Water Lost Through Breathing

Have you ever wondered how much water you lose by breathing? Do you think that it is more or less than that lost through sweating? Most of the time when we breathe, we are exhaling moister air than we breathe in. There might be a few exceptions, but normally our bodies are losing a little bit of moisture with every breath. In this blog, we’ll try to quantify that moisture loss, at least in a ballpark way. 

       The first thing that we need to get a handle on is how much air goes in and out of our lungs. Unfortunately, there is a wide variation in that number depending on body type and activity. The “tidal volume” is the amount that is moving in and out with each breath—it ignores a remainder volume that is not expelled when we breathe out since our lungs don’t collapse completely on each breath. As a first approximation, a typical number used for the tidal volume is about 500 ml for both men and women at rest. Obviously, this depends to some extent on body size, but 500 ml is an average for adults. The “inspiratory reserve” is the amount that you can force in after inhaling a normal tidal volume and averages about 3100 ml for men and 1900 ml for women. 

      Breathing rates also vary a lot. At rest, average breathing rates are around 12-20 breaths per minute, while they can go above 60 breaths per minute during really intense exercise. Combining the total volume with the breathing tempo results in a range from around 6 liters per minute to 216 liters per minute—a variation by a factor of 36! 

     We also need to get an estimate for the amount of moisture loss in each breath. The moisture lost would just be the difference between the moisture of the air coming in, and the moisture of the air going out. 

     For the case of cold air, we can pin down the incoming air condition pretty well. Recall that the shape of the psychrometric chart was much narrower to the left side (colder temperatures)

than on the right side. In fact, at 35 deg F, the maximum humidity ratio (occurring at 100 percent relative humidity) is only 0.0043 lbmv/lbmda at sea level. So if we are talking about air close to freezing, there is not much moisture, in an absolute sense, regardless of the relative humidity.

      Fixing the state of the air that is being exhaled is a little trickier, and I suspect that it may depend a little on the speed and depth at which one is breathing, but it appears that a conservative estimate might be 92 deg F and 90% relative humidity. This corresponds to a humidity ratio of 0.030 lbmv/lbmda at sea level. 

     Using those values, we can obtain constants to calculate water loss associated with breathing 35 deg F air at sea level: 

If the air is saturated (100% relative humidity):

If the air is completely dry (0% relative humidity):

Notes: 

*As an example: I breathe about 40 times per minute when I am hiking up hill, and since I am panting pretty heavily, I’ll estimate that I might be moving 2500 ml with each breath. Using the constant for dry air: 40 breaths/minute * 2.5 liter air/breath * 0.00230 liter water/hour per liter air/minute=0.23 liter/hr. Losing a quarter of a liter of water through my breath is not a lot compared with water lost through other means over the course of hiking hard for an hour, but it does provide an answer to the original question. 

*There is only about 15% difference between the two constants, so at 35 deg F, the relative humidity doesn’t matter too much, as we discussed earlier. 

*It is pretty easy to estimate your breathing rate with a stopwatch. You could get a fairly reasonable estimate for your breathing volume using a plastic bag, although you’d want to be careful not to re-breathe the same air more than a few times. 

     If you are willing to assume completely dry air as I did in the example, that constant would be pretty good for any temperature. Unfortunately, outside air is seldom completely dry, even in the desert, and as you move to warmer temperatures, the relative humidity will change the humidity ratio of the air a lot. In the next post we’ll look at the effects of temperature and altitude on those constants.