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Q: We got a lot done last weekend to clear a firebreak in the trees around the barn.  It was a long hard day’s work in the heat.


One thing I noticed that puzzles me is that I was drinking about a quart of ice-cold water on the hour- more than I usually do- and didn’t urinate until very late in the afternoon.....could I have lost all of that to perspiration?  


I was covered well with long-sleeved shirt, helmet, gloves.   I didn’t notice a whole lot of perspiration, but then, at less than 10% humidity, how much would you really notice?  Paul    (7/3/12)


A: Hi Paul.  You will consume more water than is apparent in these conditions.  Good to know that you were paying attention and drinking extra water, lest you fall behind the hydration curve.  


One thing that happens, with even mild dehydration, is that your energy level falls off quite rapidly as the body is quickly affected by dehydration.  Sometimes people just push themselves ahead anyway, which is not only unpleasant, but it can be dangerous.


Perspiration will evaporate very quickly and small amounts might not even be noticed in these very dry conditions.  Also, with the heavier exercise you will metabolize more water in the process of doing the work.  


There is another moisture loss path that you cannot see, but it is significant.  Working means you breath deeper and more rapidly.  Lately we've been running at 6% humidity.  The humidity in you lungs is around 100%, so you can imagine where some of that water goes, and it is quickly passes out of the body.  More breaths equals more water loss.


In the winter you will see the micro-Ice crystals form as a cloud of exhaled vapor, but in this heat it is invisible.


Another source of water loss is by the normal transpiration of the moisture through the skin.  Our skin is kept soft and flexible in part by the water that is passing through it all the time.  In both the cold (which is very dry even if the RH is 90% because the cold air will not hold very much water vapor) and in our current very dry heat the moisture that passes through the skin for lubrication (this is not the same as perspiration for evaporative cooling) will be very quickly drawn away into the surrounding air.  That moisture needed by the skin is then replaced from within the body.  Hence you are losing more water through normal skin transpiration than in cooler or more humid conditions.


Bottom line, you will want to drink more water even though you are not seeing lots of perspiration.


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