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Heat Stress and Hydration

Handling the heat | Heat stress risk factors

Heat stress and heat illness risk factors at mine sites

With summer fast approaching, heat stress and heat illness are again becoming an issue of concern, particularly for those working on mine sites in Australia’s harsh environment. Dietitian Kaitlyn Bruschi shares some tips to beat heat stress and stay on your feet.

Most are familiar with the traditional methods for reducing heat stress, including staying hydrated and implementing controls to keep cool. However, the lifestyle choices we make both at work and at home can also have a significant impact on our tolerance to high temperatures.

Obesity, alcohol, caffeine, and physical inactivity can all impact our susceptibility to heat stress. By increasing awareness and making small changes to our lifestyle, we are able to minimise our risk of heat illness and improve our long term health.

Working in high-temperature environments is a way of life for many Australians, with potentially serious consequences. The body produces heat through normal metabolic processes and physical exertion. Tightly controlled physiological mechanisms exist to dissipate this excess heat to the environment and maintain constant body temperature.

When environmental temperatures exceed that of dry skin, sweating becomes the primary source of thermoregulation. However, as temperatures continue to rise, particularly in humid environments with poor airflow, this may not be enough. The body’s core temperature rises, resulting in a continuum of ailments, collectively known as heat illness. This ranges from the relatively mild heat rash and muscle cramps to heat exhaustion and the potentially fatal heatstroke.

Heat illness can present as headache, nausea or vomiting, irritability, clammy skin, dizziness, fatigue, elevated heart rate, and rapid breathing rate. This translates to lower productivity, poor morale and higher rates of accidents in the workplace. When left untreated, heat illness can progress to heatstroke, where confusion, further reduced muscle coordination, convulsions and ultimately a loss of consciousness can occur.

Published data on the incidence of heat illness and heat stress in Australian mines is limited. During a 12-month study at an Australian underground mine, 106 cases of heat exhaustion requiring medical intervention were reported. This equated to 43 cases per million man-hours. A large number of these were reported in summer, when temperatures and humidity were at their highest.

Hunt, Parker and Stewart (2013) also reported high rates of heat illness in surface mine operations, with 87 per cent of workers surveyed reporting symptoms. These results suggest that this is an industry-wide health and safety concern that does not appear to be fully managed by the traditional controls currently in place.

When left untreated, heat illness can progress to heat stroke, where confusion, further reduced muscle coordination, convulsions and ultimately a loss of consciousness can occur.

Engineering controls (e.g. ventilation and air-conditioning) and regular monitoring are important steps to reduce the incidence of heat illness. However, they are not the only solution. All risk factors for heat illness need to be considered – acclimatisation, hydration, obesity, age, physical fitness, caffeine, and alcohol.


Risk factors for Heat Stress


Acclimatisation refers to the range of physiological adaptations that occur when an individual is exposed to high temperatures. The body adjusts to heat exposure by becoming more efficient at removing heat from the system. Within days of entering a new environment, acclimatisation can begin. However, it can take up to two weeks to become fully acclimatised.

The adaptations include:

  • Sweat glands grow, resulting in a higher sweat rate and improved thermoregulation
  • Sweating occurs more rapidly on exposure to heat
  • The composition of sweat changes, lowering the amount of sodium lost by up to 50 per cent
  • Decreased core temperature
  • Improved efficiency for the removal of lactic acid from muscles after physical exertion
  • Reduction in heart rate and stroke volume
  • Increased blood flow to the skin

Unfortunately, after only three days away from work, the physiological changes that occur with acclimatisation begin to revert back to their baseline state. After four weeks away from working in high temperatures, these adaptations have been completely lost. It is also important to recognise that the positive effects of acclimatisation are almost completely lost if the individual is dehydrated.


Hydration underlies some of the primary mechanisms for thermoregulation. Heat illness progression, from mild to more critical symptoms, has been found to occur in a relatively linear fashion, following that of hydration status.

Even mild dehydration can lead to impaired concentration and coordination. It is also important to recognise that an individual can be dehydrated before any adverse symptoms appear, already putting workers on the back foot. Urine colour is one of the best short-term indicators to assess hydration status as the colour changes rapidly based on fluid intake and requirements (figure 1).

Heat stress chart indicates urine concentrations for managing heat stress
Figure 1: Urine charts are common in mine site toilets to remind workers to check their urine and self-assess their hydration status

Dehydration can have a considerable impact on workplace health and safety, especially if left untreated or allowed to get worse. As your body temperature climbs, blood carries this excess heat to the skin for dissipation to the environment. When dehydrated the volume of blood in your body is reduced, known as hypovolemia. The body may not have sufficient blood volume to meet the requirements of the vital organs while also sending enough blood to the skin for heat loss, compromising vital functions and leading to the array of symptoms observed with heat illness.

At the same time, evaporation of sweat assists with heat loss and helps cool the body. However, if dehydrated, this mechanism of thermoregulation is also compromised, resulting in a build-up of heat and development of heat illness.

It is important to note that clothing can impact heat dissipation, dependant on its permeability, design, and the amount of clothing/personal protective equipment required.

The amount of sweat an individual produces per hour is extremely individual, determined by a combination of their acclimatisation, health status, environmental conditions, and work rate.

“In underground mines, workers have been estimated to sweat approximately 0.46L per hour. However, this can climb to over 1.5L per hour in very hot environments. This can equate to five to 12 litres of fluid lost over a 12-hour shift”

In underground mines, workers have been estimated to sweat approximately 0.46L per hour. However, this can climb to over 1.5L per hour in very hot environments. This can equate to five to 12 litres of fluid lost over a 12-hour shift. It is also associated with large losses of sodium – an electrolyte essential for maintaining fluid balance and normal muscle and brain function.

Further, the muscle cramping associated with heat illness is believed to be caused by this depletion in electrolytes. Sodium loss can be as high as 10g per day (average 4.8-6g/day), equivalent to 25g salt (NaCl). Both fluid and sodium need to be replaced continuously throughout the shift to maintain performance and avoid heat illness.

With such high rates of fluid and electrolyte loss, it is essential to choose the right fluid replacement beverage. The study by Hunt and colleagues (2013) adds to the importance of this premise. They found the majority of underground and surface mine workers to consume only 4.1L or 3.1L, respectively, per 12-hour shift.

With the majority of workers not consuming sufficient fluid to match that lost through sweat, the importance of selecting the correct fluid replacement beverage is even greater. It must have a composition to optimise water absorption, while also meeting electrolyte and energy requirements.

The concentration of glucose and sodium in commercially available sports drinks can vary considerably but has a significant impact on the fluid replacement capacity of each product. Many commercially available sports drinks are high in carbohydrates to maintain blood glucose during short bouts of exercise. However, in the industrial setting, the priority is to increase fluid intake and absorption to maintain hydration status over a long period of time.

Excess carbohydrates can slow fluid absorption, while also leading to weight gain and subsequent health issues long term, especially when consumed in large quantities. The maximum rate of fluid absorption in the stomach and intestines is approximately 1-1.5L per hour, depending on beverage composition and interpersonal variability. Hypotonic or isotonic solutions – those with low carbohydrate contents (less than 8%) – are absorbed the fastest and are recommended for use by those working in high-temperature environments to maximise rehydration. Education should also be provided on their appropriate use.

Other components of our diet can impact our hydration status and susceptibility to heat illness. Food provides a range of vitamins and minerals, while also replenishing sodium and encouraging fluid consumption. Fruit and vegetables are a good choice, as they are relatively low in energy but have a high water content. A very high protein diet may have a negative effect on fluid load. Nitrogen, the by-product of protein metabolism, is excreted from the body in urine. Thus, a higher intake of protein can result in a higher urine output. Consuming a healthy, well-balanced diet can help prevent heat illness while also improving overall health.

Alcohol is a diuretic (a compound that switches off the body’s water conservation methods) telling your kidneys to excrete more water thereby increasing urine output. It goes to follow, that ingestion of alcohol will impact hydration status and lead to higher rates of heat illness. Urine output has been found to increase by as much as 10mL for every 1g of alcohol consumed, potentially leading to negative water balance, despite the water/liquid consumed with the alcohol. In one standard drink, which is 10g alcohol in Australia, you would be removing an additional 100mL of fluid from the body. It is easy to see how this would deplete fluid stores quite quickly.

Caffeine is also a diuretic. In addition to increased fluid loss, caffeine also creates electrolyte disturbances by altering the processes by which electrolyte levels are controlled in the body. Excessive caffeine intake can also increase blood pressure. As the cardiovascular system is already under strain with exposure to high temperatures, this additional pressure may increase the potential for heat illness and adverse side effects.

This is highly relevant to the mining industry where caffeine is often used as a stimulant to reduce fatigue. The introduction of energy drinks and pre-workout supplements deepens this health concern, as they typically contain a higher caffeine content compared with the more common sources, such as coffee, tea, soft drink and chocolate.


Early signs of physiological strain due to high temperatures include a rise in heart rate and cardiac output, as blood is spread between the skin for heat dissipation, as well as to the working muscles and vital organs. Improved physical fitness, through regular physical activity, confers greater protection against the effects of heat by reducing resting heart rate, improving cardiac output, and enhancing the efficiency of the sweat response.

This essentially initiates the process of acclimatisation and means workers are at a better starting point before they are exposed to high temperatures. This is supported by the observation that physically fit individuals have a reduced incidence of heat illness for exposure to a given heat stimulus compared with unfit individuals. Regular exercise also assists to maintain a healthy weight, which has also been associated with a resistance against heat illness.


Research shows fatal heatstroke to be 3.5 times more common in those who are above the healthy weight range (overweight or obese) compared with those of a healthy weight. The exact mechanism for this increase is unclear. Bar-Or and colleagues (1969) investigated heat tolerance of a group of exercising women, either healthy weight or above. Participants that were overweight or obese displayed a greater rise in core body temperature for a set amount of work, compared with lean individuals.

It has been suggested that those who are overweight or obese have a smaller ratio of surface area to mass, which could reduce the effectiveness of heat dissipation through the evaporation of sweat. Fat is also a good insulator and a poor conductor of heat, impairing heat transfer from the body’s core to the surface, then the environment. Blood flow to the skin in obese individuals for a given rise in core temperature has also been found to be lower compared with healthy individuals.


Susceptibility to heat is higher again for those with Type 2 diabetes mellitus, a condition associated with weight gain and unhealthy lifestyle choices. The physiological maladaptations that occur with diabetes may also impact thermoregulation, reducing the body’s ability to deal with higher core temperatures. Those with diabetes have been found to have reduced blood vessel dilation (opening of the blood vessels to allow more fluid through) resulting in less blood able to reach the skin for heat dissipation.

They are also less reactive to heat, taking longer to respond to heat and initiate the normal physiological response such as peripheral blood vessel dilation and sweating. This means those with diabetes get to a higher core temperature before the compensatory mechanisms kick in to help relieve that heat, resulting in higher likelihood for heat illness.


Age is also a risk factor for developing heat stress and heat illness. As we age, our ability to sense heat becomes impaired. As a result, the behavioural and physiological mechanisms involved in thermoregulation are reduced. Changes in blood flow to skin, reduced sweat production, declining physical fitness, and slower recovery rates all add up to a higher risk of heat illness. These changes can occur in individuals as young as 40 years of age. This is exacerbated by the reduced thirst sensation experienced as we age, resulting in lower fluid intake and higher rates of dehydration.

Conclusions on heat stress and heat illness

Heat stress and heat illness is something that affects everyone upon overexposure to high temperatures and unfavourable conditions. However, natural resilience, as well as the protection gained from appropriate lifestyle choices, can help reduce susceptibility.

Individuals regularly exposed to high temperatures, such as many of those who work in the mining industry, should be educated on the impacts of heat illness and provided with strategies to reduce its onset.

Remember, making healthy lifestyle choices, including regular physical activity and following a balanced diet, is one of the best ways to maintain resistance to heat stress and heat illness and avoid chronic lifestyle disease.

Kaitlyn Bruschi
A love of food and human physiology lead Kaitlyn to a career as an Accredited Practising Dietitian. After experience in the acute care and community settings, Kaitlyn developed a passion for educating people on how to live a healthier life through small, realistic changes. She works with Corporate Bodies International who partner with coal mines in the Central Queensland region to develop and implement health and wellbeing programs that work towards the ultimate goal of creating workplaces free from chronic and lifestyle disease.

Read more of Mining Safety Journal or you might like to read a warning from the Mining Safety Regulator on Heat Stress

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AMSJ April 2022