Document 6bLg2gR3xKpNvM7nVYa0B9rk1

794 T. F. HATCH "predicted 4-hour sweat rate" (P4SR) can be obtained for any environment com- i juSS"i bination of air, mean radiant and wet-bulb temperatures and air velocity, and for different levels of work. From this chart one can determine the acceptabilily of a hot environment or the degree of improvement necessary to secure a predicted'j sweat rate within the acceptable limit. . E. CALCULATED LIMITS OF HEAT EXPOSURE TO MAINTAIN THERMAL BALANCE From the development of the concept of "partitional calorimetry" at tjie| John B. Pierce Laboratory of Hygiene,7 Gagge, Herrington, and Winslow8 defin|| the upper limit of tolerable heat exposure as any maximum thermal environing in which body heat balance is just maintained (for a given work rate) witho$$ significant rise in skin temperature. Using the simple heat balance equation: ...Jp M B C-E where M is metabolic rate, R the radiant heat exchange, C the convective heaf exchange, and E the heat loss by evaporation, Haines and Hatch8 outlined},) physical method for evaluating the magnitude of heat stress imposed on emplojjfjl in the hot industries. This procedure, which makes use of the coefficients of hft'`i exchange by radiation, convection, and evaporation obtained on nude men aupl Armored Medical Research Laboratory,10 may also be used in determininggW most effective control measures and the extent of controls required to Mi environmental conditions within the limits necessary to maintain heat bal'a| (with skin temperature 95 F.). For a man of average size'the approximate-fieijirB balance equations are: ^required = M + 22 (t,, - 95) + 2y/V (f,, 95) and E,, (10.3V)04 (42 - VP,,) ^required is the necessary evaporative cooling rate to maintain balance; I the maximum evaporative cooling rate for man when the body is fully W 1 The other symbols are: tw = mean radiant temperature of the surrounding)J" in 0 F.; t,, = air temperature in F.; V = air velocity in f.p.m.; VP,, = y pressure of the water vapor in the air in mm. Hg. The values 22, 2v/V, and 10 Ifj are the" coefficients of heat transfer for radiatibh, convection, reSjie'dUvei^' multiplied by the effective surface area of the bod^?-'Bjit pressure of water at the skin temperature of 95 F. is 42 mm. .I.CFE. A.. Winslow and L. P. Herrington, Temperature and Human Uje. Prinqgjg -vaittffiftBB7y35i5tonr'WrJ^ *A. P. Gagge, L. P. HeTrington, and C.-E. A. Winslow, Thermal interchangesjhiet the human body and its atmospheric environment, Am. J. Hyg., 26, 84 (1937). ' t^j * G. F. Haines and T, F. Hatch, Industrial heat exposures, evaluation and control,!! and Ventilating, 49, 93-104 (November, 1952). "N. Nelson, L. W. Eichna, S. M. Horvath, and T. F. Hatch, Thermal exchanges 'oUn at high environmental temperatures, Am. J. Physiol., 151, 626 (1947). -.-if1 HEAT CONTROL IN THE HOT INDUSTRIES 795 The external heat load, R + C, may be calculated from the difference between the "operative" temperature7 and the skin temperature: R -j- C -- K0 (to -- 95) where K,, = 22 -j- 2VV~ to = 2 2t,, + 2VT ta 22 -f 2VV' The Vernon globe temperature, ts,n may be used in place of the calculated opera tive temperature. For any given combination of globe temperature, wet-bulb temperature, air velocity, and work rate, one may quickly determine the ratio, Ereq./Fmai. with the aid of the charts in Figure 3. Values of M for different work situations are given in Table 1. The thermal balance line for t, (skin temperature) TABLE 1 Estimates of Energy Metabolism for Various Types of Activity (Vrra_liues!_ap-p!ly/for a -1-54-pound* man, and--do not include rest pauses) Activity Sleeping Sitting quietly Light work Sitting, moderate arm and trunk movements (e.g., desk work, typing)_______________________________________________ _ Fitting, moderate arm and leg movements (e.g., playing organ, driving car in traffic) Standing, light work at machine or bench, mostly arms Moderate work Sitting, heavy arm and leg movements Standing, light work at machine or bench, some walking about Standing, moderate work at machine or bench, some walking about Walking about, with moderate lifting or pushing Heavy work Intermittent heavy lifting, pushing or pulling (e.g., pick and shovel work) Hardest sustained work B.t.u./hr. 250 400 45(L550 550-650 550-650 650-800 650-750 750-1000 1000-1400 ..15002000 '2006-2400 = 400 f.p.m., calculated"qmfiKe'b'iiis of phe above coefficients of heat transfer, is compared with Eichna's lines of maximum ^physiological tolerance in Figure 2. . ' ..v . ,,v .................F. PHYS1CAL.BASIS OF TOLERANCE LINES....... _. Man's response to heat is complex, and relationships between magnitude of fstress and physiological disturbance induced by the heat involve, more than-simple |heat balance. Implicit in the establishment of tolerance limits, however, is the gf 34H. M. Vernon, The radiation experienced in factories and houses, J. Ind. Hyg. Toxicol., |l9, 498 (1937).