Why expired air often contain more water vapour than inspired air
Answers
Answer:
The air expelled from our lungs is roughly at our body temperature. Our body temperature is higher than ambient temperature in most situations and locations.
The surface area to volume ratio of our lungs is very high to be able to extract oxygen from air efficiently. This has the side effect that it makes the air that we breathe in originally at ambient humidity into our lungs become very humid very fast.
Add the fact that air at a higher temperature can hold more moisture and you will have a condition where when the air that you exhale meets a cooler surface, the air cools and is not able to hold as much moisture as it is already holding and condensation occurs on that surface. This gives the impression that we exhale water vapour.
It's basically the same effect as condensation on cold surfaces like the side of beer mugs or cold drinks.
Try creating condensation on say a metal stappler with your breaths in the summer or whenever the ambient is higher than our body temperature of 37°C and you will find that it doesn't happen. But this doesn't mean the air we exhale isn't humid. It still contains a lot of moisture due to our lungs. It's just that we can't easily create condensation on most surfaces at ambient temperatures. Breathe onto a can of cold drink in the summer and condensation will still occur if the can is cold enough.
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