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116. CHAPTER 6 1952 Guide arid air motion are most important. With dry-bulb temperature above body temperature, air motion facilitates evaporative heat loss by removing hpt humid air from contact with the skin and replacing it with relatively 'drier air. Heat regulation in man requires an intact set of sensory nerves, a riormal sympathetic nerve supply to sweat glands and blood vessels, a great -many sweat glands, and a circulatory system capable of carrying heat from muscles and viscera to the skin by circulation of the blood. Some of the phenomena of body temperature control are shown graphi cally in Fig. 2. The dotted curves, from a study at tjhe John B. Pierce Laboratory of Hygiene,10 are for subjects lightly clothed in a semi-reclining position, and give the relation between the dry-bulb temperature of the environment (with about 45 percent relative humidity) and the metabolic DRY'BUIB TEMPIRA7URL OEG FAHR (AT 45 PER CENT RELATIVE HUMIDITY) Fia. 2. Relation Between Metabolism, Storage, Evaporation, Radiation Plus Convection, and Temperature for the Clothed Subject rate (heat production), the rate of heat dissipation by radiation and con vection combined, and the latent heat loss due to evaporation from the skin and the respiratory tract. The smooth line curves from the work of the A.S.H.V.E. Research Laboratory11 give the same relationships for healthy, male subjects (18 to 24 years of age), seated at rest and cfressed in customary winter indoor clothing. The Pierce Laboratory data for the semi-reclining subjects also include the rate of heat storage (either positive or negative) due to a rise or fall in body temperature. For the normally clothed subjects, a curve gives the total heat loss (that is, the sum of the radiation, convection and evaporative losses). Here, storage is given by the difference between the metabolism and total heat loss. ' The small difference between the metabolic rates for the two groups of subjects may be accounted for by difference in activity. Heat exchange between the body and the environment by radiation and convection is greater for the lightly clothed subject, both for cool conditions where there is excessive heat loss, and for very warm , conditions where.there is transfer of heat from the atmosphere to the body. The two curves for Physiological Principles 117 .evaporative loss serve to show how physiological control uses evaporation of sweat to maintain equilibrium at high temperatures. Below 75 F, for the normally clothed subject,' and below 85 F for the lightly clothed sub ject, evaporation loss is minimal and constant. Burch12 has shown that this insensible perspiration reflects the permeability of the skin to the moisture of the body. Above these temperatures, control is obtained by the availability of sweat for evaporation. The difference in the curves above 75 F is probably largely determined by the difference in clothing and activity. > . In the zone of evaporative heat regulation, air movement facilitates heat loss if the temperature of the air is not above that of the skin.12 Under hot, dry conditions air movemerit may be of little advantage, or even of disadvantage, if it increases the addition of heat to the skin by conduction more than it promotes the loss of heat from the skin by evaporation. -I Table 2. Physiological Responses to Heat op Men at Rest and at Work* Effective -1 Temp. Actual Cheek Temp . (Fahr . Deo) Men at Rest Rise in Increase Approximate Rectal in Pulse'' Line in Body Temp Rate Weight by (Fahr Deg (Beats per Perspiration per Hr) Min per' (Lb per Hr) Hr) Men at Work 90.000 vr-LB o Work per Hour Total Work Accomplished (Ft-Lb) Rise in Body Temp (Fahr Deg per Hr) . Increase in Pulse Rate (Beats per Min per Hr) Approximate LonIn Body Wt by Per spiration (Lb per Hr) 60 70 0.0 80 96.1 0.0 0 0 85 96.6 0.1 i 90 97.0 0.3 4 95 97.6 0.9 15 100 99.6 2.2 40 105 104.7 4.0 83 110 5.9b 137,b 6.2 0.3 0.4 0.5 0.9 1.7 2.7 4.0b 225,000 225,000 209,000 190,000 153,000 102,000 67,000 49,000 37,000 0.0 .0.1 0.3 0.6 1.2 2.3 4.0b 6.0b 8.5b 6 7 11 17 31 61 103b 158b 237b 0.5 0.6 0.8 1.1 1.5 2.0 2.7b 3.5b 4.4b ? Data by A.S.H.VJ2. Research Laboratory. 0 Computed value from exposures lasting law? t.hnn one hour. HIGH TEMPERATURE HAZARDS Studies at the A.S.H.V.E. Research Laboratory14 and elsewhere during the past two decades have made available much information dealing with the physiological effects of hot atmospheres on workers, and means of alleviating the distress and hazards associated therewith. Table 2 gives some of the physiological responses of men, at rest and at work, to hot environments. Frequent and continued exposure of workers to hot envi ronments results in physiological derangement affecting the leucocyte count of the blood, and other factors dealing with man's mechanism of defense against infection. Wherever S (Equation 1) becomes strongly positive and body tem perature rises progressively, men will continue to work until body tempera ture reaches 103 F. When these body temperatures are exceeded, men work with declining efficiency and may be subject to heat stroke. Heat exhaustion is a circulatory failure in which the venous return to the heart is reduced so that fainting results.15 Early symptoms of heat exhaustion may include fatigue, headache, dizziness when erect, loss of appetite, nausea, abdominal distress, vomiting, shortness of breath, flushln of face and neck, pulse rate above 150, glazed eyes, and mental dis-