regarding intermittent exposures to heat is another major unresolved question of the effect of heat stress on the workforce. In addition, differences in sweat loss under a wide variety of industrial conditions still have not been thoroughly studied. The highly variable work loads (ภาระงาน)and the intermittency of strenuous work loads that are normal in industrial operations may also have a major effect on heat stress(10, 21).
The answers to these questions require additional research to validate the techniques presently proposed for the evaluation of heat stress conditions. Nevertheless, our current level of understanding justifies a work practices standard rather than an environmental standard for heat stress. Clearly the available environmental indices are sufficient to estimate threshold limits of heat stress that can be used to initiate work practices to protect the industrial worker adequately against heat.
5.3 Management of Heat Stress Exposures
The management of risk for heat stress exposures centers around two principles: general controls and job-specific controls. General controls are implemented when there is a reasonable chance of exposure above the TLV for unacclimatized workers. That is, at a point that is very protective of the workers. The general controls included employee and super- visor training, heat stress hygiene practices, and medical surveillance when the exposures are likely to exceed the TLV acclimatized workers, a greater depth of analysis is advised and job-specific controls should be considered(40). These are directed to specific manifestations of heat stress and follow the traditional hierarchy of engineering controls administrative controls and personal protection.
Before selecting control measures, it is crucial to identify the components of heat stress to which workers are exposed currently or expected to be exposed in new operations only then, by examining the alternatives, can the most effective means of control be selected A review of the heat balance equation described in Section 2.1 suggests that total heat stress can be reduced only by modifying one or more of the following factors: metabolic heat production, heat exchange by convection, heat exchange by radiation, or heat exchange by evaporation. Environmental heat load(C, R, and E) can be reduced by engineering controls (e.g., ventilation, air conditioning, screening insulation, process or operational modification), as well as protective clothing and equipment. Metabolic heat production can be modified by application of labor-reducing de and work practices
Clearly, heat stress control depends on understanding the contributions of all the elements of the exposure. It is important, however, to be aware of the full range of options for control in a specific situation. Because engineering controls are the first consideration, they are developed more fully in the next section.
6. ENGINEERING CONTROL OF HEAT STRESS
The control of heat in industry to ensure that exposures fall within acceptable limits requires the application of feasible engineering procedures, cost-effective engineering control of heat stress calls for an understanding of the physiological response of man in a hot environment. The underlying worker-environment thermal balance (Section 2) suggests The when a worker is exposed to elevated temperatures, the rate of heat loss decreases of engineering controls is to help sustain a rate of bodily heat loss equal to the imposition of heat from the environment. In general this can be accomplished by increasing the velocity of air across the body, a technique useful within certain limits of temperature and humidity. Above such limits it may be necessary to reduce the surrounding temperature. Barriers may be necessary when radiant heat is factor. In practice, it is often work necessary to combine some or all of these approaches to achieve an acceptable thermal work environment
The important role of engineering methods in regulating heat stress is underscored by the recommendations of the Department of Labor Standards Advisory Committee on heat stress(19). In its report, the committee sets forth special work practices needed to bring the 2-hr average heat exposure level within the limits of its recommended threshold WBGT values. The first such practice is that "the employer should adopt engineering controls which are appropriate for reducing and controlling the level of heat exposure" (19).
An engineering approach to the reduction of heat stress generally parallels the same control strategy applied for other workplace environmental stresses such as airborne gases, vapors, and dusts. Among the alternative control methods are substitution, control at the source, local controls, and general ventilation. Another approach to an analysis of engineering controls for heat stress focuses on the important environmental components of the thermal balance: convection, evaporative capacity, and radiation. This section combines approach both viewpoints and presents control both viewpoints and presents control methods that not only reflect a traditional approach to engineering control alternatives, but add appropriate emphasis to the aspects of heat control that characterize this specific physical stress.
6.1 Controls at the Source
The first and most fundamental engineering control of heat in the workplace is to examine options for eliminating heat at its point of generation. Occasionally one feasible alternative is to change the operation or substitute a process component of lower for one of higher temperature. Two well-known examples are the use of induction heating rather than direct-fired furnaces for certain forging operations, and the use of microwave ovens than convective ovens for certain heating regulated operations
As with most environmental stresses, heat is controlled most effectively if it is at the source, to prevent contamination of the space occupied by workers. In general, the options for control of heat at the source isolation, insulation in emissivity, reduction shielding and local exhaust ventilation
6.1.1 Isolation
The most practical method for limiting heat exposure from hot processing operations that are difficult to control or for operations that are extremely hot is to isolate the heat source. Such operations might be partitioned and are from the rest of the facility, located in a separated building or relocated outdoors with minimal shelter. A typical example is an industrial boiler, invariably segregated from the other operations in the same facility.
6.1.2 Reduced Emissivity of Hot Surfaces
The rate at which heat is radiated from the surface of a hot source can often be lowered boiler, the emissivity the source is reduced through surface treatment. When an oven, boiler, or other hot surface is covered with aluminum paint, the reduced emissivity of its surface offers two advantages. Less heat radiates to workers nearby; and heat is conserved inside the unit representing a substantial savings in energy costs (76)
The emissivity of a hot source can also be lowered by sheathing or by covering the source with sheet aluminum, other metals, such as galvanized steel, have relatively low emissivities but are more expensive to use than aluminum. In addition, the emissivity of galvanized steel increases faster with aging than with aluminum sheet (76)
Because of reduced emissivity, structural steel members will radiate less heat if they are painted with aluminum paint. Even though aluminum-painted surfaces have a higher emissivity than aluminum sheet, they do not radiate as much heat as oil-based painted steel at a given temperature.
6.1.3 Insulation
Insulation is not mutually exclusive of isolation in the context of engineering control of heat stress. Insulation also prevents the escape of sensible and radiant heat into the work environment. A well-known example of insulation that also has implications for energy conservation is that of-covering insulation on steam lines. By reducing the escape of heat into the environment, such insulation clearly helps conserve energy and fuel resources.
In addition to reducing radiative exchange, insulation reduces the convective heat transfer from hot equipment to the work environment by minimizing local convective currents that form when air that contacts very hot surfaces is heated.
6. 1.4 Radiation Shielding
Shielding against radiant heat represents an extremely important control measure the characteristics of radiant heat are quite different from those of high air temperatures, and the difference must be understood if engineering controls are to be effect Radiant heat passes through air without it heats heating the air; the is heat only the objects in its path that are capable of absorbing it. Shielding of radiant heat sources means putting a barrier between the worker and the source to protect the worker from being a targeted receptor of the radiant energy. Radiation shielding can be classified into reflecting, absorbing, transparent, and flexible shields.
Reflective Shields. Reflective shields are constructed from steel of aluminum, stainless steel, or other bright-surface metallic materials. Aluminum offers the advantage of about 90% reflectivity. It is used as shielding in the form of foil with insulative backing. and in aluminized paint, with reduced effectiveness.
Successful use of aluminum as shielding requires an understanding of certain principles (76) :
1. There must be an aluminum-to-air surface; the shield cannot be embedded in odd materials.
2. The shield should not be painted or enameled
3 The shield should be kept free of oil, grease, and dirt to maximize renectivity.
4 when used to enclose a hot source, the shield should be separated from the source by several inches.
5. Corrugated sheeting should be arranged so that the corrugations run vertically rather than horizontally, to help maintain a surface