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A9.3. Risk Factors.

A9.3.1. Dehydration. Following the loss of sweat, water must be consumed to replace the body’s loss of fluids. If the body fluid lost through sweating is not replaced, dehydration will follow. Under extreme conditions, maximum sweat rates can actually exceed the body's ability to absorb fluids.

Whenever consumption of water fails to keep up with output of sweat, the body will become progressively dehydrated. Other factors that can contribute to dehydration are consumption of alcohol and caffeine, which increase urination, and excessive consumption of salt. Thirst is a poor indicator of dehydration. Dehydration is possible without any signs of thirst; mental and physical performance can degrade so slowly that individuals may not recognize the problem in themselves or others. Dehydration cancels the benefits of heat acclimatization, increases the risk of heat illness, reduces work 88 AFMAN32-4005 30 OCTOBER 2001 capacity, appetite, and alertness, and increases susceptibility to other medical problems such as constipation, kidney disorders, and urinary tract infections.

A9.3.2. IPE. IPE (and other protective garments that prevent the transfer of air and moisture) restrict normal heat loss mechanisms because of their high insulation and low permeability to water vapor.

These effects occur even when ambient temperature and humidity are relatively low. Physical work tasks require more time and effort when these are worn because of added weight and restricted movement. This results in more body heat to be dissipated than normal. Variations to MOPP levels, such as opening or removing the jacket, will reduce barriers to body cooling. Therefore, commanders must conduct risk analyses to balance performing mission critical tasks, casualties due to actual NBC threat, and degraded performance due to heat stress, dehydration, and bulkiness of the protective equipment.

A9.3.3. Salt depletion. Failure to replace salt lost in sweat can lead to several heat illnesses. Salt depletion occurs when personnel are sweating heavily (especially before they acclimatize), and drinking large volumes of water without replacing salt loss. In most cases, sufficient salt can be consumed in the daily diet. If meals are skipped, or food intake is reduced considerably, salt intake may be inadequate. On the other hand, excessive salt in the diet can lead to dehydration.

A9.3.4. Lack of acclimatization. Unacclimatized personnel are those who have not built up a tolerance for working in a hot environment. The personnel most likely to be affected by the heat are those who have just arrived from cooler climates, and persons who are not in shape. They will experience degraded mental and physical performance and be highly susceptible to heat illness.

A9.3.5. Failure to observe work-rest cycles. Even in acclimated individuals, body temperatures can rise very rapidly due to the combination of excessive heat and sustained activity. To prevent a dangerous increase in body temperature, heat production must be minimized by reducing work pace and increasing rest periods. In very hot and humid conditions, or when IPE is worn, reducing physical activity may be the only way to prevent dangerous increases in body temperature.

A9.3.6. Poor physical condition. Persons who are overweight or are poorly conditioned become fatigued more easily and do not adjust to working in the heat as quickly as those in good physical condition. Overweight and fatigue also impair the body’s heat losing mechanisms; it takes work on the part of the body to lose heat, and an already tired body cannot perform this function well.

A9.3.7. Drugs which inhibit sweating, such as atropine, antihistamines, cold medicines, and some antidiarrheal medications can markedly impair heat loss.

A9.4. Prevention. Commanders must be prepared for decreased performance and heat stress casualties when acclimatization, water consumption and work/rest preventive measures are not applied. The keys to preventing heat illness and sustaining performance are knowledge of weather information for a unit’s specific location and implementation of preventive measures accordingly. Heat illness prevention guidance is often tied to a parameter known as the Wet Bulb Globe Temperature (WBGT) index. The WBGT index is a combination of measurements which take into account dry air temperature, relative humidity, and radiant heating. The Bioenvironmental Engineer has the field apparatus necessary to determine the WBGT index. Heat illness preventive measures cannot be prescribed precisely because climates, working conditions and individual capabilities will vary widely. It is therefore important to understand the principles driving these guidelines.

AFMAN32-4005 30 OCTOBER 2001 89 A9.4.1. Acclimatization. All individuals who work for the first time under heat stress will develop signs of strain such as abnormally high body temperature, pounding heart, fatigue and other signs of heat intolerance. With planned, progressively more heat exposure and physical activity each day, physiological changes occur that will allow personnel to withstand the heat. On succeeding days of working in the heat, work can be done with less difficulty. This enhanced tolerance to heat is called heat acclimatization.

A9.4.1.1. A period of acclimatization is required for all personnel regardless of individual physical condition. Typically, the mid-way point toward full acclimatization can be reached by the end of the first week. In most cases, full acclimatization will be reached in up to two weeks. The time it takes to acclimatize will vary with each individual. Those in good physical condition will acclimatize most quickly.

A9.4.1.2. Resting for three or four days in the heat, with activity limited to that required for existence, results in only partial acclimatization. Although advantage should be taken of the cooler hours in accomplishing a portion of the work, the schedule should include gradually increasing exposure during the hotter parts of the day rather than complete exclusion of work at that time.

Physical work in the heat must be accomplished for development of full acclimatization to that work level in a given hot environment.

A9.4.1.3. Work schedules should be adjusted to facilitate the acclimatization period. To start, the most strenuous tasks should be performed when it is cooler (in the early morning or evening), with lighter duty tasks performed during the remainder of the day. As the acclimatization proceeds, the ability to perform at the same level of heat stress improves, and more strenuous work can be added gradually. A day or two of intervening cool weather will not interfere significantly with acclimatization to a hot environment.

A9.4.1.4. Recreation and physical fitness training should be approached the same way. Start with less strenuous activities (e.g., softball) and gradually increase to the more strenuous (e.g., calisthenics, running).

A9.4.1.5. Personnel will retain their acclimatization for approximately one week after leaving the hot environment. If not exposed to work in high temperatures, the acclimatization will then decrease, the major portion being lost within one month. Therefore, the acclimatization process needs to be reinforced to varying degrees when ambient and workload conditions change. If there is a sudden heat spell or significant increase in humidity, increase in workload, or absence from the local area conditions for a week or more, individuals will need time to reacclimatize.

A9.4.2. Hydration. Water is critical for maintaining health and individual performance, since the human body is highly dependent on water to cool itself in a hot environment.

A9.4.2.1. All water and ice cubes consumed must be from a medically approved source to prevent waterborne illnesses. Individuals should carry as much water as possible when separated from medically approved water sources. Plain water is the beverage of choice, and personnel will be more likely to drink sufficient water if it is palatable. Whenever possible, provide cool (60-70 oF) water; if refrigeration is not feasible, water buffaloes or other distribution containers should be kept in the shade or insulated.

A9.4.2.2. Fruit flavored powdered drink mixes may be added to improve the taste of water; however, flavoring (or any drink other than potable, disinfected water) should not be added directly to 90 AFMAN32-4005 30 OCTOBER 2001 canteens or bulk water storage/central distribution containers (such as water buffaloes, tanks).

Flavoring (and other beverages) should be added to the personal drinking cup (or larger containers for group access). There are two reasons for this. First, plain, disinfected water from a canteen may be needed in an emergency, such as for irrigating eyes or wounds. Second, flavorings (or remnants of other beverages remaining in the container after use) can interfere with disinfection of central distribution containers and the ability to measure the disinfectant residual, which is often done with a colorimetric test.

A9.4.2.3. In addition to plain water, some of the fluid intake requirement can be met by almost any type of beverage (e.g., juice/fruit drinks, coffee/tea (decaffeinated preferred), soft drinks, soup, milk). If an electrolyte “sports” beverage is consumed, it should be diluted with plain water in a 1:1, or more diluted ratio. Drinks with caffeine or alcohol do not have to be totally restricted from the diet, but consumption should be moderate since caffeine and alcohol tend to increase urination which could lead to dehydration.

A9.4.2.4. Increased sweating requires additional water consumption. Acclimatization does not reduce water consumption requirements and may actually increase the requirements because acclimatization increases sweating to enhance the evaporative cooling capacity of the body.

A9.4.2.5. Thirst alone is not a good indicator of adequate fluid intake. Personnel will always need to drink before they feel thirsty. The optimum amount of water needed to prevent dehydration for any individual can not be predicted exactly due to individual physiology, climate, and physical activity, but there are “rule of thumb” guidelines for planning daily water consumption and supply needs in hot environments.

A9.4.2.6. It is much better to drink small amounts of water frequently than to drink large amounts occasionally. Following a drinking schedule may seem tedious, but in the long run it helps personnel drink more. An example drinking schedule is to drink one quart (canteen) of water in the morning, one quart at each of three meals, and routinely drink small amounts such as two cups (half a canteen) periodically throughout the work period.

A9.4.2.7. Personnel working in warm weather will need at least four to six quarts of water per day and more as work becomes more strenuous; in hot environments, 10 to 12 quarts per day. Under extreme heat, especially in an environment in which IPE is worn, water requirements can exceed over 20 quarts per day. In such cases, the rate of sweat production actually exceeds the maximum rate of water absorption from the gut (approximately 1.5 quarts/hour). Whenever guidance advises drinking more than 1.5 quarts per hour, plan for an extended rest and rehydration period to make up the deficit.

A9.4.2.8. Table A9.1. Heat Illness Prevention Guidelines, provides general recommendations for water consumption. One of the simplest ways to ensure hydration is adequate is for individuals to monitor their urine color. An adequately hydrated person will produce pale yellow urine; a dehydrated person’s urine will be dark yellow to brown. Reduction in urine volume and frequency of urination are other simple observations each person can use as indicators of hydration status. Personnel should be instructed to use these methods, and adjust water consumption accordingly.

A9.4.3. Salt Replenishment. The amount of salt lost in sweat varies depending on the degree of acclimatization. As the body acclimatizes to the heat, sweat contains less salt. To prevent heat illness resulting from excessive salt loss, there must be adequate salt in the diet. During the acclimatization period, personnel should salt their food lightly. Once acclimatized, personnel should season their food AFMAN32-4005 30 OCTOBER 2001 91 to their normal taste. In most cases, this will be adequate because the typical American diet and military rations contain sufficient amounts of salt to replenish that lost in sweat. Avoid excessive salt; this will contribute to dehydration. If food intake is cut drastically (e.g., eating only one meal per day), active personnel should salt their food lightly. Supplementation with salt tablets is not appropriate unless medically indicated and supervised by medical personnel.

A9.4.4. Work/Rest Cycles and Reduction of Heat Exposure. Body temperatures can rise very rapidly due to the combination of excessive heat and sustained activity. To prevent a dangerous increase in body temperature, heat production must be minimized by reducing work pace and increasing rest periods. In very hot and humid conditions, reducing the duration of physical activity may be the only way to prevent dangerous increases in body temperature. Acclimatization does not eliminate the need to observe work-rest cycle guidelines.

A9.4.4.1. Work schedules must be tailored to fit the mission, climate, and physical condition of

personnel. Work/Rest Cycle recommendations are provided in Table A9.1., Heat Illness Prevention Guidelines. Close supervision by commanders and medical personnel is essential in achieving maximum work output with minimum hazard. The following should be considered:

A9. Personnel should be allowed to seek relief periodically from potentially dangerous heat stress situations by resting in shaded or air conditioned areas, and by removal of IPE and other heavy equipment.

A9. Work and rest/recreation in the direct sun should be avoided as much as possible on hot days. When feasible, provide temporary shielding from the sun by using tents or improvise with canvas, ponchos, or parachutes. Shielding should allow for free air circulation.

A9. Even moderate exertion in MOPP gear can cause heat illness at lower WBGT indices. When IPE is worn, add 10 oF to the measured WBGT index and allow for additional time for tasks to be accomplished.

A9. Wear hats and keep long sleeves and long pant legs rolled down when in the sun.

Clothing fabric should be as lightweight as possible and fit loosely to let air move between the clothing and skin.

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