Cooling by Respiration

Most recent answer: 10/22/2007

Would like to determine how much energy (calories) are lost each day due to humidification of inspired air.

Normal insensible water loss with respiration is approximately 400 ml/day. Assuming ~540 calories are require to convert each gram (ml) of (lung) water to vapor, this implies a daily calorie output of >200,000 calories. Clearly, this isn’t right. So how much energy is consumed with 400 ml water loss/day in respiration?
- Craig Knoche (age 54)
Mammoth, CA, USA
Actually, I think that your calculation is probably pretty close. The trick is that the "calories" people talk about getting from food are actually kilocalaories. You’ve calculated that respiration’s evaporative cooling would use up about 200 of those per day, or about 10% of daily food intake. That sounds reasonable enough.

Mike W.

(published on 10/22/2007)