Document npm6nkqLzpYwdK8GOOYV5MZkz

798 T. F. HATCH line of thermal balance (see Figure 2). The reason for the lesser slope compared with the calculated value for thermal balance is that the skin temperature of Eichna's subjects did not remain constant in all environments, but increased wit;1 air temperature sufficiently that the necessary evaporation took place in the' extreme environments with a higher moisture content in the air than would otherwise have been permitted. These observations lend a good deal of support tq the physical basis of the heat tolerance lines. G. LIMITS OF HEAT EXPOSURE FIXED BY THERMAL BALANCE AND MAXIMUM PERMISSIBLE SWEAT RATE In recognition of the physiological cost of sweating as well as the necessit; for maintaining heat balance during sustained periods of heat exposure, Beldinp and Hatch12 have combined these two limiting requirements to define a hea TABLE 2 Physiological and Hygienic Implications of 6- to 8-Hour Exposures at Various Levels of Heat Stress1* Heat stress Implications 0 10-30 40-60 No thermal strain. :J Mild to moderate heat strain. Where a job involves higher intellectual functions, dexterity, or alertness, subtle to substantial decrements in ;,v? performance may be expected. In performance of heavy physical work, little decrement expected unless ability of individuals to perform such'irj work under no thermal-strain-is- marginal-.------------------------------------- -*3^ Severe heat strain, involving a threat to health unless men are physically,1|1'' fit. Break-in period required for men not previously acclimatized. S Some decrement in performance of physical work is to be expected;- ! Medical selection of personnel desirable because these conditions are 1 ,, unsuitable for those with cardiovascular or respiratory impairment or..# with chronic dermatitis. These working conditions are also unsuitable* for activities requiring sustained mental effort. . 70-90 Very severe heat strain. Only a small percentage,of the population mayfl be expected to qualify for this work. Personnel should be selected (a)jjs by medical examination and (b) by trial on the job (after acclimatizaS tion). Special measures warranted to assure adequate water and salt intake. Amelioration of working conditions by any feasible meansusjf'r' - ........-......... -highly. Hpairnhll_n.nH-.may,,he. expected-to decrease_the_ health hazard--, while increasing efficiency on the job. Slight indisposition, whicK|n|| most jobs would be insufficient to affect performance,' may render Wbrkl^ ers unfit for this exposure. 100 Maximum tolerable Btrain for fit, acclimatized young men. .................. . . . _...iS? stress index thatThas a value"briOO`When~E7Si'.7'EsMr=-l':OrThis-scale-holds^u| Ere,. = 2400 B.t.u./hr., which is just below McArdle's maximum acceptable-^ rate of 4.5 liters (equivalent to 2500 B.t.u./hr.) in 4 hours. In more severe enyjrray "H. S. Belding and T. F. Hatch, Index for evaluating heat stress in terms of resujjm physiological strains, Am. Soc. Heating, Air-Conditioning Engrs. Trans., 62, 213 (1950) .'f. HEAT CONTROL IN THE HOT INDUSTRIES 799 ments, the heat stress index will be greater than 100 in proportion to the ratio; re,./2400. The value of the index may be obtained from the chart in Figure 3. A stress index value of 100 represents the maximum tolerable heat load for sustained (6-8 hours) exposure for acclimatized young men in good physical condition and engaged- in elementary physical work. For older men or others with lowered physiological capacities, and in more demanding jobs requiring high degrees of skill and mental effort, the maximum level of heat stress must be set well below 100. The degrees of physiological and psychological stress associated with index jXalues ranging from 0 (comfort) to 100 are suggested in Table 2, and thesemay be used as guides in the appraisal of heat exposures and in setting limits for Vindustry. HI. Physical Evaluation of Hot Environments The following examples9 illustrate the application of the method of physical analysis to the evaluation of typical heat exposures. Example 1. An extreme heat exposure was encountered in the Forming De partment of a glass plant. Measured values of globe temperature, dry- and wet- bulb temperatures, and air velocity were: t,, = 130 F.; ta = 100 F.; tK, = `80 F.; V = 200 f.p.m.; calculated tK = 177 F.9 An average work rate was iassumed equivalent to 960 B.t.u./hr. Determine: (a) relative contributions of radiation and convection to the total . ' imposed heat load and (b) the heat stress index. Solution: R = 22(177 -- 95) = 1800 B.t.u./hr. = 62% total heat load C = 2\/200(100 -- 95) = 140 B.t.u./hr. = 5% total heat load M = 960 B.t.u./hr. Ercq, = M + R + C = 2900 B.t.u./hr. Emal. = 10.3 (200)o-4 .(42-21) = 1550 B.t.u./hr. Heat stress index = Etm./Emal. X 100 = 187 /The exposure is excessive and would lead to disaster if continued. In fact, the .operator does not remain in the heat more than a few minutes .to complete his task j.'|md then seeks relief. fe_ThisJsrao.t=dryrexpbsiUEe1.withii62ip.er.:cent,'of-the.tot,al.heat-load-imposed..by ^ filiation. Principal heat control measures must be directed toward the radiant ' y heat sources. I Example 8. The following conditions were found in a hot yi^.mine: t,, = F.; ta = 95 F.; -- 85 F.; y =.200 f.p.m.; calculated tw = 106 F.; ^average woTk rat'rarssum,ed"^"800";B7t7Dr/iTr:----------------------------------------------------Determine: (a) contribution of radiation and convection to total heat load nd (b) heat stress index value, jv Solution: f R = 22(106 -- 95) = 240 B.t.u./hr. = 23% total heat load , Mr...