The air enters at the ambient temperature of the atmosphere and must be heated to internal body temperature at an approximately constant pressure of 1.00 atm in our model. Human lung capacity varies from about 4.0L to 6.0L, so we shall use an average of 5.0L. Suppose you are outside on a winter day when the temperature is -14.0F.
How much heat would your body have had to supply in that case to raise the air up to internal body temperature? Suppose instead that you manage to inhale the full 5L of air in one breath and hold it in your lungs without expanding (or contracting) them.