It's Not the Heat,It's the Humidity!

By Tamara J. Newell, RN, MS, CS.

Division of Services for Aging and Adults with Physical Disabilities

Whew!! Sure is hot out there! Who turned the air conditioner down? Why do we get so hot? The body generates energy through metabolism which generates heat. With increased activity or environmental temperature, the body will produce even more heat. To keep the body in balance, the excess heat must be dispersed. How do we get rid off the heat? Various body mechanisms are activated by the nervous system which cause the surface blood vessels to dilate, bringing the body's internal temperature closer to the surface. Heat is then lost in the following ways:

1. Conduction: directly transfers the heat from a warm to a cooler area...a cool cloth on a hot forehead.

2. Convection: releases heat with the movement of air over the body...standing in front of a fan.

3. Radiation: heat is transferred from one surface to another and occurs without contact...standing in front of an open freezer.

4. Perspiration: fluid released through the glands on the surface of the skin cool the dilated vessels. Evaporation of this fluid causes the body to lose heat. As environmental temperatures approach the body's temperature, less heat can be lost through conduction, convection and radiation, then perspiration becomes more important.

What does humidity have to do with it? As the humidity increases, the effectiveness of perspiration decreases because the sweat doesn't evaporate. Well, is it the heat or the humidity? Decreased heat loss can result in the core body temperature rising. An adult who is not used to a hot environment can lose up to 1.5 liters of sweat per hour.

The resulting rise in body temperature and fluid loss can lead to dehydration (mild to moderate loss of fluid), heat syncope (fainting on hot days), heat exhaustion (excessive fluid loss through sweating) and heat stroke (core body temperature of 105 - 106 degrees F, which can be fatal). So it's both.

Who is at most risk? The elderly since they have an impaired heat loss mechanism to begin with. People who take certain medications (i.e. diuretics) or drink alcohol both of which contribute to dehydration. People with respiratory problems because humidity makes the air thicker and harder to breathe. Children, because thev absorb heat more rapidly, sweat less and start sweating at a higher temperature than adults.

Have fun this summer and remember.,. conduction, convection, and radiation is cool!