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Heat stress

Working in hot and/or humid environments can be uncomfortable, but more importantly lead to a heat-related illness, which can be fatal.

A heat related illness can result from these contributing factors:

  • wearing high levels of personal protective equipment (e.g. hazmat suits)
  • heat from extremely hot or molten material (e.g. foundries, steel mills, bakeries, smelters, glass factories, and furnaces)
  • sunshine (e.g. outdoor work such as construction, road repair, open-pit mining and agriculture)
  • high humidity (e.g. laundries, restaurant kitchens, and canneries)
  • internal body heat (e.g. from heavy manual work).

On this page

How the body controls heat gain and heat loss

The human body gains and loses heat in two ways:

  • body heat – the internal heat generated by metabolic processes
  • exchange with the environment – the body exchanges heat with its surroundings mainly through:
    • radiation - the process by which the body gains heat from surrounding hot objects (e.g. hot metal, furnaces or steam pipes), and loses heat to cold objects (e.g. chilled metallic surfaces) without contact with them
    • convection – the process by which the body exchanges heat with the surrounding air. The body gains heat from hot air and loses heat to cold air which comes in contact with the skin
    • evaporation of sweat – the cooling effect is more noticeable with high wind speeds and low relative humidity. In hot and humid workplaces, the body cooling due to sweat evaporation is limited because the air cannot absorb more moisture. In hot and dry workplaces, the cooling due to sweat evaporation is limited by the amount of sweat produced by the body.

The body also exchanges small amounts of heat by conduction and breathing, which can usually be discounted when assessing the heat load on the body.

Not everyone reacts to heat in the same way

The way heat affects people varies from person to person and is influenced by:

  • general health
  • body weight (being overweight or obese can make it more difficult to cope with heat)
  • age (particularly for people about 45 years and older)
  • poor general health
  • a low level of fitness will make people more susceptible to feeling the extremes of heat
  • certain prescription and illicit drug use
  • medical conditions (can also increase how susceptible a person is). People with conditions such as heart disease, high blood pressure, pregnancy, respiratory disease and diabetes may need to take special precautions. In addition, people with some types of skin diseases and rashes may be more susceptible to heat.

Other factors include circulatory system capacity, sweat production and the ability to regulate electrolyte balance.

Thermal comfort

Most people feel comfortable when the air temperature is between 20°C and 27°C and when the relative humidity ranges from 35 to 60 per cent.

People may feel uncomfortable when air temperature or humidity is higher than this. However, it is important to note that such situations do not cause harm as long as the body can adjust and cope with the additional heat.

Common terms

Heat stress is the total heat load on the body from all sources including:

  • ambient air temperature
  • radiant heat from other sources (e.g. vehicles, equipment and hot-work processes)
  • air movement
  • relative humidity
  • individual task requirements such as a need to wear a respirator or multiple layers of clothing
  • metabolic heat produced by the body because of physical activity.

Heat strain is how the body responds to heat stress. These responses are focussed on removing heat from the body and are influenced by:

  • physical fitness
  • degree of acclimatisation
  • medical conditions
  • age
  • general health
  • weight
  • hydration state
  • certain drugs/medications.

Health effects

People have an average core body temperature of around 37˚C. In hot environments, or where internal temperature is raised through exercise for example, body responses include:

Normal responses

  • vasodilation (widening of blood vessels)
  • sweating
  • increased respiratory rate
  • increased heart rate.

Responses due to heat-strain

  • electrolyte changes
  • dehydration
  • elevated core temperature.

Excessive heat-strain can occur when the work environment, task or individual health prevent these cooling mechanisms from working properly, and can lead to heat-related illnesses including:

  • Heat rash – sometimes called ‘prickly heat’. It is a skin irritation caused by excessive sweating and:
    • can occur at any age
    • is most commonly associated with humid /dusty tasks in which the skin pores become blocked
    • looks like a red cluster of pimples or small blisters
    • is most likely to occur ogn the neck and upper chest, and in skin folds and creases
  • Heat cramps – these include muscle pains or spasms, usually in the abdomen, arms or legs. They may occur after strenuous activity in a hot environment, when the body gets depleted of salt and water. They may also be a symptom of heat exhaustion.
  • Dizziness and fainting – heat related dizziness and fainting results from reduced blood flow to the brain. Heat causes an increase in blood flow to the skin and pooling of blood in the legs, which can lead to a sudden drop in blood pressure.
  • Heat exhaustion – is a serious condition that can develop into heatstroke. It occurs when excessive sweating in a hot environment reduces the blood volume. Warning signs may include paleness and sweating, rapid heart rate, muscle cramps (usually in the abdomen, arms or legs), headache, nausea and vomiting, dizziness or fainting.
  • Heatstroke – is a medical emergency and requires urgent attention. Heatstroke occurs when the core body temperature rises above 40.5 °C and the body’s internal systems start to shut down. Many organs in the body can suffer damage and to rectify it, the person’s body temperature must be reduced quickly. Most people will have profound central nervous system changes such as delirium, coma and seizures. The person may stagger, appear confused, have a fit or collapse and become unconscious. As well as effects on the nervous system, there can be liver, kidney, muscle and heart damage. The symptoms of heatstroke may be the same as for heat exhaustion, but the skin may be dry with no sweating and the person’s mental condition worsens.

Heat stroke requires immediate first aid and medical attention. Delayed treatment may result in death.

Watch the film A Current Affair - Melting Man, below on this webpage, that outlines the tragic consequences that can occur if heat strain factors are not adequately controlled.

Workplace exposure standards

There is no workplace exposure standard or limit for heat stress.

Setting of a safe/unsafe limit simply based on ambient air temperature is not appropriate due to the many variables (i.e. task, environment and individual) associated with the onset of heat strain.

It is important to note that heat stress indices are not safe/unsafe limits and should only be used as guides.

Managing exposure

Identification and assessment

Identifying and managing heat stress is not just about the air temperature.

Assessing risks of heat related illness requires accurate identification and assessment of:

  • workplace conditions
  • job requirements
  • individual worker attributes.

Conduct a walk through survey of the workplace, and ask your workers about their heat stress issues. Consider the work location’s previous history of heat stress issues, including what time of day or year and under what circumstances they occurred.

Some standard questions in any heat investigation include:

  • What are the potential sources of heat?
  • What heat stress problems have been experienced?
  • What type of action has the worker taken to minimise his or her exposure to thermal energy?
  • Is there a thermal stress management program in place?
  • Does it include awareness training and information on heat stress?
  • What type of remedial action has been taken to reduce the site’s thermal loading to workers?
  • What comments or concerns have been raised by workers?

Use the following three level approach to identify heat stress factors in your workplace (Di Corleto et al 2013, refer figure 1).

Heat stress flowchart - Figure 1
Figure 1 [Click on image to enlarge]

Level 1 (Non-technical): is a basic risk assessment, which can use a simple index such as the Heat stress (basic) calculator.

Heat stress (basic) calculator is a simple online tool designed for use by anyone with basic knowledge of the work area and process. There is no direct measurement of environmental conditions required to use the calculator.

The online calculator uses a number of factors to make a basic assessment of heat stress including:

  • the state of acclimatisation
  • work demands, location
  • clothing
  • sun exposure
  • hot surfaces
  • task complexity
  • air movement, and other factors
  • apparent temperature (this is calculated using ambient temperature and relative humidity).

Note: As the basic assessment is a qualitative assessment, instrumentation for measurement of wet bulb glove temperature (WBGT) is not mandatory meaning people with limited technical expertise and equipment can identify and assess the risk factors. The basic assessment can therefore be performed using readily available information at the workplace (i.e. ambient air temperature and relative humidity).

The online calculator is an initial qualitative review of potential heat stress situations and is not a definitive assessment tool.

A level 2 (technical) assessment will be needed when:

  • the online calculator identifies an unacceptable level of heat stress risk
  • impermeable clothing is worn.

Level 2 (Technical): needs additional data collected and the use of a second level index such as ISO 7933: Predicted Heat Strain (PHS) or Thermal Work Limit (TWL). A range of measuring equipment and expertise is required for this type of assessment.

Additional data needed for this type of assessment will include:

  • dry bulb temperature
  • globe temperature
  • air velocity
  • humidity
  • metabolic load
  • clothing type
  • posture.

This type of assessment and gathering of required data should only be undertaken by a competent person such as an occupational hygienist or occupational physician.

The University of Queensland (UQ) has developed a smart phone app based on ISO 7933, called PHS, visit The Thermal Environment for more information.

A level 3 (expert) assessment will be needed when either:

  • data cannot be collected
  • where impermeable clothing is used.

Level 3 (Expertise): Physiological monitoring

Physiological monitoring is used in cases of high heat stress risk such as:

  • work involving the use of encapsulating suites (e.g. hazmat suits) or high levels of external PPE
  • environments where the level 2 assessment indicates allowable exposure times of less than 30 minutes
  • where a work environment cannot be accurately assessed.

The risk and severity of heat related illness will vary widely among workers, even under identical heat stress conditions. Individual monitoring measures a worker’s physiological responses (such as elevated body temperature and/or heart rate) to assess the effectiveness of implemented controls.

Control the risks

There are two approaches to controlling the risk:

  • modify the environment to suit the work
  • modify work to suit the environment.

Look carefully at the control measures that can be used, some are more practical for certain situations than others. In most situations, multiple controls from the following will be required.

Modifying the environment to suit the work

Control the source of the heat

  • Reduce the temperature of the source of heat (e.g. allow the section of plant or equipment to cool before work commences)
  • Insulate hot surfaces (can also provide protection from contact burns)
  • Clad sources of radiant heat with bright or polished materials (e.g. clad steel or brick surfaces with aluminium or tin)
  • Use radiant heat shields or barriers (need to have good insulation properties and low emissivity/high reflectivity so they don’t become hot)
  • Use shade barriers (e.g. temporary gazebos) over the work area to block heat from the sun.

Ventilation, air conditioning and air movement

  • Remove or dilute hot/humid air and replace it with cooler/drier air. This is the most efficient method and can be achieved by either forced mechanical ventilation or naturally. It is especially important in hot and humid environments. Examples include:
    • mechanical ventilation which draws cool air from outside the work area to displace the hotter air
    • exhaust or extraction fans to remove hot air
    • natural ventilation via windows, doors and roof vents/louvres
  • Increase air movement in the work area e.g. fans.
  • Use artificial cooling such as evaporative coolers, air conditioning, vortex tubes, or chillers.

Modifying the work to suit the environment

Modify the work process

  • use mechanical aids such as cranes, forklifts and earthmoving plant to reduce the workload
  • conduct work at ground level or organise work to minimise climbing up and down stairs and ladders
  • do the work indoors or in a shaded area.

Provide rest areas/refuges to escape the heat

  • use refuges for workers to escape the effects of hot environments.
  • locate as near to the work area as possible.
  • provide shelters (shelters can range from temporary gazebos through to insulated structures or cabins which are air-conditioned.

Administrative controls

  • Worker selection – workers who have become acclimatised to the work environment are at less risk than unacclimatised workers.
  • Scheduling of work – where possible, conduct the work:
    • in cooler parts of the year, especially where the work requires protective clothing
    • at night, early morning or late afternoon rather than midday.
  • Work-rest intervals:
    • spend rest periods in a cool place with a plentiful supply of cool water for fluid replacement.
    • remove protective clothing during breaks to allow workers to cool off.
  • Fluid replacement – critical when working in a hot environment, especially where hard work (metabolic work) is required:
    • drink small volumes as frequently as possible during work
    • provide cool drinks or water as close as possible to the work area (if supplying drinks, make sure the workers actually like them)
    • help workers to self-monitor their hydration status via hydration test strips and urine colour charts
    • encourage workers to avoid diuretic drinks immediately prior to starting work and to rehydrate between shifts
    • encourage workers to consume water at the start of the shift so as to maximise their hydration status
    • encourage the replacement of electrolytes in high sweat scenarios.
  • Buddy systems – trained workers can keep an eye on each other for signs of heat effects, reducing risks compared to isolated workers.

Personal protective equipment (PPE)

Clothing and particularly protective clothing can often have an adverse effect on the body’s heat balance in hot environments by insulating the body and reducing evaporative heat loss. Impervious clothing can impede heat loss.

PPE can also help to reduce the risk of heat strain in some circumstances.

Examples include:

  • wearing a hat to protect from sunlight
  • vented safety eye wear (where safe to do so)
  • protective clothing (especially if made from natural fibre, will provide some protection against contact burns and radiation)
  • wearing light coloured (especially white, and reflective clothing e.g. aluminised) which absorb comparatively little radiant heat energy
  • respiratory protective equipment (RPE) with an exhalation valve
  • wearing cotton gloves as a lining under regular gloves
  • wearing lightweight/cotton undergarments
  • using specialised cooled or conditioned personal protective clothing including:
    • air circulating systems: these usually incorporate the use of a vortex cooling tube. Depending on the size of the vortex tube, they may be used to cool a large volume system such as a tank or may be utilised as a personal system whereby the vortex is worn on the belt and cool air is fed into an air supplied helmet and/or vest (e.g. powered air purifying respirators (PAPR)). Breathing quality air should only be used for such air -supplied systems
    • liquid circulating systems: these systems rely on the principle of heat dissipation by transferring heat from the body of the liquid and then the heat sink (which is usually an ice water pack). Liquid (water) cooling suits must be worn close to the skin and the chilled liquid is pumped through fine capillary tubing from either a battery powered pump worn on the belt or through an ’umbilical cord’ from a remote cooling unit
    • ice cooling systems: these involve the placement of ice or other materials such as n-tetradecane in pockets in an insulating garment, typically a vest, worn close to the skin such that heat is conducted away. This in turn cools the blood in the vessels close to the skin surface, which then helps lower the core temperature
    • reflective systems: reflective clothing is utilised to help reduce radiant heat load on an individual. It acts as a barrier between the person’s skin and the hot surface reflecting away the infrared radiation. The most common configuration of reflective clothing is an aluminised surface bonded to a base fabric.

Long work clothing is not a cause of heat related illness. Rather multiple factors, as outlined above, collectively cause heat related illness. Outdoor workers should wear long pants and work shirts with a collar and long sleeves to cover as much skin as possible. This is because outdoor workers are exposed to high levels of direct sunlight over long periods of time, generally receiving five to 10 times more ultraviolet radiation exposure per year than indoor workers. Therefore, outdoor workers are at an increased risk of skin cancer and other health issues related to excess UV radiation. Read more about sun safety for outdoor workers.

Train workers

Training is required for all workers likely to be working in hot environments, undertaking strenuous work at elevated temperatures and those who wear impermeable protective clothing.

Training should include:

  • mechanisms of heat exposure
  • potential heat exposure situations
  • recognition of predisposing factors
  • importance of fluid intake
  • the nature of acclimatisation
  • effects of alcohol and drugs
  • early recognition of symptoms of heat illness
  • prevention of heat illness
  • first aid treatment of heat illnesses
  • self-assessment (this is a critical key element)
  • how medical surveillance programs work and the advantages of employee participation.

Use the Heat stress (basic) calculator as part of training by encouraging your workers to input combinations of different controls to achieve an improved risk score.

Maintain and review the controls

The right controls may be in place, but are they working properly?

  • When did you last check the effectiveness of the controls?
  • Are available controls used by workers when needed?
  • Is there a better control that could be implemented?

Use the Heat stress (basic) calculator to assess the benefit of implementing new controls by inputting combinations of the above controls to see if the risk score has been improved.

Pre-start checklist

The American Industrial Hygiene Association (AIHA) provide a checklist to be considered before commencing work in hot environments:

  • Are adequate supplies of palatable cool drinks available?
  • What is the major source of heat stress and how can it be mitigated (e.g. protective clothing requires particular strategies)?
  • If radiant shielding (including shade) is possible, is it in the most strategic location?
  • Is temperature monitoring equipment available at the work site?
  • Are work guidelines that are appropriate to the situation available to workers and supervisors?
  • Are first aid supplies available that are appropriate to heat/cold emergencies?
  • Has an appropriate work rate been determined, and is there sufficient manpower to stay on schedule despite a slower work pace?
  • Have supervisors been instructed to remove workers at the first sign of problems?
  • Have workers been properly and thoroughly acclimatised (or reacclimatised after a time away from the stressing environment)?
  • Is a cool recovery/rest area available?
  • Are workers and supervisors trained in recognising the symptoms, and providing first aid treatment of heat injury?
  • Is there a means of calling emergency medical support? Do workers know how and where to call emergency medical support?
  • Is the clothing appropriate (minimal obstruction of sweat evaporation and maximal protection from radiant heat i.e. use the lightest, most permeable clothing that provides adequate safety)?
  • Is the air velocity as high as practical?
  • Are workers well hydrated at the beginning of work?
  • Is spot cooling available?
  • Is microclimate cooling (e.g. cool type vests) available as needed?
  • Have workers been reminded of appropriate safety precautions?
  • Have workers who are pregnant, have relevant medical conditions or who are taking medication that increases their risk, have had previous heat injuries, and who have fever, been protected from elevated internal body temperatures?

Seek medical advice about a person’s fitness for work in hot environments if you know they have predisposing medical conditions.

Heat stress risk is not being properly managed

If you are concerned that you are working in an unsafe and hot environment, you should alert your immediate supervisor to the problem and discuss some of the possible ways of reducing the impact on you and other workers. Use the Heat stress (basic) calculator to highlight your concerns to your supervisor.

If the supervisor will not deal with the problem or you believe the problem hasn’t been fixed, you should contact your health and safety representative (if there is one), or the supervisor’s manager.

If the matter is still unresolved and you continue to believe that an unsafe situation exists, contact Workplace Health and Safety Queensland on 1300 362 128.

Resources

AIOH – A guide to Managing Heat Stress: Developed for Use in the Australian Environment.

ISO 7933:2004 Ergonomics of the thermal environment -- Analytical determination and interpretation of heat stress using calculation of the predicted heat strain.

W502 Student Manual: Thermal Environment. February 2016. Occupational Hygiene Training Association. The Thermal Environment.

Heat stress (basic) calculator

A Current Affair - Melting Man

This story shows the tragic consequences that can occur if heat strain factors are not adequately controlled. Story provided to Workplace Health and Safety Queensland courtesy of the Nine Network's A Current Affair program.

Download a copy of this film (MP4, 221MB)

  • Read transcript
    • Speakers:

      Mr Martin King, Current Affair Journalist

      Mr Alan Byrne

      Mrs Bernadette Byrne, Mother

      Mr Des Byrne, Father

      Mrs Karen Simpson, Lawyer, Slater & Gordon

      Dr Andrew Heggie, Associate Professor, Facial Surgeon

      Martin King: Hey mate, how are you going?

      Alan Byrne: Good.

      Martin King: Can you stand up?

      Alan Byrne: Yeah.

      Martin King: Want to have a crack? Do you want me  to – are you right?

      Alan Byrne: I'm right.

      Martin King: Good man.

      Alan Byrne: Okay.

      Martin King: How does that feel?

      Alan Byrne: Good.

      Bernadette Byrne: Alan was always a great worker,  always. He had rung me the weekend before and said "Mum I'm working really  hard".

      Martin King: What is your memory of that day in  Queensland?

      Alan Byrne: Oh God, none.

      Martin King: No memory at all?

      Alan Byrne: No.

      Martin King: Who would want to remember? On a  stinking hot Queensland day Alan Byrne almost worked himself to death. He can't  remember because he's brain damaged from heat stroke.

      Bernadette Byrne: Alan's been through everything you  could imagine, from being in a coma, on death's doorstep, to now going through  a process of rehabilitation to get his life back.

      Des Byrne: G'day Alan, how are you?

      Alan Byrne: Good.

      Bernadette Byrne: What are we watching today?

      Des Byrne: These soapies again.

      Martin King: The days of Alan's life are so  different now. The big, strong brickie's labourer is back in Melbourne with his  parents, Des and Bernadette. What is it like for you two to see  your son like this?

      Bernadette Byrne: Oh, it's heartbreaking, it's  heartbreaking, because we know what he was like and we want him back that way,  but we can't.

      Martin King: What happened that day on a building  site is a warning to all of us about the dangers of heat stroke. It was 42  degrees. Alan had been working in the full sun all day. He hadn't had enough  water, he hadn't had any food, he wasn't wearing a hat. At 3:00 o'clock in the  afternoon he started feeling sick and began walking home, five kilometres. He  didn't make it. He was found unconscious at a roundabout by a bus driver, who  called police.

      Bernadette Byrne: This is in Queensland.  Said he's critically ill, you need to get here. But don't come here, go to  Townsville because we're airlifting him because he's critically ill. We just  went into panic, we were all just shaking and crying.

      Martin King: When Des and Bernadette arrived at  Townsville Hospital their son was in a coma.

      Bernadette Byrne: It hit us, didn't it?

      Des Byrne: Yeah.

      Bernadette Byrne: We saw our son there, dying really.  They told us he was dying. On total life support, total life support. There  wasn't one part of his body that wasn't on some machine.

      Martin King: Tell me what happened to your son's  body.

      Bernadette Byrne: It completely shut down, everything  shut down. His body, well, it boiled, it boiled. Even they told us the bone  marrow boiled. It was so overheated. His body temperature was 42 degrees,  that's why he couldn't – they said he wouldn't live.

      Karen Simpson: In Alan's case the employer has  failed to ensure that he was adequately hydrated and protected from such  extreme conditions.

      Martin King: Karen Simpson from Slater &  Gordon in Brisbane is Alan's lawyer. His case is now one of Australia's biggest  WorkCover claims.

      Karen Simpson: It's certainly a catastrophic sized  claim. Because of the lack of residual earning capacity for Alan, it will  result in very significant monetary compensation for him.

      Des Byrne: One of the surgeons said "You don't  know it's happening to you until someone comes up behind you with a baseball  bat and goes whack, and that's it and you're down".

      Martin King: It was the capable hands  of doctors in Townsville that saved Alan's life, but then a shocking complication.  He lost blood supply to his face and his facial bones began to dissolve.

      Dr Andrew Heggie: Absolutely unusual, quite unique.

      Martin King: Dr Andrew Heggie specialises in  facial surgery.

      Dr Andrew Heggie: The three major components have been  replaced. There's the bone from the hip to replace the upper jaw. A rib graft  to reconstruct the nasal bridge and the contour. Then subsequently these  titanium implants so that a full bridge can be made to reconstruct his jaw and  bite.

      Martin King: What is the one thing you would like  to do with your body?

      Alan Byrne: Walk.

      Martin King: Walk. Alan's legs are strong and  capable, it's the brain damage that's the problem.   What are your hopes for the future?

      Alan Byrne: Work.

      Martin King: What would you like to do for work?

      Alan Byrne: Anything.

      Martin King: Just anything? If you  can't work you might as well work out. Or go shooting. Or sailing. What is Alan's future?

      Bernadette Byrne: Who knows? You've got to leave all  the doors open. Who knows? He's still our son and he's still absolutely  beautiful.

      ~ The End ~

      Run time: 4:36

Last updated
18 April 2017

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