6-5. Comfortable limits in the cockpit are 68 to 72 degrees Fahrenheit and 25 to 50 percent relative humidity. To maintain these temperatures and this humidity range, aircraft must have extra heating and cooling equipment. This equipment is expensive in both performance and cost. (A rule of thumb is that one pound of extra load requires nine pounds of structure and fuel to fly it.)

6-12. When the temperature increases to about 82 to 84 degrees Fahrenheit, sweat production increases abruptly to offset the loss of body cooling through radiation, convection, and conduction. By the time the temperature reaches 95 degrees Fahrenheit, sweat evaporation accounts for nearly all heat loss.

6-13. Many factors affect the evaporation process. Some of these factors are—

• Protective clothing.
• Availability of drinking water.
• Relative humidity above 50 percent.
• Environmental temperature above 82 degrees Fahrenheit.

6-14. Relative humidity is the factor that most limits evaporation; at a relative humidity of 100 percent, no heat is lost by this mechanism. Although the body continues to sweat, it loses only a tiny amount of heat. For example, a person can function all day at a temperature of 115 degrees Fahrenheit and a relative humidity of 10 percent if given enough water and salt. If the relative humidity rises to 80 percent at the same temperature, that same person may be incapacitated within 30 minutes.

6-16. Individuals vary in their response to heat stress. Some serious reactions are heat cramps, heat exhaustion, and heatstroke. Factors that influence the physiological responses to heat stress include the amount of work that individuals perform and their physical condition as well as their ability to adapt to the environment. Old age, excessive alcohol ingestion, lack of sleep, obesity, or previous heatstroke can also diminish tolerance to heat stress. A previous episode of heatstroke can predispose an individual to repeated episodes.

• Error rates increase.
• Short-term memory becomes less reliable.
• Perceptual and motor skills slow, and the capacity to perform aviation tasks decreases.

6-20. Salt loss is high in personnel who either have not adapted to the environment or have adapted but are subjected to strenuous activity under heat stress. Replenishing this salt is important. Normally, adding a little more salt to food during preparation is enough to replenish the salt level. If larger amounts are required, the flight surgeon should be consulted.

6-27. Many factors influence the incidence of cold injury. If troops are in a static defensive position, the incidence of injury drops because they have time to take care of their bodies. Individuals under 17 or over 40 years of age seem to have a predisposition to suffer cold injury as do those who have previously suffered from it. Fatigue level, organizational discipline, individual training and experience, and physiological factors all affect the tendency of individuals to experience cold injury. Nutrition, activity, and the ingestion of certain drugs and medications also influence the incidence of cold injury.

6-29. Superficial cold injury usually can be detected by numbness, tingling, or pins-and-needles sensations. By acting on these signs and symptoms, individuals often can avoid further injury simply by loosening boots or other clothing and by exercising to improve circulation. In more serious cases involving deep cold injury, people may not be aware of a problem until the affected part feels like a stump or a block of wood.

6-30. Outward signs of cold injury include discoloration of the skin at the site of the injury. In light-skinned persons, the skin first reddens and then becomes pale or waxy white; in dark-skinned persons, the skin looks gray. An injured foot or hand feels cold to the touch. Swelling may also indicate deep injury. Soldiers should work in pairs—buddy teams—to check each other for signs of discoloration and other symptoms. Leaders should also be alert for signs of cold injuries.

6-31. First aid for cold injuries depends on whether the injury is superficial or deep. A superficial cold injury can be adequately treated by warming the affected part with body heat. This warming can be done by covering cheeks with hands, placing hands under armpits, or placing feet under the clothing of a buddy and next to his abdomen. The injured part should not be massaged, exposed to a fire or stove, rubbed with snow, slapped, chafed, or soaked in cold water. Individuals should avoid walking when they have cold-injured feet. Deep cold injury (frostbite) is very serious and requires more aggressive first aid to avoid or to minimize the loss of parts of the fingers, toes, hands, or feet. The sequence for treating cold injuries depends on whether the condition is life threatening. That is, removing the casualty from the cold is the priority. The other-than-cold injuries are treated at the same time as cold injuries while casualties are awaiting evacuation or are en route to a medical-treatment facility.

• Keep their body dry.
• Limit exposure to the cold.
• Avoid wearing wet clothing.
• Monitor the windchill factor.
• Keep activity below the perspiration level.
• Avoid the direct contact of bare skin and cold metal.
• Use the buddy system to check for early signs of cold injury.
• Wear several layers of loose-fitting clothing to increase insulation and cold-weather headgear to prevent loss of body heat.
• Avoid alcohol intake because it dilates surface blood vessels; this dilation initially causes the body to feel warmer but, because of heat loss, actually chills it.

6-33. The windchill chart in Table 6-1 gives the time limits for exposure to the cold before individuals experience injury. This chart correlates wind velocities and ambient air temperatures and shows the resulting temperatures from the windchill factor. The same data apply when wet boots or wet clothing is worn or flesh is exposed. This chart also indicates the level below which frostbite becomes a real hazard. Trench foot, or immersion foot, can occur at any temperature shown on the chart, given the right combination of wind velocity and ambient air temperature.