Thermal work limit
Thermal work limit (TWL) is an index defined as the maximum sustainable metabolic rate that well-hydrated, acclimatized individuals can maintain in a specific thermal environment within a safe deep body core temperature (< 38.2 °C or 100.8 °F) and sweat rate (< 1.2 kg or 2.6 lb per hour).[1] The index is designed for self-paced workers and does not rely on estimation of actual metabolic rates. The index has been introduced into the United Arab Emirates[2] and Australia, resulting in a substantial fall in the incidence of heat illness in the latter.[3]
History
[edit]The idea of a thermal work limit (TWL) was developed by Graham Bates and Derrick Brake in 1997.[4] TWL and its management protocols have been introduced into several industrial operations where workers are subject to thermal stress. Since the introduction of TWL-based policies in the Australian mining industry, the amount of man-hours lost due to serious heat illness has fallen from 12 million to 6 million, and the amount lost due to all heat illness incidences has fallen from 31 million to 18 million.[5]
Theory
[edit]The thermal work limit index calculates the maximum metabolic rate, in watts of metabolic heat per square meter of body surface area, that can be continuously expended in a particular thermal environment in order to keep the body within safe physiological limits. The TWL is an integrated measure of the dry-bulb, wet-bulb, wind speed, and radiant heat.[2] From these variables and taking into consideration the type of clothing worn and the acclimatization state of the worker, the TWL predicts the maximum level of work that can be carried out in a given environment without workers exceeding a safe core body temperature 38.2 °C (100.8 °F) and sweat rate.
Sweat rates are also calculated, so the level of fluid replacement necessary to avoid dehydration can be calculated. The thermal work limit algorithm builds on work originated by Mitchell and Whillier who developed an index "specific cooling power", which subsequently became known as "air cooling power" (ACP).[6]
Application
[edit]
The TWL has been included in the Abu Dhabi EHSMS code of practice.[7] If TWL is too low, then work cannot be carried out continuously.[5]
References
[edit]- ^ Brake, D. J.; Bates, G. P. (2002). "Limiting Metabolic Rate (Thermal Work Limit) as an Index of Thermal Stress". Applied Occupational and Environmental Hygiene. 17 (3): 176–186. doi:10.1080/104732202753438261. PMID 11871754.
- ^ a b "The Thermal Work Limit Heat Stress Index". Health Authority - Abu Dhabi. Archived from the original on May 15, 2013. Retrieved May 15, 2013.
- ^ Miller, V. S.; Bates, G. P. (2007). "The Thermal Work Limit is a Simple Reliable Heat Index for the Protection of Workers in Thermally Stressful Environments". Annals of Occupational Hygiene. 51 (6): 553–561. doi:10.1093/annhyg/mem035. PMID 17878259.
- ^ Brake, D.J.; Donoghue, M.D.; Bates, G.P. (August 1998), A New Generation of Health and Safety Protocols for Working in Heat (PDF)
- ^ a b Brake, D. J.; Bates, Graham (2000). Occupational heat illness: an interventional study. International Conference on physiological and cognitive performance in extreme environments. Canberra.
- ^ Mitchell, D.; Whillier, A. (1971), "Cooling power of underground environments", Journal of the South African Institute of Mining and Metallurgy, 71: 93–9
- ^ Abu Dhabi EHSMS Regulatory Framework (AD EHSMS RF): Technical Guideline: Safety in the Heat (PDF), 2.1, April 2013, archived (PDF) from the original on 23 April 2013