Heat stress and WBGT - workers protection?



What is WBGT, why do we need to measure, what is it used for?

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History of WBGT (an article coming from http://www.bom.gov.au/info/thermal_stress/#wbgt)

The Wet Bulb Globe Temperature (WBGT)

The WBGT was developed in the late 1950s for the US Marine Corps Recruit Depot on Parris Island in South Carolina. Humidity in this region can be quite high and Marines have to undergo vigorous training exercise in military clothing, under full sun. There is a significant risk of heat injury if precautions are not taken.

The WBGT was later used by researchers as an easily measured general heat-stress index. In time its use widened. Because its use is recommended in the Standard, ISO 7243, it is often used in Occupational Health and Safety guidelines for working in hot environments. It has been advocated for use in sports requiring continuous exertion, such as the marathon. It is also used for horses in equestrian events.

Also read this article, coming from the Canadian Centre for Occupational Health and Safety

The Wet Bulb Globe Temperature (WBGT)

The wet bulb globe temperature is calculated using a formula that takes into account air temperature, speed of air movement, radiant heat from hot objects, sunshine and body cooling due to sweat evaporation.

Air temperature is measured using a conventional thermometer.

The contribution due to radiant heat is measured using a black globe thermometer. A conventional thermometer is inserted through a rubber stopper into a hollow, six-inch diameter copper ball which is coated with a flat black paint. The thermometer bulb is positioned at the centre of the copper ball. The black globe thermometer normally requires at least 20 minutes to come to equilibrium reading.

The cooling effect of evaporation and air movement is taken into account using a natural wet bulb thermometer. A natural wet bulb thermometer is a conventional thermometer with its bulb wrapped with an absorbent cotton wick. The wick extends 30 to 35 millimetres above the thermometer bulb, and the lower end of the wick is immersed in distilled water. About 25 mm of moistened wick is exposed between the water and the bulb of the thermometer. The moist wick continuously provides water for evaporation. As with the black globe thermometer, the natural wet bulb thermometer also requires at least 20 minutes to reach equilibrium.


There are several solutions to measure, most common is to use a complete setup (climate - tree) to determine the actual WBGT. You will find an example here of this measurement.

An example of a portable instrument with direct reading, is being shown here: WBGT