Heat from Breath, part 2

In the last post we considered heat lost through breathing and determined that breathing air at 35 °F results in a heat loss of about 2 Watts for every liter/min of air flow.  Of course, this depends strongly on the temperature and relative humidity of the ambient air.  In this post, we’ll consider both the effect of the condition of the ambient air, and also how much of the heat is latent (due to moisture) and how much is sensible (due to temperature).

This figure

shows the heat loss (in W/(liter/min)) as a function of both the temperature and relative humidity of the ambient air.  At 35 °F, the range from 1.9 to 2.1 W/(liter/min) is pretty small which is why we just approximated it as 2 W/(liter/min) in the last post.  At 75 °F, even though the heat loss is considerably less, the dependence on relative humidity is much more pronounced.

This figure

shows the portion of the total heat loss that is latent heat as a function of temperature and relative humidity.  The remainder, of course, is sensible heat loss.  It is interesting to note that in all cases, over half of the heat loss is latent.