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242 CHAPTER 20 1962 Guide And Data Book f, and to all four of the thermal characteristics of the en vironment: M + 22(1. - t.) + 2 VV(U - O - 10K-`(P. - P.) (6) Equation 6 provides a rational Haag on which to construct a heat stress index and to establish thermal standards for different industrial work situations. Two physiological criteria are important in fixing the limit of sustained heat exposure: 1. The increase in body heat content must not exceed a cer tain level. Practically, this is reflected in the rise io skin tem perature. This temperature may be limited to 95 F. 2. Thermal balance must be accomplished with a rate of Sweating not greater than one liter per hour (equivalent to evaporative cooling of 2400 Btu per hr). Studies on young ac climatized subjects have shown that this maximum rate of sweating can be maintained over a workday without Undue strain or physiological cost. The Heat Stress Index (HSI) proposed by Belding and Hatch* makes use of these criteria together with the previous biophysical equations. A so-called standard young man, in good physical condition and acclimatized to heat, can safely engage in simple physical work over a normal eight-hour day in a particular thermal environment so long as his skin tem perature does not exceed 95 F and he maintains thermal balance with a sweat rate not exceeding 1 liter per hr. By definition, this degree of stress has an (HSI) value of 100 and is given by any combination of M + R + C which is balanced by within their defined limits. For other situa tions where is greater or less than (2? 2400 Btu per hr), the value of (HSI) is proportionately different from 100: (HSI) 100 xe. (7) By using the globe thermometer temperature of the en vironment to represent the combined effects of mean radiant temperature and air temperature, the Heat Stress Index chart (Fig. 1) has been constructed, to permit quick de termination of (HSI) when M, 4> **, and air velocity are given. The procedure for finding (HSI) will be obvious by following the dash line in Fig. 1. It should be noted that the globe thermometer temperature reading is made directly. A satisfactory globe thermometer can be constructed from a 4 in. diameter float ball (a toilet float will do) painted a flat black. A laboratory thermometer should be inserted through a rubber stopper and this assembly inserted in an opening in the ball so that the thermometer bulb is at the center of the sphere. For additional information on globe thermometers, see Chapter 16 in the 1961 Guide And Data Book. The Heat Stress Index was developed from research ex perience with young men in good physical condition and well to heat. Table 2 assigns suggested physiological meaning to (HSI) values above and below 100, for older men as well as the standard young man and, also, for various kinds of jobs which differ in their demands upon mental and physical effort. From these it becomes possible to select (HSI) levels appropriate to each work situation. However, the maximum allowable index level for a specific job condition must be selected with caution. The research for the (HSI) was conducted on nude subjects.* Recent studies indicate that the insulating effect of clothing may reduce the effects of radiation and convection by 40 percent.1 The (HSI) is lying re-evaluated to allow for the effects of clothing. Provisionally it is recommended that R and C be estimated by taking 60 percent of the values indicated by Equations 3 and 4 and inserting the results in Equation 6, to determine the heat bad, En*, and (HSI) by Equation 7. Where possible it Table 2 .... Evaluation of Index of Heat Stress tad** of] Hw* Siren PbjraoJogkaf and Hygmme layfaefioai of 9-Ht Expoww to VsrioM Hoof SIrenes Mild cold strain. This condition frequently exists i areas where men recover from exposure to heat. No thermal strain. +10 20 Mild to moderate heat strain. Where a job involve higher intellectual functions, dexterity, or alert ness, subtle to substantial decrements in perform ance may be expected. In performance of heavy physical work, little decrement expected unl^ ability of individuals to perform such work under no thermal stress is marginal. 40 Severe heat strain, involving a threat to health SO men are physically fit. Break-in period required 60 for men not previously acclimatized. Some decre ment in performance of physical work is to be ex pected. Medical selection of personnel desirable because these conditions are 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 Very severe heat strain. Only a small percentage of the 80 population may be expected to qualify for this work. 90 Personnel should be selected (a) by medical exami nation and (b) by trial on the job (after acclimati sation). Special measures are needed to assure adequate water and salt intake. Amelioration of working conditions by any feasible means is highly desirable, and may be expected to decrease the health hazard while increasing efficiency on the job. Slight "indisposition" which in most jobs would be insufficient to affect performance may render workers unfit for this exposure. 100 The maximum strain tolerated daily by fit, acclimatized young men. is desirable to establish the maximum allowable (HSI) value from field testa of the workers under normal conditions of work load, duration'of work and clothing. Once established, this limit may then be extended to similar working conditions. The Heat Stress Index (ESI) differs from the Effective Temperature (ET) scale in the rational rather than subjec tive basis of its construction. Practically, the commonly recommended ET=80 F for heavy work at moderately high air velocity agrees very well with the (HSI) limit of 100 for acclimatized young men. In Fig. 2 are shown the combina tions of dry- and wet-bulb temperature (air and wall tem perature alike) for three different air velocities and three different grades of work, all of which give (HSI) -- 100. Points representing ET = 80 F with an air velocity of 300 fpm are also shown for comparison. CONTROL OF HEAT EXPOSURES Measures for control of heat exposures will be treated un der the following headings: Control at Source, Local Exhaust Control of the Industrial Environment 243 that all surfaces in a shop or building are hot because of ra diant heat received from a few primary sources. Under these circumstances, the ventilation air is heated rapidly as it circulates through the building. The result is a very unsatis factory thermal environment, all stemming from the few pri mary heat sources. Consequently if the radiant heat load from these is reduced there will be a great improvement in the entire .building. A shield is simply a sheet of material, opaque to the infra red waves (or essentially so), placed between the hot object such as a furnace, and the cooler surroundings. The closer it can be placed to the hot object, the greater will be the effective coverage. F-ssential to most shielding is the absence of physical contact and the presence of an air space between the hot object and the shield to avoid heating of the shield by conduction. This is not a vital matter if the shielding ma terial has a low coefficient of emission for infra-red rays as does aluminum, for example. Under such circumstances, very little heat is radiated even if the shield becomes very hot. Radiation shields in the following 5 forms are very effec tive: Ventilation, Radiation Shielding, General Ventilation, Dilu tion Ventilation, Local Relief Ventilation. Control at Source The magnitude of heat exposure can be reduced by insulat ing hot equipment, locating such equipment most favorably (in zones of good general ventilation within buildings or eves outdoors), covering steaming water tanka, providing covered drains for direct removal of hot water, and maintaining tight joints and valves where steam may escape. This method is an obvious one and requires no particular comment other than to emphasize the benefits to be derived from elimina tion of heat sources wherever possible. Local Exhaust Ventilation The natural convection column of heated air rising from a hot process may be captured by means of ventilated enclo sures or exhaust hoods and removed with a minimum of dilu tion by air from the surrounding space when local exhaust ventilation can be used. Chapter 21 provides detailed infor mation on the design of exhaust hoods and duct systems for local exhaust. Radiation Shielding In some industries, commonly referred to as the hot in dustries, there are many hot objects and surfaces such as furnaces, ovens, furnace flues and stacks, boilers, molten ma terial, and hot ingots of metal, castings, or forgings and, in consequence, the major environmental heat load is in the form of. radiant beat. Since air temperature has no significant influence, on the flow of radiant heat, ventilation is of no help in controlling such exposures. The only effective control is the direct oDe of decreasing the amount of radiant heat impmgmg on the exposed worker. This frequently can be ac complished by radiation shielding. Radiant heat exposures an be reduced by lowering the surface temperature of hot equipment by using insulation or water-cooled surfaces or radiation shields. Hot surfaces emit infra-red waves which reach all objects within visible range. This is true not only if the hot surface B a primary one such as a furnace wall, a high temperature riack, or a hot ingot, but *lsn if it is secondary in nature, that b, one which is reradiating heat after receiving it by direct radiation from a high temperature source. It may well be 1. Sheets of reflective metal or insulating board, semi-permaaeotly attached to the hot equipment (such as furnace buck stays) or arranged as semi-portable floorslands. 2. Aluminum foil-faced cloth curtains raised or lowered on spring rolls. 3- Transparent shields, including heat reflective tempered plate glass, reflective metal chain curtains, and close mesh wire screens. These have lower efficiency than opaque shields in items 1 and 2. 4. Water-cooled shields for absorbing the heat. 5. Reflective garments, such as aprons, or in the form of a sandwich in cases of continuous front and back exposures. For continuous wear the apron or sandwich width should be limited to from 14 to 18 in. in order to assure adequate continuous side openings for air circulation and body ventilation. In addition, gauntlets and faoe shields are particularly applicable to opera tions such as the pouring ana of not metal. Supply houses dealing in safety clothing for industry offer Suitable light-weight flame proofed foil-faced cotton drill or denim of excellent reflectivity. For repairs inside hot coke ovens and in-* dustrial furnaces, a complete suit is available using forced ven tilation from a small blower or a compressed air source. These suits may be made of asbestos cloth farad with a reflective metal in atomized form. If the shield is a good reflector, it will remain relatively cool in the presence of severe radiant heat. This reflectivity is a surface characteristic and is not dependent on thickness. A thin foil is effective. Bright or highly polished tinplate, stainless steel, and ordinary fiat or corrugated aluminum sheets are efficient and long-lived. See Chapter 4 of the 1961 Guide And Data Book. Foil-faced plaster board, though less durable, gives good reflectivity on one side. Since the best radiation shields are effective reflectors of infra-red rays, they must be used intelligently lest a radiant heat load merely be transferred from one place to another. To be efficient, the reflective type shield must be maintained in the bright con dition. The objective is to have the shield reflect the radiant heat hack to the primary source where it may be removed*by local exhaust. However, unless the shield completely sur rounds the primary source, some of the infra-red energy will be reflected into the cooler surroundings and possibly into an occupied area. It is imperative, therefore, to study well where the reflected heat will go before shielding is installed, to avoid merely getting rid of the problem in one area by transferring it to another. GENERAL VENTILATION General ventilation involves sweeping a building area with relatively large quantities of air in order to provide an im-