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About WBGT (Wet Bulb Globe Temperature)

Guideline

Air temperature
(typical value)
WBGT Guides to how much exercise can be safely performed
≥ 35 ℃ ≥ 31 ℃ Danger
(exercise prohibited)
At a WBGT of 31 ℃ or above
the actual temperature is higher than the skin temperature, so body heat cannot escape, and except for special cases, all exercise should be stopped.
31 - 35 ℃ 28 - 31 ℃ Severe Warning
(heavy exercise prohibited)
At a WBGT of above 28 ℃
the danger of heatillness is high, so events that require heavy exercise or events where the body temperature will rise, like endurance races should be avoided. When such events are held, rest periods should be provided often and water replenishment conducted aggressively. People who are weak or not used to the heat should stop the exercise.
28 - 31 ℃ 25 - 28 ℃ Warning
(rests should be provided often)
At a WBGT of above 25 ℃
the danger of heatillness increases, so rest periods should be provided often and water replenishment conducted. Rest periods should be provided every 30 minutes for events requiring heavy exercise.
24 - 28 ℃ 21 - 25 ℃ Caution
(water should be replenished often)
At a WBGT of above 21 ℃
there is the danger of fatal accidents due to heatillness, so caution is advised, and water replenishment should be promoted during exercise.
< 24 ℃ < 21 ℃ Almost safe
(appropriate water replenishment suggested)
At a WBGT below 21 ℃
normally the danger of heatillness is small, but appropriate water replenishment is necessary. Heatillness can occur even under these conditions in events such as citizen marathons, so caution is advised.


Reference: Japan Sports Association, "A Guidebook for the Prevention of Heat Disorder During Sports Activities", 2013.