Heat exposure for a human depends on the temperature, humidity, wind speed, solar radiation, clothes worn and the intensity of the work. We use two heat indexes: WBGT (wet bulb globe temperature) that has been used for decades and has solid empirical backing; UTCI (Universal Thermal Climate Index) is a modern heat stress index developed from a physiological model.
When the body generates heat (by work) faster than it can lose it, the core body temperature rises. While some increase in core temperature above 37°C/98.6°F is acceptable, an increase beyond 39°C/102°F creates health risks, which vary from person to person, depending on ethnic group, age, gender, the duration of high heat exposure, and the degree of acclimatisation. International Standards Organisation (ISO) sets guidelines to keep core body temperature at 38°C/100.4°F. It uses WBGT as the heat stress index to specify recommended rest/work cycles at different physical work intensities (ISO Standard 7243).
As solar radiation and wind speed are very variable from day to day (and hour to hour), we first calculate indoor, or in shade WBGT (no solar radiation and constant 1m/s wind). The maximum outdoor WBGT is calculated by adding a value derived from short-wave solar radiation (between 1 and 2°C/1.8-3.6°F) to the indoor/in-shade WBGTmax.
References
- Lemke, B., and Kjellstrom,T. 2012. Calculating Workplace WBGT from Meteorological Data: A Tool for Climate Change Assessment, Industrial Health, 50, 267–278.
- ISO (1989) Hot environments − Estimation of the heat stress on working man, based on the WBGT-index (wet bulb globe temperature). ISO Standard 7243. International Standards Organization, Geneva
Heat exposure for a human depends on the temperature, humidity, wind speed, solar radiation, clothes worn and the intensity of the work. We use two heat indexes: WBGT (wet bulb globe temperature) that has been used for decades and has solid empirical backing; UTCI (Universal Thermal Climate Index) is a modern heat stress index developed from a physiological model.
UTCI is a bioclimatic index for describing the physiological comfort of the human body under specific meteorological conditions. It takes into account not just the ambient temperature but also other variables like humidity, wind and radiation, all factors significantly affecting our physiological reaction to the surrounding environment.
UTCI is based on the UTCI-Fiala model, which combines a dynamic thermoregulation model of the human body together with a temperature-varying clothing insulation model, both describing distinct states depending on different ambient factors. Altogether, UTCI provides an estimation of the "apparent" temperature that our body would feel under a given environmental condition specified by the air temperature, wind, humidity and radiation. This apparent temperature is in reality the temperature that a reference environment, defined by fixed values of humidity, wind and radiation, should have in order to produce in our body the same physiological reaction of the one produced by the initial given environment.
The concept of UTCI was then developed as an equivalent temperature. This involved the definition of a reference condition to which all other climatic conditions will be compared. The equivalence between real and reference conditions was based on an equivalence of the dynamic physiological response between both. As this dynamic physiological response is multidimensional (body core temperature, sweat rate, skin wetness etc. at different points in time), a single dimensional strain index was calculated based on principal component analysis.
The associated assessment scale was developed from the simulated physiological responses and comprises 10 categories that range from extreme cold stress – very strong cold stress – strong cold stress – moderate cold stress – slight cold stress – no thermal stress – moderate heat stress – strong heat stress – very strong heat stress – extreme heat stress.
References
- Fiala, D. et al, 2011. UTCI-Fiala multi-node model of human heat transfer and temperature regulation. International Journal of Biometeorology 56: 429-441.
- Bröde, P. et al, 2011. Deriving the operational procedure for the Universal Thermal Climate Index (UTCI), International Journal of Biometeorology volume 56, 481–494.
The heat index was developed in 1979 by Robert G. Steadman. It contains assumptions about the human body mass and height, clothing, amount of physical activity, individual heat tolerance, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.
Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as a last resort, by panting, when blood is heated above 37°C/98.6°F. Sweating cools the body through evaporation. However, high relative humidity retards evaporation, robbing the body of its ability to cool itself.
The heat index (HI) is an index that combines air temperature and relative humidity in shaded areas, to suggest a human-perceived equivalent temperature, i.e. what it feels like to the human body. The result is also known as the "felt air temperature", "apparent temperature", "real feel" or "feels like". If you are exposed to direct sunlight, the heat index value can be increased by up to 8°C/15°F.
Physiological effects of the Heat Index:
Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
Extreme danger: heat stroke highly likely with continued exposure.
References
- Steadman, R. G. (July 1979). The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. Journal of Applied Meteorology. 18 (7): 861–873.
- Steadman, R. G. (July 1979). The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature. Journal of Applied Meteorology. 18 (7): 874–885.