Last edited 18 Dec 2015

Wet bulb globe temperature

The Health and Safety Executive (HSE) suggest that wet bulb globe temperature WBGT (or the wet bulb globe temperature index) can be used to indicate the likelihood of heat stress.

Heat stress is a form of overheating that the occupants of a building may experience when the measures their bodies use to regulate internal temperature begin to fail. This can occur, for example, in buildings where an industrial process is being carried out, such as smelting, brick-firing, cooking and so on.

Heat stress is dependent on a number of factors, such as air temperature, metabolic rate, humidity, clothing, length of exposure, acclimatisation and so on. This means that it is a personal condition. Some occupants may experience heat stress whilst others may not.

Where humidity is high, or occupants are wearing protective clothing that cannot be adjusted or removed, the body may be unable to lose heat through the evaporation of sweat and so the individual may begin to experience elevated deep-body temperature, excessive sweating and increased heart rate. This can cause; dehydration, headache, difficulty concentrating, confusion, muscle cramp, heat rash, fatigue, giddiness, fainting, nausea, convulsions and ultimately, if it is allowed to persist, death.

The Workplace Regulations, the Management of Health and Safety at Work Regulations 1999 require that employers assess the risks to the health and safety of their workers, and take action where necessary and reasonably practicable.

Wet bulb globe temperature is a relatively simple indicator of heat stress. It includes components of natural wet-bulb temperature (tnw), globe temperature (tg) and air temperature (ta - dry bulb temperature).

Where there is no solar radiation, wet bulb globe temperature = 0.7tnw + 0.3tg
Where there is solar radiation, wet bulb globe temperature = 0.7tnw +0.2tg +0.1ta

The recorded values can be compared to recommended values for a specific metabolic rate and state of acclimatisation (found in BS EN 27243:1994, ISO 7243 Hot environments. Estimation of the heat stress on working man, based on the WBGT-index (wet bulb globe temperature)).

However, this will only provide an indication of whether heat stress is likely. Wet bulb globe temperature is a compromise, offering a measure that is relatively simple to assess, whilst still recording the key components of heat stress. Where the recomended values are exceeded, a more accurate measure such as Required Sweat Rate (also found in BS EN 12515) should be used for further assessment.