Document aJmBLVdLRr5mwZka4xGJJy9rR

212 KENNETH E. ROBINSON in the normal situations of comfort there is no change in body temperati re;' zero. It has a positive value when the body heats up and is negative wr cooling. R and C may be positive or negative, depending upon the direjiti the skin-environment temperature difference. Evaporative exchange is positive since a negative value (condensation) could only occur with a deW. K temperature higher than the surface temperature and this would be incompa with life. j1 Heat exchange by radiation is proportional to the difference in the] powers of absolute radiant temperature of the body surface and of surrbuif surfaces. As a first approximation, however, R may be expressed in tin the simple temperature difference: j (,)R -- R~rA,(tB -- j where Kr -- K(T,B -j- T,2TW -f- T,TW2 + Twa) = approximate constant modest ranges in T, and Tw (K being the universal radiation constant, andj'I's Tw absolute radiant temperatures of body surface and surrounding respectively); Ar = effective radiation surface area of body (about 0.8 X a' surface area for sitting subject); and t, and ta = radiant temperatures <j>fiS surface2 and surrounding surfaces,respectively. j' ,, Heat exchange by convection depends upon the temperature differ between the body surface and surrounding air and upon the air velocity, vl; "still" room air always has significant motion, the theoretical minimum eicl .with. Iree,,.con-vection..can-.be_.neglected-.and,.practically, the_conv.ective. may be expressed as follows: ; ij, C = A KaVV(t. - ta) where A = surface area of body in square feet; K,, = coefficient of heat excli by convection at unit velocity; V = effective air velocity in feet per minut? t, and ta -- temperatures of the body surface and air, respectively. Evaporative heat loss is directly proportional to the amount of watdr :e ` rated from the body surface and respiratory tract. For a completely jv surface, the evaporative rate depends upon the vapor pressure difference VPJ, where VP, is the vapor pressure of water at skin temperature -Iffi.^g^^^r^ig^o^y^r^^.orin-tiipraiiHA^yBny'r^itrenrtvap'Qg^. (Mere|^^^|^t^^r4j|e^mcreMesMth^ir velocity. For conditions of n|p evaporative'cooling with the body surface completely wetted, the J'B expression has been derived:3 Emox. = AKeV < (VP, - VP,,) 'Human skin and ordinary clothing radiate like black bodies; the radiant tempfe..., ia, therefore, the actual surface temperature. M. Nelson, L. W. Eichna, 8. M. Horvath, and T. F. Hatch, Thermal exchanges iq at high environmental temperatures, Am, J. Physiol., 151, 626 (1947). ilRp ant CONDITIONING 213 Ig^^^tJ^^rface area of body in square feet; K, = coefficient of heat exchange ^^^v^ijinSSoii'at'unit velocity; V = effective air velocity in feet peT minute;, and >:; -- as Siven above. aBliwKfiSeitfilltgaifo these several equations, it is seen that thermal exchange V--`uman body and the environment depends upon four external air temperature, radiant temperature, air velocity, and moisair. The several rates of exchange are modified, of course, by of clothing worn. 28 a*so object to delicate physiological control. The metaincreased spontaneously by shivering to counteract body ^^^'oqhrm'fVa:id|qwith continued cooling, the individual may engage in deliberate Bg|jfeijS?^Vipy:ea8e heat production. In the heat, on the other hand, the experi- reduces his activity to keep M within acceptable limits. In ^ufjiff^iaMpnents, R and C are automatically reduced by a decrease in the rate yje surface tissues, which thereby limits the amount of ,yto the surface. The skin temperature is thus lowered but internal SSoci'^^^Sifconserved. Conversely, in warm environments, blood flow to the JPplased; this raises the skin temperature and R and C go up if dfi* WItjV^r^i^fe&aaecs iiff Tn-eimgrroa-ftiixvreo. TT^-hVeio igrrroeoa+tneos+t rdJenrgrTrOeOe rotff rp\VhvyiresiAiol<lVofgini/caa1l 0a/d^ijnuCs+t.-- vSi1'1>yid'ed through the mechanism of sweating and evaporation. With jn cooj environments and under normal conditions of comfort, for active sweating and the only evaporative heat loss is the bV evaDoration in the resDiratorv tract (proportional to volume ^^f^bjeauhgml gandrdrifrfer_e_n_ceabetween tEe constant water vapor _pr_e_s_s_u_re in li^aiS^the p` artia-l vaipor pressu-re- in inhaled air)' and b--y the rate of atHfcransfeffto the skin (insensible perspiration). Since insensible perspiration is fflBMftitft. heat loss by this avenue is ordinarily: independent of the mois^^^^S|he air. Evaporative heat loss under comfortable conditions amounts 20,5.10^2iSper cent of total output. With increasingsheat load, metabolic or mal^hqTOver, active sweating is initiated by the . need-for greater evapora- ,Mlffigpbr-maintain thermal balance. Up to a certain point the desired and the sweating and evaporative rates will thus "be auto- ^*TM^^mit|d without regard to the moisture content of the air. A maximum ^Tippling rate is reached wjth higher heat loads;-when sweat is proKi*iQtie''evapoi.ative rate. At this point, the body-becomes coihpleteiy IHefe can be no further increase in the cooling capacity; which is now iBair velocity and the vapor pressure difference, as.outlined above. Sjajm^fe^human response at rest to different environmenlal'tleinperatures _i^^M?mPi'gufe`T.''SKbwn''`Sepafatgly`arer^^ffieta'b61i`c`'rSt%,i''tlre'''siIih' of tfhiiiii:ej'by radiation and convection, heat loss by evaporation, and changes i^hcjit'kohtent (storage). Over a considerable range .in-environmental change occurs in metabolic rate. In environments up to 85,F., JMffffiheat loss remains at a low level since M is adequately offset by R ip. % .... i msk