Wet Bulb Globe Temperature or Heat Index?

Yuri Hosokawa, PhD, ATC, Vice President of Education, Vice President of Communication


On February 27th, KSI’s Vice President of Education and Communication, Yuri Hosokawa, PhD, ATC was invited to give a presentation on wet bulb globe temperature (WBGT) based activity modification guidelines at the Athletic Trainers’ Society of New Jersey.

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In the CAATE Athletic Training Education Competencies [5th Edition] under the Prevention and Health Promotion section, it is stated that athletic trainers should be able to “explain the principles of environmental illness prevention programs to include acclimation and conditioning, fluid and electrolyte replacement requirements, proper practice and competition attire, hydration status, and environmental assessment (e.g., sling psychrometer, wet bulb globe temperatures [WBGT], heat index guidelines).” That being said, we, as Athletic Trainers and clinicians have all been exposed to the utilization of a sling psychrometer, WBGT, and heat index to monitor and assess environmental heat risk. But do you know the differences in how they work? Without the proper understanding of these indices, you may not be capturing the heat strain appropriately. For example, WBGT of 82°F and heat index of 82°F represent very different environmental conditions because of how these numbers are derived.

To calculate WBGT, you will need: wet bulb temperature (Tw), globe temperature (Tg), and dry bulb temperature (Td). Wet bulb temperature is a measurement of humidity, globe temperature is a measurement for amount of solar radiation, and dry bulb temperature is a measurement for air temperature. In addition, wet bulb temperature and globe temperature are influenced by wind speed. WBGT equation (see bellow) weighs heavily on the Tw (70%) because the air saturation dictates the capacity for the body heat dissipation through sweat evaporation. Since evaporative heat loss accounts for the majority heat dissipation during exercise, an environment that hinders this process will pose an extreme heat strain.


WBGT= 0.7Tw + 0.2Tg + 0.1Td


On the other hand, heat index is a number that shows “how hot it feels” when relative humidity is factored into the air temperature. It also assumes that the environment is under shade (i.e., not full sunshine) and that the person is walking at 3-mph, which is does not depict the heat stress of someone performing intense exercise in the heat. Therefore, it is apparent that activity modification guidelines that rely on heat index is not appropriate in an athletics context. Lastly, the measurement taken from the sling psychrometer is reflective of the Tw and Td. Typically, a sling psychrometer unit comes with a conversion scale, which allows the clinicians to use the Tw and Td values to calculate the heat index.

Athletic trainers, who are interested in checking or improving current activity modification guidelines, are encouraged to review Table 5 from the NATA Position Statement on Exertional Heat illness, which shows an example from the Georgia High School Athletics Association’s activity modification policy using WBGT.