Last week I spent three peaceful and fantastic days volunteering at the Festival for the Eno.  The weather was steamy with high humidity and temperatures flirting with 100 oF.   In the course of delivering drinks and ice to hot and thirsty people, I got into a conversation about sweat with my fellow Drink Booth Coordinator, Chip Gentry, and the outline for this column just sort of popped into my head.
Your body has a regulation system designed to keep your core temperature at 98.6 oF.  When your body gets too cold it converts energy stored in your cells into heat and increases body movement through shivering.  When your body gets too hot you sweat.  Sweating removes heat from your body when the liquid water on your skin evaporates, carrying away the energy required to vaporize it.
Even when you are sitting still your body is generating heat through normal metabolic processes.  Therefore, in order to maintain its normal temperature, your body must reject heat into the surrounding air.  Movement or exercise increases the amount of heat generation within the body, thereby increasing the amount of heat which must be removed to maintain body temperature.  Whenever your body can’t reject sufficient heat to the air through normal heat transfer mechanisms, including conduction, convection, and radiation, you sweat. 
The discussion above can be summarized by the following heat balance equation:

(heat generated by the body) = (heat lost to the air) + (heat removed by evaporation)

The rate of evaporation of sweat from your body is controlled by several parameters as shown in the equation below.

(Rate of Evaporation) = (K)(A)(100-RH)

The parameter K is called a mass-transfer coefficient.  I’ll spare you a potentially long discussion of this parameter.   All you need to know to follow this column is that K increases with higher air speed.  You already know this intuitively.  When you have sweat on your skin and a breeze picks up you can feel the rate of cooling by evaporation increase dramatically. 
The parameter A is area.  While sweat on your skin can, and does, evaporate through your clothing, the rate is much lower than from exposed skin.  So A in the equation is effectively the area of exposed skin.  So, as you already know, you stay cooler on hot days when you wear fewer clothes.
The parameter, RH, is relative humidity which requires a bit more explanation.  First, you need to know that warm air can hold more water vapor than cold air.  The maximum amount of water vapor which can be held in the air at a particular temperature is called the saturation concentration.  To calculate relative humidity, you take the actual concentration of the water vapor in the air and divide it by the saturation concentration at the current temperature to calculate a percentage. The relative humidity on a hot summer day in Chapelboro tends to be around 90%, while the relative humidity in an air-conditioned room is normally in the range of 40-50%.
Relative humidity has a strong influence on the rate that sweat evaporates from your skin.  Therefore, it also strongly influences the rate at which your body can cool itself. Consider the rate of evaporation at 90% relative humidity versus 40%.  From the equation above we can calculate this as:
Rate of Evaporation at 90% RH     =              KA(100-90)          =              KA(10)   =      0.167
Rate of Evaporation at 40% RH                     KA(100-40)                          KA(60)
So with all other parameters remaining equal, the rate of sweat evaporation at 90% RH is only 16.7% of what it is at 40% RH.  This phenomenon explains why we sweat much more on humid days.  Your sweat glands sense that your skin is too hot so they pump some water out on the surface.  When it is really humid, the water doesn’t evaporate very quickly so not much heat is removed.  So your sweat glands figure they better pump out even more water in an attempt to cool your body. Pretty soon your skin is glistening, your clothing is soaked, and you need to replace the lost fluids by drinking.
While there are days when I think the local weather forecasting can be a bit sensationalistic, hot weather can be a serious health threat. Consider our heat balance equation again.

(heat generated by the body) = (heat lost to the air) + (heat removed by evaporation)

On a day where the temperature exceeds 98.6 oF, not only can your body no longer reject heat into the air, the air adds heat to your body meaning that you have to rely almost exclusively on sweating to maintain body temperature.  As relative humidity approaches 100% then the rate of evaporation of sweat goes to zero since no water can evaporate into air which is already at its saturation concentration.  Therefore, if you are exerting yourself on a day where it is 100 oF and 99% relative humidity, your can get into trouble in a hurry since your body will continue to generate heat but almost none is being removed.  If your body temperature reaches 104 oF your body will conserve fluids by ceasing to sweat.  This condition is known as heat stroke and, if the body temperature is not reduced, can lead to organ damage and death.
I hope you enjoyed this short dissertation on sweat. In the end your Common Science advice for the next heat wave is to stay hydrated, look for a breeze, provide plenty of surface area for sweat evaporation and watch out for heat stroke.
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