Heat from Breath

In an earlier post, we looked at how much water is lost through breathing.  Today, we’ll consider the amount of body heat lost through breathing.  Of course, depending on surroundings, air movement, and  how one is dressed, a lot of (or a little) heat might be lost through the skin, but we’re just considering the part that goes out with your breath.
We can use some of the things that we developed in the earlier post.  For example, in that post we decided to use 92 °F, 90 % rh as the typical condition for exhaled air.  We also determined that the volumetric air flow might vary a lot depending on body size and activity level: usually between about 6 and 216 liters/min.

For the purposes of calculating heat loss, we can just go to the psychrometric chart and determine the difference in enthalpy of incoming air and exhaled air.  Our exhaled breath carries both sensible (due to temperature) and latent (due to moisture) heat with it, but both of those are included in the enthalpy of the air.  Using 35 °F outside air, we get an enthalpy difference of 41.8 Btu/lbm_da for saturated air and 46.4 Btu/lbm_da for completely dry air.

This results in roughly 2 W/(liter/min) for total heat loss.

Now using the volumetric air flow, we can estimate heat loss at 12 W for a small person breathing slowly to 430 W for a large person engaged in vigorous activity.  Remember that this is just the part of heat loss from breathing air at 35 °F and does not include heat loss through the skin. For rough context, a common rule of thumb holds that total metabolic heat generation for an average person at rest is around 100 W.