix of gases with water in it will have less of the other ingredients than if there were no water. At 298 K/77 F, saturated water vapor pressure (corresponding to 100% humidity) is 3.17 kPa, which would imply that 3.1% of oxygen (and nitrogen) is missing, compared to 0% humidity. For comparison, at 3050 m/10000 ft 31% of oxygen (by mass) is missing compared to the sea level. The decrease in oxygent content due to the extra water at 77F/100% is approximately equal to the decrease due to altitude at 1000 ft.

I would look at this picture from dynamical point of view as well as physiological. The later will have probably more adverse affect as body tries to cool body down more at higher humidity and high temperatures.

Vlad, would I be correct in concluding from your scientific explanation that the humidity in the air would directly affect your oxygen intake, making it harder to run? I have definitely experience that when it is extremely humid I sometimes have the sensation of running under water, as if I am breathing water and not enough oxygen.