Heat stress management
In Australia's northern regions, Rio Tinto operations such as Comalco Weipa, Robe, Dampier Salt and Hamersley Iron, are located in areas of very high ambient summer temperatures as well as either very high or very low humidity. Heat stress can become a major issue if not adequately managed. Heat stress initially causes discomfort and reduced productivity, and can lead to more serious health effects such as accidents, illness, and even death. Prolonged exposure to high temperatures alone can lead to excessive fluid loss, heat exhaustion, or heat stroke.
Various tools or indices of thermal stress are used to assess the potential for adverse effects of heat. The most widely used index, the wet bulb globe temperature (WBGT), was developed for more temperate climes and is inappropriate for people acclimatised to Australian conditions. The challenge was to develop a practical scale that was still protective of employees' health.
A new heat stress index, thermal work limit (TWL), which generates a single figure specifying a maximum work limit, based on extensive physiological monitoring for underground mining exposures, was developed by Australian researchers. The TWL had been shown to be simple to use, reliable and less prone to interpretive error. It predicts the maximum rate of work that can safely be sustained in a given environment, and thus allows the development of realistic guidelines and protocols to minimise the human and financial costs of heat illness.
However, further validation of the TWL was required for use in the outdoor environment where radiant heat is a significant contributor to environmental heat stress. Rio Tinto with Pilbara Iron thus sponsored further research on TWL with one of the co-developers of the index, Dr Graham Bates of Curtin University, Western Australia.
The first part of the project consisted of using TWL to conduct a thermal risk assessment of the working environment at a variety of above ground sites operated by Rio Tinto at Dampier and West Angelas during the summer months of 2005. The second part of the study evaluated the thermal strain experienced by workers in these environments. Habitual fluid intakes were quantified and hydration levels monitored for groups of workers at a number of sites. For a smaller group at one site sweat loss was measured and physiological strain monitored by continuous recording of heart rates during work. The third part of the study investigated the effect on fluid consumption habits, hydration status, and perceived wellbeing of replacing the usual beverage with a fluid and electrolyte replacement product developed specifically for industrial use.
The project demonstrated that the WBGT index is an excessively conservative index of environmental heat stress in an aboveground environment where the convective and evaporative effect of air movement contributes significantly to cooling. It also emphasised the need for adequate fluid intake before reporting for work in order to start the shift in a well hydrated state, as the hydration level is unlikely to improve during the work shift. It appears individuals have a hydration "set point" and that fluid intake is unconsciously adjusted to maintain this. A conscious effort needs to be made to increase this set point by increasing fluid intake prior to and throughout the work shift.
The research validated the TWL and its associated protocols developed for surface operations. It provides managers with a workable strategy for minimising the risk to workers posed by environmental heat stress.
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