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.

This page has information about the causes and sources of heat stress, types of heat-related illness and control measures.

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.

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.

Heat awareness

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
  Explains difference between heat stress and heat strain.
• Health effects
  Heat-strain can prevent normal cooling mechanisms from working properly and can lead to heat-related illnesses.
• Managing exposure
  Heat stress indices are not safe/unsafe limits and should only be used as guides.
• Pre-start checklist
  The American Industrial Hygiene Association (AIHA) provides a checklist of things to consider before starting work in hot environments.
• More information
  More information and resources on heat stress.