Throughout the summer, we’ve talked about temperatures and the heat index, but there’s another measurement that’s been used to talk more specifically about heat stress in direct sunlight.
The Wet-Bulb Globe Temperature (WBGT) was originally introduced for the safety of workers outside. OSHA and the International Organization for Standardization recognize this measurement as a standard for occupational environments.
The University Interscholastic League, or UIL, is also recommending coaches use the WBGT to protect their athletes from heat-related illnesses when exercising outdoors.
Here’s why this measurement is used to keep you safe in direct heat:
What does the WBGT measurement take into account?
According to the National Weather Service, the WBGT measures the heat stress on the body based on four parameters:
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Temperature
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Relative humidity
The measurement’s output is a number in degrees (much like the heat index) and is categorized into four danger thresholds by the U.I.L: low, moderate, high, and extreme. Here’s a chart of the risk level, WBGT, and U.I.L Guidelines.
| RISK LEVEL | WBGT | U.I.L GUIDELINES |
|---|---|---|
| EXTREME | Above 92° | No outdoor practice. Delay practices until a cooler WBGT is reached. |
| HIGH | 90° to 92° | Maximum practice time: 1 hour; FOR FOOTBALL: No protective equipment may be worn during practice, and there may be no conditioning activities. FOR ALL SPORTS: There must be 20 min of rest breaks distributed throughout the hour of practice. |
| MODERATE | 87° to 89° | Maximum practice time is 2 hours; FOR FOOTBALL: players are restricted to helmets, shoulder pads, and shorts during practice. If the WBGT rises to this level during practice, players may continue to work out wearing football pants without changing to shorts. FOR ALL SPORTS: Provide at least four separate rest breaks each hour with a minimum duration of 4 min. each. |
| LOW | 82° to 86° | Use discretion for intense or prolonged exercise; Provide at least three separate rest breaks each hour with a minimum duration of 4 min. each. |
How is it different from the heat index?
The heat index is a calculation of what it feels like to our bodies based on the humidity and the temperature in the shade (i.e., not in direct sunlight).
On the other hand, the WBGT takes into account those two parameters while also including solar radiation and wind speed. The output is then based on how all four parameters affect individuals outside.
We know that hotter temperatures can be more dangerous for those working or exercising outside, but here’s why the other three parameters are also important to consider:
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Relative humidity is important to the measurement because the greater the humidity, the harder it is for the body to cool down.
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Wind also helps the body cool down; if there’s no wind, you immediately feel hotter.
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Solar radiation takes into account how much direct sunlight is present. If clouds are out when measuring the WBGT, the output will be lower because the clouds reflect a portion of the sun’s radiation back into space.