HeatGuard field guide
WBGT vs. heat index: which number should you use?
Heat index and WBGT both describe heat stress, but they answer different questions. Heat index communicates how hot warm, humid air feels in the shade. WBGT is designed to support decisions about exertion and exposure.
01
What goes into each measure
Heat index uses air temperature and relative humidity. The National Weather Service calculation assumes shaded, light-wind conditions, so direct sunlight can add exposure beyond the published number.
WBGT incorporates temperature, moisture, radiant heat and air movement. That broader view makes it useful for strenuous exercise, sports and work when a protocol links readings to activity changes.
02
A simple comparison
On a humid afternoon in full sun, heat index can show the moisture-driven burden while an on-site WBGT meter also captures radiant load on an open field. Move into deep shade and air temperature may barely change while the globe-temperature component falls.
A lower raw WBGT number does not mean lower risk. WBGT and heat index have different units, formulas and thresholds; never subtract one from the other.
03
Choose the right tool
For a neighborhood forecast, start with heat index and hourly UV. For practice or a jobsite, follow the governing WBGT protocol. A modeled WBGT trend can help choose a time in advance, but it cannot replace a required on-site reading.
- Check whether the value is measured or estimated.
- Confirm the units before applying a threshold.
- Combine the number with workload, clothing and acclimatization.
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This guide provides environmental planning information. It is not medical advice, an emergency service, or a substitute for an applicable workplace, school, league or governing-body protocol.
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