Question

how can we prove the air we breath is warmer

Answer 1

It’s simple. Place a small thermistor or other fast-response temperature sensor into the airstream, inserted into a short length of wide-bore respiratory tubing for the subject to breathe through. Temp versus time shows a crude square wave corresponding to the respiratory cycle. I saw this technique used in a lab in the early 1980s before capnography was readily available.

You could calibrate inspired air temperature to ambient temp, then expect an expiratory plateau at body temperature. The lungs are wonderful heat exchangers, so exhaled alveolar gas leaves the lungs at core temp. It hardly needs proving!

Since the mouth is slightly cooler (about 1 degree F) than lungs, in principle you might measure a slightly lower temperature due to passage of exhaled gas through the oral cavity, as well as mixing of alveolar gas with anatomic deadspace gas remaining in upper airway since the previous breath. Breathing pattern—rapid & shallow vs. slow & deep—might plausibly play a role as well in determining the exact temperature. If you’re running a fever then your breath will be correspondingly warmer.

All of this presupposes that ambient temperature is below body temperature. Otherwise it’s the reverse: exhaled breath might be cooler than ambient temperature—at least until you succumb to hyperthermia, heat stroke & death.


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