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790 T. F. HATCH I. Effects of Heat on Man The varying effects of heat on employees are portrayed in Figure l.*1 In-| creasing- degrees of psychological disturbance are noted as the level of thermall stress goes up above the comfort zone, and some of the well-known consequencesjL are noted on the chart. These range from purely subjective discomfort to demon-|P strable impairment of efficiency in the performance of mental tasks. A further in-.| crease in heat load results in a higher order of psycho-physiological disturbance! with an increase in frequency of errors, higher accident frequency, and reduced!1 efficiency in the performance of skilled physical tasks. At still higher levels ogl fiHrSlClO<f/C^L Acvrr iMHHViTiQH aioutto *oa* cAAAGirr cmtuLAToor VATt* AMO JAlT mOAiAMCt PirCHO PHrUOLQGKAL ___ li tMCAtAtIMa rfCC/o/ffTJ pitovtto trrtutoctfxtuto ve**) * 5 TMCMtAtMt **0*$ w tortroRT ZONE (** AMOOLt*) !*W iNToienaiie, roue (uaiAFO rv urrjfQ 54 5? V *f*fMoaToMhoaihfoiutuy rftormr FUUrtfaHT Ml FFFFFFttlOO OF IFOFFFFt meo/cal Figure 1. Effects of heat on man, ranging from no effect (comfort) to extreme physiologies ------------------------------------- -breakdown-(heafexhaustion,"strokerdeathlr **"* exposure there is a definite loss of work capacity with disturbance of physiol 1 ^`j well-being, strain upon the heart and circulatory system, and overloading - mechanisms of water and salt balance in the body. These. ;djsturba.nces^culmii^ "in acute heat exhaustion or stroke and may terminate fatally. In parallel with the three (overlapping) categories of response to heat, psy$| 1T. F. Hatch, Standards for industrial heat control, in symposium on "Man and-| Relationships to Air," Heating, Piping, Air Conditioning, 23, 140-142 (April, 1951). HEAT CONTROL IN THE HOT INDUSTRIES 791 'logical, psycho-physiological, and physiological, the areas of primary responsiHfeility for fixing limits of exposure are also shown on the chart. At the lower levels jll'f-thermal stress there is no demonstrable effect on health. The consequences of jHheat exposure are reflected in personnel problems and indirect production losses, ^e middle zone, still without evidence of direct ill-effects upon employees, the ` cost of thermal stress is reflected in increased production costs, and these must be . _ compared with cost of heat control to determine the extent to which thermal stress fell to be reduced. This is the concern of industrial and production engineering. It . tei-the top zone of physiological disturbance with which we are concerned here. pfThermal standards are required that will insure safe working conditions for the j||nprmal industrial population working at the rates and for the periods of time ^gnecessary to the performance of their tasks without physiological distress or g|impairment to health. tIE H. Physiological Limits of Heat Exposure Body heat balance is a physiological requirement for comfort and health, ler normal conditions of comfort, the rate of heat production (metabolism) is NIP8*' balanced by heat loss to the environment (see Chapter VIII). This comfort- balance is maintained without active sweating and with optimum skin and l|lep-body temperatures. An increase in heat .production with work must be offset grjifi|||jy an equal increase in rate of heat loss, which is accomplished by sweating and ^yjfPffiftPorative cooling. In environments above skin temperature, heat is brought to ^^^W'budybyrsdifftion and convection, ancTffiis quantity together witK the meta- fftolic heat must be eliminated by the sole remaining avenue, evaporative cooling. SpFiDS t0 hiS flexible sweating mechanism, man can dispose of substantial amounts iofr'heat in this manner. An upper limit of evaporative cooling is reached, however, Sh'en the body surface becomes fully wetted.2 If the metabolic heat plus that re^^P'ived from the environment exceeds the maximum evaporative capacity, the must heat up. With the rise in skin temperature thbieifernal. heat lqad goes and evaporative cooling capacity increases so that .thbrfhal balance may be |erestablished- at an elevated body temperature. A modest rise is physiologically Ii)f,cePl'ablG, but any substantial increase in body temperature is accompanied by HIMous strain. A. HALDANE'S WET-BULB TEMPERATURE LIMITS The necessity to maintain heat balance at an acceptable level, of body tem||rature has long been recognized as one criterion for fixing limitstp'het/t expo- ^.--Early-observationswyere^ihlMe^bygaldane in 1 llpurance as the most extreme heat exposure that could be withstood without jii: "A. Gagse, A new physiological variable associated with sensible and insensible per oration, Am. J. Physiol, 120, 277 (1937). ./ *J. J. Haldane, The influence of high air temperatures, J. Hyg., 5, 494 (1905). <