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Wet Bulb Globe Temperature

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Other Names

  • Wet Bulb Globe Temperature (WBGT)


  • Wet Bulb Globe Temperature: measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover
    • Used by employers, athletes, sporting events and military to determine appropriate exposure levels to high temperatures
    • Can reliably predict environmental heat stress

Table of alert levels and recommendations

ACSM 5 Colored Flags

  • Black flag level: WBGT greater than 82°F (28°C),
    • races should be cancelled or recommends voluntary withdrawal
    • Races that are always held in these conditions should acknowledge the extreme heat risk to the poorly acclimated and non-resident competitors
    • Announcement should be in the pre-race literature and the pre-race announcements
  • Red flag level: WBGT is between 73°F to 82°F (23°C to 28°C).
    • Participants who are at increased risk for heat collapse are recommended to withdraw from the race
    • It is recommended that all other participants slow their pace to match the conditions
  • Yellow flag level: WBGT between 65°F to 72°F (18°C to 22°C).
    • ACSM recommends that participants at increased risk for heat collapse slow their pace
    • Entrants should be warned of increased risk of heat collapse
  • Green flag level: WBGT less than 65°F (18°C).
    • Although collapse can still occur, there is a decreased risk of hyperthermic and hypothermic collapse
    • This is an ideal level for a marathon
  • White flag level: WBGT less than 50°F (10°C).
    • There is increased risk of hypothermic collapse
    • participants should be advised to wear appropriate clothing

See Also


Created by:
John Kiel on 30 June 2019 22:52:18
Last edited:
19 August 2022 06:37:27