The following paper has been included on this site with the permission of the Queensland Mining Council.

This is a very good example of the professional approach required when dealing with heat in the working environment.







Rick Brake

B.E. (Mining), MBA, MAUSIMM

Enterprise Mine Project

Michael Donoghue


Chief Occupational Physician, Mount Isa Mines

Graham Bates


Senior Lecturer in Physiology, School of Public Health, Curtin University



The existing safety and health management protocols for the management of heat stress in the Enterprise mine are those that currently apply to the other Isa mines. These date to the 1960s when the last full review into working in heat was undertaken, and are based on experimental work conducted by the military in England and Singapore during and shortly after WW2, although the concept of the "six hour shift" dates to 1942 where it came to Isa from the Broken Hill mines. In turn, the short shift came to Broken Hill in the 1920s, probably from Europe, although this origin has been lost in time. The Isa and Broken Hill protocols have formed the basis for much of the existing working-in-heat protocols in Australia and are therefore quite dated. The main body of work done in an occupational setting since WW2 has been undertaken in South Africa, where labour-intensive mining methods, employment practices and education levels are very different to the more capital-intensive and productive Australian mines and the much better educated Australian workforce. Even in Australia, since the mid 1960s, mining methods, personal protective equipment requirements, and statutory and societal attitudes towards health and safety have changed. It is now possible to draw on a vast amount of work that has been done over the past 20 years in the area of sports physiology, in terms of exercise-related heat disorders. Moreover, solid state technology, miniaturisation of electronic components and development of such technology as Erasable Programmable Read Only Memory (EPROM or "firmware") have provided the means to have much better and sophisticated measurement of environmental conditions and the resulting heat stress on workers. Finally, much better monitoring of occupational diseases including heat illness and monitoring of environmental conditions in the workplace have greatly improved the understanding of both the cause and effect of heat illness.

This Paper focuses on the Health and Safety Medical Protocols adopted in the Enterprise Mine to deal with working in heat. Extensive monitoring of heat-related illness and environmental conditions at Isa has shown that existing protocols do not adequately protect the workforce from heat related illness. A comprehensive, integrated approach to managing working-in-heat is presented.


The Enterprise mine is located at Mount Isa and is wholly owned by Mount Isa Mines Limited (MIM). It is being developed, at a cost of $331 million, from about 1000 m below surface to almost 2000 m below surface. Expected completion for the construction program is late calender 1999 with the mine producing at 3.5 Mtpa from about 2002 to 2015. The effects of high surface ambient temperatures in summer, combined with "autocompression" in the intake airways, high virgin rock temperatures and heavy use of diesel equipment, results in heat stress in the working place that, without intervention, would exceed the levels that human physiology can withstand. It would also result in significant decreases in productivity and high accident rates even where work is possible.

Enterprise Mine recognises that working in heat potentially affects health, safety, productivity and morale. It also recognises that a "systems approach" must be used to effectively tackle such a complicated issue. This involves setting limits of thermal stress for workers, understanding productivity decrements when working in heat, establishing productive and healthy environmental targets, developing engineering solutions to meet these targets, creating a well-educated workforce with respect to working in heat and then developing health protocols to ensure workerís health is protected.

Refrigeration must be added to the mine to provide acceptable and productive conditions underground. At full production, Enterprise mine will have an installed refrigeration capacity of almost 40 MW. Some of this is provided by way of surface bulk air cooling, some by way of underground air cooling and some by way of chilled service water used in the operation of equipment. A detailed review of the engineering issues involved in the working in heat protocols are given elsewhere.

These developments when taken together have provided both the need for a complete review of the safety and health management protocols for working in heat and the ability to come up with new solutions to many of these problems.

Human Physiology, Heat and Underground Mines

The human body has several mechanisms for rejecting heat and remaining cool. However, once the air temperature exceeds about 34 degrees Celsius, the only effective mechanism is by evaporation of sweat from the skin.

In this scenario, heat generated in the deep body organs and muscles is picked up by the blood and pumped to the skin. The skin is cooled by evaporation of sweat. The system operates similarly to a car, with the heart being similar to the water pump, the blood similar to the carís coolant, and the skin operating as the radiator.

The ability of the body to dissipate heat is a function of many factors. The key ones are:

Of equal significance is the amount of heat that the body needs to reject. Obviously this is primarily a function of the work rate. The maximum efficiency of the human body is about 20%, but it is rare to be this high. The harder the body works, the greater is the metabolic heat generation rate.

The System

The control of hazards related to in working in heat were reviewed under three categories:

Hazard management was reviewed in three respects:

Enterprise Mine then developed a series of statements (Figure 1) that embodies this philosophy in terms which management and the workforce could relate to.

wpe1.jpg (39295 bytes)


Heat Illness

Prolonged moderate elevations of core temperature can cause Heat Exhaustion. This is characterised by: weakness or extreme fatigue, faintness, nausea, vomiting, headache and restlessness. Replacement of lost water, together with rest in a cool environment, will cure heat exhaustion in a short time.

When the bodyís capacity to lose heat is exceeded and the core temperature rises above 390 Celsius, the risk of serious heat illness increases rapidly.

Heat Stroke is a state of thermoregulatory failure: a condition in which an uncontrolled rise in core temperature occurs and the bodyís tissues start to break down. It occurs when the body core temperature is above 410 Celsius. Symptoms include: confusion, staggering, delirium, coma and, often, the absence of sweating. Death is imminent. Permanent tissue damage usually occurs, although the longer-term health impact depends on which tissues are damaged and the severity of the damage.

About 1 in 5 cases of heat stroke is fatal in underground mines in South Africa, even with hospitalisation. Medical treatment requires the most rapid cooling of the body.

The Isa mines have never had a case of heat stroke, let alone a fatal case of heat stroke, despite working at least ten million manshifts in hot conditions (more than 280 WB) over the past 30 years. However Isa continues to have many cases of lesser forms of heat illness, with over 100 cases during summer 97/98, and many cases still believed to be unreported. A continuing campaign to improve workforce education about heat illness and its symptoms, is resulting in many of these previously unreported incidents now being reported.

Obesity and poor cardiovascular fitness are risk factors for heat stroke. Hence the need for obesity (Body Mass Index or BMI) and aerobic fitness (oxygen uptake or VO2 max) testing for persons working in heat. Due to the need to drink large volumes of fluids and concentrate the urine when working in heat, persons with poor kidney function should not work in heat. People who are diabetic or anaemic are more susceptible to heat as are those with circulatory problems, atherosclerosis, previous head injuries, some skin diseases, thyroid disease or blood pressure problems. Depending on the severity of these conditions, some individuals with these conditions may not be able to work safely in heat.

People can also develop an intolerance to heat following recovery from a severe heat illness although this is very rare in the occupational setting. Irreparable damage to the bodyís heat-dissipating mechanisms has been noted in many of these cases.

Other heat illnesses include: heat collapse (heat syncope or fainting), heat cramps, heat rash (prickly heat), heat oedema (swelling of the limbs) and chronic heat fatigue.

Safety, Shift Length and Rosters When Working in Heat

This is a large topic in itself and will not be discussed here except to make a few observations:

"If during the first trimester of pregnancy, a female workerís core temperature exceeds 390 C for extended periods, there is an increased risk of malformation to the unborn foetus. Additionally, core temperatures above 380 C may be associated with temporary infertility in both females and males".

Therefore MIM does not currently allow women of child-bearing capacity to work in very thermally stressful situations.

Health and Safety Medical Protocols

Dehydration Test and Heat Illness Protocol

The MIM Dehydration and Heat Illness Protocol has two basic objectives:

The MIM Medical Centre is currently monitoring heat illness in the Isa workforce in line with the introduction of the new Dehydration and Heat Illness Protocols. For "fit", healthy adults, dehydration is responsible for almost all of the deleterious effects of working in heat.

Heavy (stressful) physical exertion in heat will result in sweat rates of about 1 litre per hour.

Very stressful environmental conditions will result in sweat rates of about 1.5 litre per hour.

Sweat rates up to 2.2 litres per hour have been recorded and are sustainable over periods of one to two hours in fit, healthy individuals with access to plenty of water.

The limit of the stomach and gut to absorb water is about 1.6 to 1.8 litres per hour on a continuous basis, providing the individual is not dehydrated.

Sweat starts to drip off the body when the body is about 50% fully wet. Sweat rates about 70% above the evaporation rate are required to attain and sustain 100% skin surface wetting.

The sweat glands do fatigue over a period of hours when sweat rate is close to the maximum. When the skin is wet for a period of hours, swelling of the duct opening in the skin can occur, which occludes the sweat gland, a condition called hydromeiosis. These conditions are temporary but will affect sweat rate and therefore the ability to work at high sweat rates for long periods in heat. Allowing a break in which the body can cool down (preferably in an air-conditioned lunch room) and sweating can cease allows the sweat glands to recover.

Some studies have shown :

The education campaign about dehydration and drinking when working in heat has been so effective that most workers now maintain or even improve their hydration state when working in heat. However, a large number (almost 50%) are coming to work dehydrated, especially after their roster break, and an education focus on this aspect is required for next summer.

For workers with no symptoms of heat illness, oral rehydration (using diet cordial) has been found to take no longer than two hours, with the median being one hour.

Dehydration is affected by any diuretic, including alcohol and beverages containing caffeine (coffee, tea, colas). Some miners, who have developed repeat cases of heat illness, have been found to drink six litres or more of Coke per day on their days off (and nothing else). This will result in dehydration, even while the individual continues to urinate regularly.

Many over-the-counter cold and flu remedies contain codeine (hence the warning on some medications that the substance can cause drowsiness and should not be taken if "driving"). Most contain either a sedative or a stimulant and some contain drugs that cause vaso-constriction, which is a serious problem if working in heat.

There are many prescription drugs that also affect the ability to work in heat. Examples include "beta-blockers" which are often prescribed for high blood pressure. These restrict the maximum heart rate and can also "clamp down" the blood vessels. Persons on such medication probably should not be working in heat. Diuretic medications will affect the ability of a worker to maintain hydration levels.

There are many herbal remedies which also affect the ability to work in heat.

For these reasons, Mount Isa Mines requires:

The workplace temperatures and airflow are measured for all persons who develop a heat illness.


Acclimatisation is very real and the sum total of all the ways the body has to adapt to working in heat. In an early and seminal clinical study, the following results were obtained during acclimatisation:

It is recognised that acclimatisation also results in the sweat glands becoming much more resistant to hydromeiosis and sweat gland "fatigue".

Most of the effects of acclimatisation are developed within 7 days, but continue to 14 days and beyond.

Acclimatisation has a major impact on the ability to work in heat (both cognitive abilities and physical abilities). Less is known on the time required to lose acclimatisation, but 7 to 21 days is a consensus. For Isa, loss of acclimatisation is considered to occur after 14 days. Therefore MIM considers any person who is a new starter (from outside the Tropics), or has been on leave or sick or otherwise away from work for 14 days (unless within the Tropics) to be unacclimatised.

Persons acclimatise to the level of heat stress they experience. Acclimatisation is therefore not an "on or off" state. Care needs to be taken when a person in one job with one degree of acclimatisation is exposed to work requiring a higher level of acclimatisation.

Formal acclimatisation programs in South Africa typically involve persons working in a controlled environment at a set work rate and needing to be able to keep their core temperature below a threshold. Some workers can do this on their first shift, most in about three shifts, and "heat intolerant" persons (3%) are rejected after five days.

Acclimatisation is being required in an increasing number of codes, e.g. ISO7933. For example, the ACGIH code now states:

"Acclimatisation and Fitness

Acclimatisation to heat involves a series of physiological and psychological adjustments that occur in an individual during the first week of exposure to hot environmental conditions. The recommended TLVs are valid for acclimatised workers who are physically fit. Extra caution must be employed when unacclimatised or physically unfit workers must be exposed to heat stress conditions".

In reality, "acclimatisation" runs in parallel with two other important adaptations:

Work hardening is very important for workers that have heavy physical demands on certain muscle groups. Many workers in traditional "blue collar" jobs would have relatively "low fitness" in the aerobic sense; however, their work hardening means they could do the work their muscles are adapted for long after an endurance athlete had been forced to retire in the same circumstances.

Work hardening is therefore an issue if it is decided to "job share" some work, or use one type of worker as a "back-up" for another type of worker. For example, to use a forklift driver used to low physical activity in an air-conditioned cabin as the back-up for a bricklayer would result in a high probability of "work hardening" injuries for the forklift driver on those occasions when he is required to lay bricks at "bricklayers intensity". Similarly, the forklift driver may be fully acclimatised to the level of heat stress regularly experienced in his job, but insufficiently acclimatised for the job with higher heat stress. Heat illness is likely to result if this person is required to work alongside a team of workers acclimatised to more heat stress.

A simple, easily administered protocol has been developed to ensure only acclimatised persons work in heat. The major components of this protocol are:

Note that the positive effects of acclimatisation are almost entirely lost if the worker is dehydrated.

Short Shift ("6 hour job" or "hot job") Protocol and Temperature and Airflow limits

Mount Isa Mines currently has a "short shift" ["hot job"] protocol, which reduces the working shift length to six hours where workers have worked in thermally stressful situations for more than two hours. Workers are paid for the full shift (8, 10 or 12 hours as the case may be). The concept of a "short shift" came to Mount Isa in 1942 when it was introduced from Broken Hill where it was introduced in the 1920s and is believed to ultimately have originated in Europe. This protocol has been thoroughly reviewed and significant changes are expected to the protocols in 1998 as it is now recognised that the work/rest "cycling" required when working in heat is very much less than 6 hours on and 2 hours off and that heat illness can and does develop in as little as ten to 30 minutes of exposure. The new protocols not only build on the much better understanding MIM now has of the health and safety effects of working in heat, but also build on the new technology now available for measuring environmental conditions (thermal stress) and predicting the impacts on human physiology (thermal strain). The new protocols cover the whole range of relevant issues (dehydration, acclimatisation, clothing, pacing, etc) but in terms of environmental (thermal) stress will establish four limits:

All limits will be expressed in terms of Air Cooling Power (ACP) which is the cooling power of the air on a sweating person, measured in watts per square meter of body surface area.

The AREL is effectively a "buffer zone". Work can be done safely in the AREL region but will not be allowed unless the ventilation on the job is up to standard, and must be reported to the Manager within 24 hours.

The AREL and withdrawal limits will be significantly reduced from (i.e. more conservative than) the existing "six hour job" and "stop job" limits and the design of the limits will drive the underground mines towards higher airflows on the job, as low air speed on the job (less than 0.5 m/s) has been found to occur in 75% of all workplaces where symptoms of heat illness commenced. Substantial use of "air movers" (compressed-air venturi fans) will be required in future.

With much reduced temperature limits, current six hour environmental conditions will effectively become "withdrawal" (i.e. stop job) limits in future. A major program of measuring deep body core temperatures and fatigue levels in underground workers most exposed to heat stress shows that heart rates, work rates, core temperatures and fatigue levels on 12 hour shifts in these new limits are within acceptable parameters.

There is a strong correlation between heat illness and surface and underground workplace wet bulb temperatures. However, it is important to understand that focussing on a temperature protocol alone will not eliminate heat illness, unless workplace temperatures can be brought down to "office conditions". For example at Isa, there were about 100 cases of heat illness from underground operations in summer 97/98, but only 6 of these occurred in persons who were granted a "hot job" [i.e. who were "caught" in the formal temperature/time protocol, which has been designed to protect workers from heat illness]. Conversely, there have been over 1000 "hot jobs" granted this past summer, but only 4 of these resulted in heat illness. At Isa, there has been no correlation between the incidence of heat illness and the incidence of short shifts granted, although many workplaces where heat illnesses started were in hot job conditions, but workers did not qualify because they failed to be there for more than two hours.

Clothing and Personal Protective Equipment (PPE)

Clothing has a major impact on the ability of the body to cool itself via sweating as the following Air Cooling Power (ability of the air to cool the body) calculations shows:

Environmental conditions: 280 C wet bulb air temperature, air speed 1 m/s:

ACP for unclothed body 13 W/m2

ACP for lightly clothed body 168 W/m2

ACP for heavily clothed body 127 W/m2

Key issues for clothing include: fabric vapour permeability and conductivity, clothing design (ventilation, e.g. "bagginess"), amount of clothing and personal protective equipment and type of PPE, e.g. wearing leather boots is better than impermeable rubber boots.

Pre-Employment, Pre-Transfer and Periodic Health Assessments

The purpose of health assessments is to ensure persons who cannot work healthily and safely in heat are excluded from working in the Enterprise Mine under MIMís Duty of Care. These assessments are generally in line with Guidelines for Safe Mining.

It does this by excluding those with known risk factors for heat illness from working in the Enterprise Mine.

Periodic health assessments are every two years but managers can refer an individual for medical review at any time if they have concerns.

The two key tests, based on ability to work in heat, which would exclude work are:

Note that a 5í10" male in the "ideal" weight range [Figure 2] would weigh no more than 80 kg (BMI of 25). At a BMI of 35 (the MIM exclusion point), he weighs 110 kg (i.e. is carrying a surplus 30 kg constantly) and at a BMI of 45, he weighs 142 kg (i.e. is constantly carrying a surplus 62 kg).

Contrary to popular opinion, physical work rates of underground miners in Australia are relatively low and heavy musculature does not produce high BMIs. Even the burliest of forwards in the Brisbane Broncos does not exceed a BMI of 30.

About 3% of blue-collar workers fail the BMI requirement to be under 35, and about 12% fail the aerobic requirement to be over 30 ml oxygen per kg per minute. The average BMI of Isa blue-collar workers is 26 and the average aerobic capacity is 39 ml/kg/min. Financial assistance (dietary counselling, membership of a local gym, attendance at "Gutbusters") is offered to employees who fail their health screen on these tests.

For all new employees, or transferees from other MIM mines, the specific health requirements for the Enterprise Mine are written in to their letter of offer and contract of employment, i.e. BMI, VO2 max, etc and a medical must be passed, even for "internal transfers".

Education is provided for the workforce and also, optionally, for the spouses. Given a largely male workforce, it is the wives who prepare most of the meals for workers underground.

Permit to Work In Hot Locations

Sometimes, work in very hot conditions is unavoidable, e.g. to fix the ventilation itself, or for operating or other emergencies. To ensure proper precautions are taken when persons work in very hot conditions, a Permit to Work in Hot Locations has been introduced. The key components of this system are:

  1. No person is allowed to work alone under this permit.
  2. No unacclimatised person can work under this permit.
  3. No woman of child-bearing capacity can work under this permit.
  4. Expected water consumption, work-rest cycles, maximum working time are to be provided before the work commences, except in emergency (e.g. rescuing another worker).
  5. Sufficient water must be kept on the job at all times.
  6. A supervisor or other competent person must be on the job at all times.
  7. Ready access must exist to emergency services (e.g. phone or radio on the job).
  8. Each person covered under this Permit must have a dehydration test at the end of the shift.

Emergency Egress (Escape) and Entrapment Procedures

The depth and heat levels within the Enterprise mine add a number of quite serious complicating factors to emergency escape and entrapment procedures. Survival times both with and without access to drinking water have been identified. This impacts on the sizing, location and specification of emergency refuge bays and the selection of self-rescuers. Procedures in the event of power failures, which will bring the volume of ventilating air to a standstill within about three hours in mid summer conditions, when Natural Ventilating Pressure is at its lowest, have also been reviewed.

Visitors Health Screening Protocol

Until recently, there were no medical restrictions on any private or "official" visitors to the Enterprise mine, although some warnings were provided. This was inconsistent with MIMís Duty of Care to all persons and has been rectified by getting each visitor to fill in a health assessment in the form of a self-assessed medical questionnaire. This questionnaire is now:

Chronic Heat Fatigue

In cold climates such as Canada, a common ailment that was historically and colloquially known as "cabin fever", has now been called the SAD syndrome: Seasonal Affective Disorder. A related phenomenon, in effect a longer-acting form of Heat Fatigue, has been recognised in the Mount Isa community for many years, and is locally and colloquially known as "Mango Madness". It is noticeable in the occupational context by an increase in work related incidents over the hot summer months. It is described as resulting in longer term impairment in work performance and social behaviour. A lack of motivation, alcoholic overindulgence and an inability to concentrate are symptoms. As for the SAD syndrome, a number of factors working together create the setting for "mango madness":

The best way to combat "mango madness" is to educate managers, supervisors and workers alike.


The Enterprise Mine will be Australiaís deepest underground mining operation and will be among the hottest underground mines in the world. It will employ several hundred workers for at least fifteen years. With high virgin rock temperatures and the adverse Mount Isa surface summer conditions, the workforce will be exposed on a continual basis to adverse thermal conditions. Mount Isa Mines has developed a comprehensive series of health protocols to dovetail with a complete system of engineering and other solutions to managing the risks associated with working in heat. These have been developed from first principles but have been extensively validated by on-site studies.



Brake, D and Nixon, T. 1998. The Development of Ventilation and Refrigeration Systems and Engineering Controls for the Enterprise Mine. Proc 1998 Annual Conf of AUSIMM. p 187-197.
The Thermal Environment. 1990. Brit Occ Hyg Soc. Technical Guide # 8.
Hygge, S, 1992. Heat and Performance in Handbook of Human Performance (Ed: Smith & Jones). Chapter 4, pp 79-104. Academic Press.
ACGIH, 1991. Heat Stress in Documentation of the Threshold Limit Values For Physical Agents in the Work Environment.
Howes, M and Nixon, C, 1997. Development of Procedures for Safe Working in Hot Conditions, paper presented to the 6th International Mine Ventilation Congress. Pittsburg.
Shapiro, Y, Magazanik, A, Udassin, Pl, Ben-Baruch, G, Shvartz, E and Shoenfeld, Y, 1979. Heat intolerance in former heat stroke patients. Annals Inter Med. 90: 913-916.
Bates, G. The Industrial Worker: a Different Breed of Athlete, 1997. Record of Sports Science Exchange Roundtable. Gatorade Sports Science Institute. Vol 8:2.
Stewart, J, 1989. Fundamentals of Human Heat Stress in Environmental Engineering in South African Mines. Chp 20, page 503. The Mine Ventilation Society of South Africa.
Lee, D H (ed), Falk, H L (ed) and Murphy, S D (ed), 1988. Handbook of Physiology Ė Environmental Physiology, American Physiological Society. pp 204-205.
ISO7933. Hot Environments-Analytical determination and interpretation of thermal stress using calculation of required sweat rate. 1989. Int Org for Standardization. Geneva.
Bates, G. & Matthew, B, 1996. A New Approach to Measuring Heat Stress in the Workplace, paper presented to The Aust Inst of Occ Hyg 15th Ann Conf. Perth. 30 Nov to 4 Dec.
McPherson, M J, 1992. The Generalisation of Air Cooling Power, paper presented to the 5th International Mine Ventilation Congress. Johannesburg.
Guidelines for Safe Mining. 1996. Hygiene and Health Facilities. Chp 4, p 85-86. NSW Department of Mineral Resources.
Stewart, J, 1989. Practical Aspects of Human Heat Stress in Environmental Engineering in South African Mines. Chp 21, page 539. The Mine Ventilation Society of South Africa.
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