Scarlet Tech Ltd.

Empirical validation of a new heat stress index

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Courtesy of Courtesy of Scarlet Tech Ltd.

Thermal stress is a well-recognised health hazard in the workplace. In addition to the health deficits, working in the heat can impact significantly on the productivity of some industries which are located in harsh environments. A long-standing dilemma in OHS has been the specification of what constitutes a safe working environment. The current indices used to evaluate the environment are either flawed or are difficult to implement. A new heat stress index, the thermal work limit, has now been developed which incorporates all needed inputs, and generates a single figure specifying a maximum work limit. Initial validation of the index demonstrates it to be simple to use, less prone to interpretive error, reliable and far superior to currently recommended indices as an indicator of thermal stress.


It is now generally accepted that working in conditions of thermal stress has associated risks and consequences. From a health perspective, heat stroke (a serious condition with a mortality rate of around 80%) is the classical end point of the inability to maintain a stable core temperature. The lesser condition of heat exhaustion is relatively common and has recently been shown to have a clear biochemical and haematological profile.1 In addition, chronic hypohydration leads to increased risk of renal calculi and bladder cancer.2 From an economic perspective, high environmental stress contributes to safety incidents, lowered productivity and poor morale.

These considerations are particularly relevant to Australian industry, especially in the tropical northern region where operations are frequently located in areas of high ambient summer temperatures and humidity. Under the current OHS legislation there is a duty of care to protect workers in these environments. However, the guidelines to ensure this protection are scant, inappropriate and often non-specific. This situation has persisted as a consequence of minimal research into human thermal homeostasis in high stress environments and limiting technology. Currently, the most widely recommended index of thermal stress is the wet bulb globe temperature (WBGT). In common with other heat stress indices in use in industry, the WBGT is relatively insensitive to the cooling effect of air movement, although, due to its impact on both evaporative and convective cooling, increased ventilation is one of the most cost-effective ways of reducing physiological strain to the individual working in a thermal environment. In practice the WBGT is difficult to apply as it requires estimation of the workers' metabolic rates, which will not only vary throughout a shift widi different tasks performed, but may be voluntarily altered by self-pacing workers. The measured WBGT, estimated metabolic rate and assessed acclimatisation status must be applied to a standard chart to arrive at a thermal index. Personnel with the necessary knowledge and training to correcdy apply die WBGT are not present at all times in all workplaces, and in practice the index is often not used. An alternative is ISO 7933-1989: Analytical detenninator and interpretation of thermal stress using calculation of required sweat rate, but it is extremely difficult to apply and in reality is rarely used.1

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