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.
[edit] Find out more
[edit] Related articles on Designing Buildings Wiki
- Dry bulb temperature.
- Globe temperature.
- Heat stress.
- Operative temperature.
- Relative humidity.
- Temperature.
- Thermal comfort.
- Wet bulb temperature.
[edit] External references
- HSE, Heat stress.
- 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).
Featured articles and news
Designing sustainability and performance into buildings
Specifying and selecting sustainable resilient timber products.
Modifying wood to improve resistance to decay and movement.
A last minute, long look for built environment professionals.
The architecture of creative reuse. Book review.
Installing solar panels on listed structures.
Sustainable development global goals, history in progress?
"Unless we act now, the 2030 Agenda will become an epitaph for a world that might have been."
Mike Kagioglou FCIOB named CIOB President
'Sustainable Development Goals must be focus for construction'
BSRIA training; a look at what's on offer
From energy management to compliance training.
TESP video warns to beware of rogue trainers.
Highlighting the slippery tactics of non-approved providers.
New Building Safety Wiki launched
Boosting awareness and understanding of the new fire safety regime.
New playbook on AI in construction published by CIOB
How to get to grips with, and the best from AI.
Digital Construction Report NBS
BIM, cloud, off-site, immersive tech, AI, twins and sustainability.
ECA learning zone and industry focus video series
From updates and amendments to circular economy, emergency lighting and much more.