Document ZBxp449L9wL8v6ba9JwwaN2dZ

Table 3. Upper Limits op Environmental Conditions for Acclimatized, Healthy, Young Men in Military Service #- Environment Reactions at the end of 4 hb Rectal Temp F Pulse rate Below 101 101 to 102 Above 102 Below 130 130 to 145 Over 145 turbances as apathy, poor judgment, and irritability which usually precede fainting (syncope). Recovery is usually prompt when the man is removed to a cool place and kept lying down for a time, unless he has some other illness such as heart disease. Heat cramps are painful muscle spasms in extremities, back and abdo men due, at least in part, to excessive loss of salt in sweating. Formerly common in hot industries, this manifestation of illness due to heat is now greatly reduced by drinking water containing 0.1 percent salt, or by proper use of salt tablets. Heat cramps are readily alleviated by administration of salt solution intravenously. Heat stroke is a serious effect of exposure to great heat. The body temperature climbs rapidly to excessive levels often above 105 F when for unknown reasons free sweating suddenly stops. At such high temperatures, coma appears and death may be imminent. Emergency measures are required to reduce the excessive body temperature by cooling quickly to avoid irreparable damage to the brain.1 The deleterious physiologic effects of high temperatures exert a power ful influence upon physical activity, accidents, sickness and mortality. Both laboratory and field data show that physical work in warm atmos pheres is a great effort, and that production falls progressively as the temperature rises. ACCLIMATIZATION When men move to deserts or to jungles some adaptation to the climate takes place. If work is gradually increased day by day, and if the men can get plenty of water and salt, and can sleep each night, acclimatization may be complete in 7 to 10 days. The acclimatized man works with a lower heart rate, lower skin and rectal temperature, and more stable blood pres sure than when unacclimatized. The process of acclimatization requires work in the heat.17 During the recent war, white troops lived and did hard physical work for long periods in tropical conditions when disease hazards were controlled. In recent tests made at the A.S.H.A.E. Research Laboratory,18 subjects were required to perform fight work under very hot conditions for a 4-hr period each day. It was found that the ability of a new subject to endure these conditions showed daily improvement for a period of at least 2 weeks. However, after acclimatization was completed, a recess of several days had no effect on the endurance of the subject. Individuals differ widely in their capacity to acclimatize. Acclimatized men lose most of these improvements in a few weeks of temperate climate, even though they are vigorously active. In the course of acclimatization, the sweat glands come to secrete fluid less rich in salt.18 For all except those carrying out really hard work in hot, dry atmospheres, this effects an important saving in Physiological Principles IIP Fig. 3. Heat Endurance of Acclimatized Subjects Working at a Specific Rate50 salt loss, and makes all the difference between being exposed and not being exposed to the risk of heat cramps. The adaptive level changes somewhat with the season. There are also marked differences between the sexes. In the cold zone the thickness of thermal insulating tissues of women is almost double that of men, although the sensory responses to cold are similar. In the hot zone, the threshold of sweating is higher for women. The thickness and insulating value of the clothing worn are also important factors in the determination of the comfort level. UPPER LIMITS OF HEAT FOR MEN AT WORK In very hot conditions humidity is the limiting factor, and the wet-bulb temperature assumes great importance. In 1905 Haldane recognized that 88 F wet-bulb was the limit of endurance for coal miners, and later observers have concurred. A study was made at the Armored Medical Research Laboratory50 to determine the upper limits of environmental conditions under which a man can perform certain work. Thirteen enlisted men, thoroughly accli matized to the hot conditions, served as subjects. During each test, the subjects were required to march for 4 hr at the rate of 3 mph, carrying