Living in Australia, we've all gotten a spiel about the heat, whether it's 'toughen up, sunshine', or 'stay hydrated, kids!'. Of course, working in hot and/or humid environments can be uncomfortable, but more importantly can lead to a heat-related illness, which may be fatal. However, the risks of working in heat, and the source of the heat, can vary significantly between jobs, so it's important to highlight all the aspects of heat stress.

A heat related illness can result from these contributing factors:

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

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.

How heat affects people differently

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.

Identification and Assessment

There is no workplace exposure standard or limit for heat stress. Heat stress indices are not safe/unsafe limits and should only be used as guides.

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

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

  • workplace conditions
  • job requirements
  • individual worker attributes.

There are two approaches to controlling the risk:

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

Of course, staying hydrated is vitally important to preventing heat stress, but it's not the be-all end-all solution. Staying hydrated will always help, but it won't prevent heat stress in all environments, and hydration should just be one component of your heat management program.

Environmental Modification

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 or cover sources of radiant heat
  • 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.

Work Modification

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:

  • 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 eyewear (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 e.g. aluminised) workwear, which absorbs 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.

Worker Training:

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.

 

Worksafe Queensland recommends their heat stress calculator for a simple diagnosis of the dangers of heat stress in your workplace and an assessment of your control measures.