Document LJkmekg4BYJ6gDKrma9RR5b8z

106 CHAPTER 7 Table 2 .... High Environmental Dry. and Wet-Bulb Temperofures* That Can Bo Tolerated in Dolly Work by Healthy, Acclimatized Men Wearing Warm Weather Clothing Aefmf)' ffcfofive Humid- ity % 15-25 (pro Dry Bulb Wti Mb Air Movenefrf 100 (po Dry Bulb Wot Bulb 300 fpm Dry Wef Mb Mb season Light sedentary activities (85 ET) Summer season Heavy Work (80 ET) 80 60 40 20 5 80 60 40 20 5 89 94 100 109 119 83 88 93 100 107 84. 91 82 96 79 101 75 110 69 118 78 86 76 90 73 95 69 101 64 107 85 93 84 98 81 103 75 110 69 117 81 89 78 93 75 97 70 102 64 106 87 85 82 75 68 83 80 76 70. 63 Winter season Lighter, 80 78 73 81 77 85 79 heavy - 60 81 71 85 74 88 76 work 40 86 68 89 70 91 72 (75 ET) 20 91 63 93 65 94 66- 5 97 58 97 58- 97 59 . Inctedio* radiation effect. (Reprinted by permusioa of Committee on industrial Ventilation, American Conference of Coreramentel ladustriat HypemsU.) .. 1965 Guide. And Data Boole Table 2 summarizes the conditions under which young, healthy individuals are capable of working and maintaining heat balance provided the metabolic rate is not excessive; Le., above 88 Btu per (sq ft) (hr). Long-continued exposure is known- to lead to deterioration of performance and morale even in the most fit and weliaccliinatized individuals. Any safe limits that coay.be set must, therefore, allow for a wide margin; of safety to be generally-acceptable. Where exposure to heat beyond the limits shown in Table 2 is necessary, short periods .of- work should be alternated with resting periods in a cooler environment! The length of recovery periods in a relatively cool environ ment will depend' upon, the cate and duration of work, the se verity of heat, and, individual constitutional,factors. As a rub, the workers themselves set the pace and length of recovery period in industries. Reference shouldbe made to fig. 6 to determine safe periods of exposure for extreme environmental temperature. Fig. &isbased upon the average time required to raise the pulse rate 50 points from an initial rate of about 75 to 125, and the rectal temperature to 101 F from initial' levels of 98 to 99 F. Symptoms of heat distress do not appear until the pulse rate exceeds 130 and: the rectal temperature. 101 F, in exposure lasting;more than 1 hr. In mines where temperatures approach 95 F, with humidi ties close to the saturation point, men work one-half,to onethird of the shifttime, alternating short periods of work in the ais differ widely in their capacity to acclimatize. Men lose most of their acclimatisation characteristics in a few weeks of temperate climate, even though- they are vigorously active. In the course- of acclimatization, the sweat glands come to secrete fluid less-rich in salt." For all men except those carry ing out really hard work in hot, dry'atmospheres, this effects an important reduction in salt loss, and the difference .between being and not bring subject to the- risk of beat cramps. 1 The adaptive- level changes somewhat with the season. There are also marked differences between the sexes. In the cold zone,- the thickness-of the thermal-insulating tissues of women is almost double that of men, although the sensory responses to cold are similar. In the hot zone,, the threshold of sweating is higher for women. The- thickness- and insulating yahie of the. clothing worn are also important factors, in the determination of the comfort..tevel.. UPPER LIMITS OF HEAT FOR MEW AT WORK Hot Spaces ' . .In some'industries it-is impossible, due to-process, require ments, to maintain optimum.conditions of comfort- Unless the whole process can be mechanized or isolated, the problem resolves.itself into one of alienating conditions of discomfort to the greatest extent feasible. Several approaches: are possibles (l)i the source of heat,. (2)-reduce tem perature by bringing in cool air, (3) reduce, humidity,. e.g,, vent, steam,generated to the outside,, (4) strip, down to mini mum amount of clothing suitable, for the job, (5) limit hours of exposure, and (6) condition, men to the assignment, he., employ heat adaptation: All' these factors-will assist, in. main taining'efficiency without impairinghealth. . Pig: dr...... Safe Periods, of Exposure for Extreme Environmental) Temperatures-. c l - A- Ckrtbang* Cudomoff Moor cfo&feg. B. AcfMfyt SoJeakuy or Eght metcvlor work. C Methods Convection type, i-e, wore, air, direct steam or fioi water ndiaton, pteoua tyriecit Fig. 7... - Effective Temperature Chart Showing Normal Scale of Effective Temperature, Applicable to Inhabitants of the United States Under Conditions Stated heat with rest periods of 40 to 45 min in cooler areas. In aluminum and steel manufacturing processes, in which en vironmental temperatures between 120 and 160 F with rela tive humidities of 3 to 15 percent (T 90 to 100 F) are en countered, men doing light duties work for about 1 hr and rest for an equal period, white those engaged in'heavy occupations work for about 30 min and rest during toe next 30 min. Sometimes men have to be exposed to temperatures as high as300 F, near kilns, in front of open furnaces, or in experimen tal laboratories, but only for 2 to 3 min at a time. Since air cannot be breathed at temperatures higher than 240 F, it is necessary to protect the upper respiratory passages as well as the eyes and hands, if hot objects must be bandied. Sweat Rate Index (P43R)' and toe Belding-Hatoh Heat Stress Index (HSI). There is presently no one proven method for combining all of the component'heat loads into a single value that would accurately indicate the degree of heat stress as perceived by an individual working or resting in a hot environment. The difficulty lies in the inability to 'simulate human response. Physical instruments can* accurately intograte, but the human body has the ability, to"differentiate between component thermal effects'and to make prompt adapf tive changes which the instruments cannot do.' Nonetheless, each of the indices will supply. valuable_inforination .on which to base an informed opinion.' The index or indices selected for . use should depend upon the nature and extent of the, problem, ^ the equipment and facilities at hand, and the.availability of-' personnel trained in the field of thential stress. Assessment of Heat Stress The thermal load to which man is subjected in a hot en- viro&ment is toe sum of toe environmental heat load and the metabolic load. There are a number of indices winch may be used to assess (measure) heat , stress, but no single index seems to be foolproof at present. Two of the older indices still in use and still valuable in making a first approximation are the Effective Temperature Index (ET) and the Robinson Index of Physiological -Effect (Ep). ''Recently, two' other indices were developed, namely, the- Predicted Four-Hour Dry-Bulb Temperature For practical purposes, the best index of warmth under ordinary room conditions with.stih air is the reading obtained with the dry-bulb thermometer. . ' Effective Temperature (ET) . Effective Temperature (ET) is an empirical sensory;index, combining into a single value the thermal effect of tempera^ 1