Preventing heat injuries Published July 8, 2009 By Maj. Darryn Bryant 19th Aerospace Medical Squadron Aerospace and Operational Physiology Flight Commander LITTLE ROCK AIR FORCE BASE, Ark. -- First of a three part series Whether deployed to Southwest Asia or operating stateside during the summer heat, many of our military operations take place under extremely hot conditions. Because of the insidious nature of heat illnesses, heat injuries frequently result because people often don't recognize the symptoms until it's too late. Our body's protective cooling mechanism against heat injury is sweat. As long as we can sweat and the sweat can evaporate, we can continue to cool ourselves efficiently. If either the sweating mechanism begins to fail or the sweat cannot evaporate, then the cooling mechanism will fail and heat injuries may occur. On hot, humid days, our cooling system is extremely inefficient. It becomes relatively easy to overheat because the sweat cannot evaporate. The evaporation of sweat accounts for 90 percent of our cooling ability. Additionally, our ability to sweat diminishes as we become dehydrated. We lose body fluids in many ways every day. Sources of fluid loss include respiration, perspiration, urination and defecation. The loss rate from each of these varies according to activity levels, air temperature, humidity and altitude. With normal daily activities, we typically lose about one to two liters just from respiration, and another 1-2 liters from normal perspiration. During heavy exertion, we can lose eight to 10 liters of fluid over an afternoon of exercise or heavy activity. A 150-pound person can lose two percent of his body weight - three pounds - of fluid in just one hour! Since muscles are made up of about 70 percent water, this can definitely affect our ability to continue to do both aerobic and anaerobic work. How much fluid do you need? One of the best ways to judge your hydration status is to check the color of your urine. It should be relatively odorless and no darker than the color of straw. The rule of thumb is "clear fluids in, clear fluids out." A dehydrated person is more susceptible to developing a heat-related illness. Early symptoms of dehydration include thirst, fatigue, loss of appetite, lightheadedness and flushed skin. Later symptoms include difficulty in swallowing, stumbling, numbness, blurred vision, painful urination, muscle spasms and delirium. It's extremely important to pay attention to these early symptoms so heat illnesses, such as heat exhaustion and heat stroke can be averted.