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Wednesday, 16 March 2011 21:41

Prevention of Heat Stress

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Although human beings possess considerable ability to compensate for naturally occurring heat stress, many occupational environments and/or physical activities expose workers to heat loads which are so excessive as to threaten their health and productivity. In this article, a variety of techniques are described which can be used to minimize the incidence of heat disorders and reduce the severity of cases when they do occur. Interventions fall into five categories: maximizing heat tolerance among exposed individuals, assuring timely replacement of lost fluid and electrolytes, altering work practices to reduce exertional heat load, engineering control of climatic conditions, and use of protective clothing.

Factors outside the worksite which may affect thermal tolerance should not be ignored in the evaluation of the extent of exposure and consequently in elaborating preventive strategies. For example, total physiological burden and the potential susceptibility to heat disorders will be much higher if heat stress continues during off-duty hours through work at second jobs, strenuous leisure activities, or living in unremittingly hot quarters. In addition, nutritional status and hydration may reflect patterns of eating and drinking, which may also change with season or religious observances.

Maximizing Individual Heat Tolerance

Candidates for hot trades should be generally healthy and possess suitable physical attributes for the work to be done. Obesity and cardiovascular disease are conditions that add to the risks, and individuals with a history of previous unexplained or repetitive heat illness should not be assigned to tasks involving severe heat stress. Various physical and physiological characteristics which may affect heat tolerance are discussed below and fall into two general categories: inherent characteristics beyond the control of the individual, such as body size, gender, ethnicity and age; and acquired characteristics, which are at least partly subject to control and include physical fitness, heat acclimatization, obesity, medical conditions and self-induced stress.

Workers should be informed of the nature of heat stress and its adverse effects as well as the protective measures provided in the workplace. They should be taught that heat tolerance depends to a large extent upon drinking enough water and eating a balanced diet. In addition, workers should be taught the signs and symptoms of heat disorders, which include dizziness, faintness, breathlessness, palpitations and extreme thirst. They should also learn the basics of first aid and where to call for help when they recognize these signs in themselves or others.

Management should implement a system for reporting heat- related incidents at work. Occurrence of heat disorders in more than one person—or repeatedly in a single individual—is often a warning of serious impending trouble and indicates the need for immediate evaluation of the working environment and review of the adequacy of preventive measures.

Human traits affecting adaptation

Body dimensions. Children and very small adults face two potential disadvantages for work in hot environments. First, externally imposed work represents a greater relative load for a body with a small muscle mass, inducing a greater rise in core body temperature and more rapid onset of fatigue. In addition, the higher surface-to-mass ratio of small people may be a disadvantage under extremely hot conditions. These factors together may explain why men weighing less than 50 kg were found to be at increased risk for heat illness in deep mining activities.

Gender. Early laboratory studies on women seemed to show that they were relatively intolerant to work in heat, compared with men. However, we now recognize that nearly all of the differences can be explained in terms of body size and acquired levels of physical fitness and heat acclimatization. However, there are minor sex differences in heat dissipation mechanisms: higher maximal sweat rates in males may enhance tolerance for extremely hot, dry environments, while females are better able to suppress excess sweating and therefore conserve body water and thus heat in hot, humid environments. Although the menstrual cycle is associated with a shift in basal body temperature and slightly alters thermoregulatory responses in women, these physiological adjustments are too subtle to influence heat tolerance and thermoregulatory efficiency in real work situations.

When allowance is made for individual physique and fitness, men and women are essentially alike in their responses to heat stress and their ability to acclimatize to work under hot conditions. For this reason, selection of workers for hot jobs should be based on individual health and physical capacity, not gender. Very small or sedentary individuals of either sex will show poor tolerance for work in heat.

The effect of pregnancy on women’s heat tolerance is not clear, but altered hormone levels and the increased circulatory demands of the foetus on the mother may increase her susceptibility to fainting. Severe maternal hyperthermia (over-heating) due to illness appears to increase the incidence of foetal malformation, but there is no evidence of a similar effect from occupational heat stress.

Ethnicity. Although various ethnic groups have originated in differing climates, there is little evidence of inherent or genetic differences in response to heat stress. All humans appear to function as tropical animals; their ability to live and work in a range of thermal conditions reflects adaptation through complex behaviour and development of technology. Seeming ethnic differences in response to heat stress probably relate to body size, individual life history and nutritional status rather than to inherent traits.

Age. Industrial populations generally show a gradual decline in heat tolerance after age 50. There is some evidence of an obligatory, age-associated reduction in cutaneous vasodilatation (widening of the cavity of blood vessels of the skin) and maximal sweat rate, but most of the change can be attributed to alterations in lifestyle which reduce physical activity and increase the accumulation of body fat. Age does not appear to impair heat tolerance or ability to acclimatize if the individual maintains a high level of aerobic conditioning. However, ageing populations are subject to increasing incidence of cardiovascular disease or other pathologies which may impair individual heat tolerance.

Physical fitness. Maximal aerobic capacity (VO2 max) is probably the strongest single determinant of an individual’s ability to carry out sustained physical work under hot conditions. As noted above, early findings of group differences in heat tolerance which were attributed to gender, race or age are now viewed as manifestations of aerobic capacity and heat acclimatization.

Induction and maintenance of high work capacity require repetitive challenges to the body’s oxygen transport system through vigorous exercise for at least 30 to 40 min, 3 to 4 days per week. In some cases activity on the job may provide the necessary physical training, but most industrial jobs are less strenuous and require supplementation through a regular exercise programme for optimal fitness.

Loss of aerobic capacity (detraining) is relatively slow, so that weekends or vacations of 1 to 2 weeks cause only minimal changes. Serious declines in aerobic capacity are more likely to occur over weeks to months when injury, chronic illness or other stress causes the individual to change lifestyle.

Heat acclimatization. Acclimatization to work in heat can greatly expand human tolerance for such stress, so that a task which is initially beyond the capability of the unacclimatized person may become easier work after a period of gradual adjustment. Individuals with a high level of physical fitness generally display partial heat acclimatization and are able to complete the process more quickly and with less stress than sedentary persons. Season may also affect the time which must be allowed for acclimatization; workers recruited in summer may already be partly heat acclimatized, while winter hires will require a longer period of adjustment.

In most situations, acclimatization can be induced through gradual introduction of the worker to the hot task. For instance, the new recruit may be assigned to hot work only in the morning or for gradually increasing time periods during the first few days. Such acclimatization on the job should take place under close supervision by experienced personnel; the new worker should have standing permission to withdraw to cooler conditions any time symptoms of intolerance occur. Extreme conditions may warrant a formal protocol of progressive heat exposure such as that used for workers in the South African gold mines.

Maintenance of full heat acclimatization requires exposure to work in heat three to four times per week; lower frequency or passive exposure to heat have a much weaker effect and may allow gradual decay of heat tolerance. However, weekends off work have no measurable effect on acclimatization. Discontinuing exposure for 2 to 3 weeks will cause loss of most acclimatization, although some will be retained in persons exposed to hot weather and/or regular aerobic exercise.

Obesity. High body fat content has little direct effect on thermoregulation, as heat dissipation at the skin involves capillaries and sweat glands which lie closer to the skin surface than the subcutaneous fat layer of skin. However, obese persons are handicapped by their excess body weight because every movement requires greater muscular effort and therefore generates more heat than in a lean person. In addition, obesity often reflects an inactive lifestyle with resulting lower aerobic capacity and absence of heat acclimatization.

Medical conditions and other stresses. A worker’s heat tolerance on a given day may be impaired by a variety of conditions. Examples include febrile illness (higher than normal body temperature), recent immunization, or gastroenteritis with associated disturbance of fluid and electrolyte balance. Skin conditions such as sunburn and rashes may limit ability to secrete sweat. In addition, susceptibility to heat illness may be increased by prescription medications, including sympathomimetics, anticholinergics, diuretics, phenothiazines, cyclic antidepressants, and monoamine-oxidase inhibitors.

Alcohol is a common and serious problem among those who work in heat. Alcohol not only impairs intake of food and water, but also acts as a diuretic (increase in urination) as well as disturbing judgement. The adverse effects of alcohol extend many hours beyond the time of intake. Alcoholics who suffer heat stroke have a far higher mortality rate than non-alcoholic patients.

Oral Replacement of Water and Electrolytes

Hydration. Evaporation of sweat is the main path for dissipating body heat and becomes the only possible cooling mechanism when air temperature exceeds body temperature. Water requirements cannot be reduced by training, but only by lowering the heat load on the worker. Human water loss and rehydration have been extensively studied in recent years, and more information is now available.

A human weighing 70 kg can sweat at a rate of 1.5 to 2.0 l/h indefinitely, and it is possible for a worker to lose several litres or up to 10% of body weight during a day in an extremely hot environment. Such loss would be incapacitating unless at least part of the water were replaced during the work shift. However, since water absorption from the gut peaks at about 1.5 l/h during work, higher sweat rates will produce cumulative dehydration through the day.

Drinking to satisfy thirst is not enough to keep a person well hydrated. Most people do not become aware of thirst until they have lost 1 to 2 l of body water, and persons highly motivated to perform hard work may incur losses of 3 to 4 l before clamorous thirst forces them to stop and drink. Paradoxically, dehydration reduces the capacity to absorb water from the gut. Therefore, workers in hot trades must be educated regarding the importance of drinking enough water during work and continuing generous rehydration during off-duty hours. They should also be taught the value of “prehydration”—consuming a large drink of water immediately before the start of severe heat stress—as heat and exercise prevent the body from eliminating excess water in the urine.

Management must provide ready access to water or other appropriate drinks which encourage rehydration. Any physical or procedural obstacle to drinking will encourage “voluntary” dehydration which predisposes to heat illness. The following details are a vital part of any programme for hydration maintenance:

  • Safe, palatable water must be located within a few steps of each worker or brought to the worker every hour—more frequently under the most stressful conditions.
  • Sanitary drinking cups should be provided, as it is nearly impossible to rehydrate from a water fountain.
  • Water containers must be shaded or cooled to 15 to 20ºC (iced drinks are not ideal because they tend to inhibit intake).

 

Flavourings may be used to improve the acceptance of water. However, drinks that are popular because they “cut” thirst are not recommended, since they inhibit intake before rehydration is complete. For this reason it is better to offer water or dilute, flavoured beverages and to avoid carbonation, caffeine and drinks with heavy concentrations of sugar or salt.

Nutrition. Although sweat is hypotonic (lower salt content) compared to blood serum, high sweat rates involve a continuous loss of sodium chloride and small amounts of potassium, which must be replaced on a daily basis. In addition, work in heat accelerates the turnover of trace elements including magnesium and zinc. All of these essential elements should normally be obtained from food, so workers in hot trades should be encouraged to eat well-balanced meals and avoid substituting candy bars or snack foods, which lack important nutritional components. Some diets in industrialized nations include high levels of sodium chloride, and workers on such diets are unlikely to develop salt deficits; but other, more traditional diets may not contain adequate salt. Under some conditions it may be necessary for the employer to provide salty snacks or other supplementary foods during the work shift.

Industrialized nations are seeing increased availability of “sports drinks” or “thirst quenchers” which contain sodium chloride, potassium and carbohydrates. The vital component of any beverage is water, but electrolyte drinks may be useful in persons who have already developed significant dehydration (water loss) combined with electrolyte depletion (salt loss). These drinks are generally high in salt content and should be mixed with equal or greater volumes of water before consumption. A much more economical mixture for oral rehydration can be made according to the following recipe: to one litre of water, suitable for drinking, add 40 g of sugar (sucrose) and 6 g of salt (sodium chloride). Workers should not be given salt tablets, as they are easily abused, and overdoses lead to gastro-intestinal problems, increased urine output and greater susceptibility to heat illness.

Modified Work Practices

The common goal of modification to work practices is to lower time-averaged heat stress exposure and to bring it within acceptable limits. This can be accomplished by reducing the physical workload imposed on an individual worker or by scheduling appropriate breaks for thermal recovery. In practice, maximum time-averaged metabolic heat production is effectively limited to about 350 W (5 kcal/min) because harder work induces physical fatigue and a need for commensurate rest breaks.

Individual effort levels can be lowered by reducing external work such as lifting, and by limiting required locomotion and static muscle tension such as that associated with awkward posture. These goals may be reached by optimizing task design according to ergonomic principles, providing mechanical aids or dividing the physical effort among more workers.

The simplest form of schedule modification is to allow individual self-pacing. Industrial workers performing a familiar task in a mild climate will pace themselves at a rate which produces a rectal temperature of about 38°C; imposition of heat stress causes them to voluntarily slow the work rate or take breaks. This ability to voluntarily adjust work rate probably depends on awareness of cardiovascular stress and fatigue. Human beings cannot consciously detect elevations in core body temperature; rather, they rely on skin temperature and skin wettedness to assess thermal discomfort.

An alternative approach to schedule modification is the adoption of prescribed work-rest cycles, where management specifies the duration of each work bout, the length of rest breaks and the number of repetitions expected. Thermal recovery takes much longer than the period required to lower respiratory rate and work-induced heart rate: Lowering core temperature to resting levels requires 30 to 40 min in a cool, dry environment, and takes longer if the person must rest under hot conditions or while wearing protective clothing. If a constant level of production is required, then alternating teams of workers must be assigned sequentially to hot work followed by recovery, the latter involving either rest or sedentary tasks performed in a cool place.

Climate Control

If cost were no object, all heat stress problems could be solved by application of engineering techniques to convert hostile working environments to hospitable ones. A wide variety of techniques may be used depending on the specific conditions of the workplace and available resources. Traditionally, hot industries can be divided into two categories: In hot-dry processes, such as metal smelting and glass production, workers are exposed to very hot air combined with strong radiant heat load, but such processes add little humidity to the air. In contrast, warm-moist industries such as textile mills, paper production and mining involve less extreme heating but create very high humidities due to wet processes and escaped steam.

The most economical techniques of environmental control usually involve reduction of heat transfer from the source to the environment. Hot air may be vented outside the work area and replaced with fresh air. Hot surfaces can be covered with insulation or given reflective coatings to reduce heat emissions, simultaneously conserving heat which is needed for the industrial process. A second line of defence is large-scale ventilation of the work area to provide a strong flow of outside air. The most expensive option is air conditioning to cool and dry the atmosphere in the workplace. Although lowering air temperature does not affect transmission of radiant heat, it does help to reduce the temperature of the walls and other surfaces which may be secondary sources of convective and radiative heating.

When overall environmental control proves impractical or uneconomical, it may be possible to ameliorate thermal conditions in local work areas. Air conditioned enclosures may be provided within the larger work space, or a specific work station may be provided with a flow of cool air (“spot cooling” or “air shower”). Local or even portable reflective shielding may be interposed between the worker and a radiant heat source. Alternatively, modern engineering techniques may allow construction of remote systems to control hot processes so that workers need not suffer routine exposure to highly stressful heat environments.

Where the workplace is ventilated with outside air or there is limited air-conditioning capacity, thermal conditions will reflect climatic changes, and sudden increases in outdoor air temperature and humidity may elevate heat stress to levels which overwhelm workers’ heat tolerance. For instance, a spring heat wave can precipitate an epidemic of heat illness among workers who are not yet heat acclimatized as they would be in summer. Management should therefore implement a system for predicting weather-related changes in heat stress so that timely precautions can be taken.

Protective Clothing

Work in extreme thermal conditions may require personal thermal protection in the form of specialized clothing. Passive protection is provided by insulative and reflective garments; insulation alone can buffer the skin from thermal transients. Reflective aprons may be used to protect personnel who work facing a limited radiant source. Fire-fighters who must deal with extremely hot fuel fires wear suits called “bunkers”, which combine heavy insulation against hot air with an aluminized surface to reflect radiant heat.

Another form of passive protection is the ice vest, which is loaded with slush or frozen packets of ice (or dry ice) and is worn over an undershirt to prevent uncomfortable chilling of the skin. The phase change of the melting ice absorbs part of the metabolic and environmental heat load from the covered area, but the ice must be replaced at regular intervals; the greater the heat load, the more frequently the ice must be replaced. Ice vests have proven most useful in deep mines, ship engine rooms, and other very hot, humid environments where access to freezers can be arranged.

Active thermal protection is provided by air- or liquid-cooled garments which cover the entire body or some portion of it, usually the torso and sometimes the head.

Air cooling. The simplest systems are ventilated with the surrounding, ambient air or with compressed air cooled by expansion or passage through a vortex device. High volumes of air are required; the minimum ventilation rate for a sealed suit is about 450 l/min. Air cooling can theoretically take place through convection (temperature change) or evaporation of sweat (phase change). However, the effectiveness of convection is limited by the low specific heat of air and the difficulty in delivering it at low temperatures in hot surroundings. Most air-cooled garments therefore operate through evaporative cooling. The worker experiences moderate heat stress and attendant dehydration, but is able to thermoregulate through natural control of the sweat rate. Air cooling also enhances comfort through its tendency to dry the underclothing. Disadvantages include (1) the need to connect the subject to the air source, (2) the bulk of air distribution garments and (3) the difficulty of delivering air to the limbs.

Liquid cooling. These systems circulate a water-antifreeze mixture through a network of channels or small tubes and then return the warmed liquid to a heat sink which removes the heat added during passage over the body. Liquid circulation rates are usually on the order of 1 l/min. The heat sink may dissipate thermal energy to the environment through evaporation, melting, refrigeration or thermoelectric processes. Liquid-cooled garments offer far greater cooling potential than air systems. A full-coverage suit linked to an adequate heat sink can remove all metabolic heat and maintain thermal comfort without the need to sweat; such a system is used by astronauts working outside their spacecraft. However, such a powerful cooling mechanism requires some type of comfort control system which usually involves manual setting of a valve which shunts part of the circulating liquid past the heat sink. Liquid-cooled systems can be configured as a back pack to provide continuous cooling during work.

Any cooling device which adds weight and bulk to the human body, of course, may interfere with the work at hand. For instance, the weight of an ice vest significantly increases the metabolic cost of locomotion, and is therefore most useful for light physical work such as watch-standing in hot compartments. Systems which tether the worker to a heat sink are impractical for many types of work. Intermittent cooling may be useful where workers must wear heavy protective clothing (such as chemical protective suits) and cannot carry a heat sink or be tethered while they work. Removing the suit for each rest break is time consuming and involves possible toxic exposure; under these conditions, it is simpler to have the workers wear a cooling garment which is attached to a heat sink only during rest, allowing thermal recovery under otherwise unacceptable conditions.

 

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