Water Lost Through Breathing, Part 2

In the last post we talked about the water loss from breathing and used the assumptions of exhaled air being at 92 deg F and 90% relative humidity, and inhaled air close to freezing, to develop a constant to estimate the water loss per hour for any breathing rate. This time, we’ll look at the effect of temperature and relative humidity and elevation on that constant.

     Last time we said that close to freezing, the whole span of humidity ratio from completely dry air to saturated air doesn’t make too much difference in the constant. However as we move away from the freezing point, the relative humidity starts to make a bigger difference in the humidity ratio. Recall that for dry air at 35 deg F, the constant was:

     This figure shows the constant (at sea level) as a function of temperature and relative
humidity: 

   For completely dry air, the constant barely changes at all, but for saturated air (100% relative humidity) the constant drops all the way to zero over the temperature range shown. Of course, anybody exercising hard at 90 deg F and 100% relative humidity has a lot of other things to worry about besides how much water might be lost through their breath. 

      This figure shows, in a general way, how the constant changes with altitude.

   I used the standard atmosphere for temperatures and pressures. Of course, there is some ambiguity because the standard atmosphere specifies both temperature and pressure as a function of altitude, and at higher altitudes, the temperatures are way below freezing. Since I just wanted to show the approximate effect, this figure shows a hazy band representing the constant for temperatures close to freezing, and including all relative humidities, up to about 20,000 feet.