Measurements · Meteorology
WBGT (Heat Stress)
Also known as wet bulb globe temperature, heat stress index
Wet-Bulb Globe Temperature (WBGT): the standard composite index for occupational heat-stress assessment.
What it is
What is WBGT (wet bulb globe temperature)?
Wet bulb globe temperature is the international standard index for heat stress. Ordinary air temperature is a poor guide to how dangerous heat is to a working or exercising body, because humidity, radiant heat from the sun or hot surfaces, and air movement all change how effectively a person can shed heat by sweating. WBGT combines these into one number by weighting three thermometers: a natural wet-bulb temperature (humidity and evaporation), a black-globe temperature (radiant heat), and the ordinary dry-bulb air temperature. Outdoors the standard weighting is 0.7 wet-bulb + 0.2 globe + 0.1 dry-bulb, so humidity dominates, which is why a humid 30 °C day is far more hazardous than a dry one.
Health and operational effects
Heat stress ranges from cramps and exhaustion to life-threatening heat stroke, and it kills outdoor workers, athletes, and military personnel every year. The body's ability to cool itself collapses as WBGT rises, especially during physical exertion, so the risk depends jointly on the environment and the workload. Managing it means adjusting work-rest cycles, hydration, and acclimatization as WBGT climbs. Those decisions require an actual measurement, since perceived temperature is an unreliable guide.
For employers, sports bodies, event organizers, and armed forces, WBGT is the basis of enforceable heat-safety programs and, increasingly, of legal duty-of-care obligations.
Limits
WBGT (Heat Stress) Limit Values
The published values, by averaging period; each links to the standard that sets it.
TLV, acclimatized, light work (75–100% work)
TLV, acclimatized, moderate work (75–100% work)
TLV, acclimatized, heavy work (50–75% work)
Action limit, unacclimatized, moderate work (75–100% work)
- ACGIH WBGT 27.5 °C: no TLV is set for heavy work at 75–100% work time, because that work is not recommended
- ACGIH WBGT 25 °C: ACGIH screening values; adjust for work/rest allocation and clothing
WBGT is reported in °C, and its limits are workload-dependent. The ACGIH heat-stress TLV and similar frameworks set lower WBGT thresholds for heavier work and for unacclimatized people, and prescribe work periods that shorten as WBGT rises. A WBGT value is never read in isolation. It is compared against the threshold for the current metabolic workload and acclimatization state. Rising WBGT triggers escalating controls: more frequent breaks, mandatory hydration, rescheduling to cooler hours, and, at the top, stopping work.
How it is measured
How WBGT (Heat Stress) Is Measured
A WBGT meter integrates the three sensors in one instrument: a wetted wick around a temperature probe for the natural wet-bulb, a 150 mm black globe housing a probe for radiant heat, and a shielded dry-bulb probe. It then computes the weighted index directly. Outdoor (in-sun) and indoor (no solar load) versions use different formulas, and the black globe must be the correct size and color for a valid reading. Continuous WBGT monitors log the index over a shift so that exposure can be tracked against the work-rate limits.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing WBGT monitoring
Measure at the work or activity location, at roughly the height of the torso, in the same sun and wind conditions the people experience: a WBGT reading taken in shade does not represent a crew working in full sun. For fixed sites, position the monitor where it captures the most exposed working area. Because radiant load matters, nearby hot surfaces, machinery, or reflective ground can raise real WBGT above what a simple weather forecast implies.
Every device and option that measures WBGT (Heat Stress) 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 TSI · QuestTemp-32, QuestTemp-34, QuestTemp-36… | Heat Stress & Comfort | Thiamis |
Where it is monitored
Solutions That Monitor WBGT (Heat Stress)
Each one links to the solution page showing what that industry measures it for.
Heat Stress & Outdoor Work
Core parameterWet bulb globe temperature is the standard index for managing heat stress in outdoor work, athletics, military training, and events, because it combines humidity, radiant heat, air movement, and air temperature into the single number that heat-safety programs are written around.
Cabin Air Quality
Often addedRespirable dust, gases, and blast monitoring
Occupational Safety & Industrial Hygiene
Often addedWorker exposure, confined space, and heat stress
Weather Stations & Meteorology
Often addedProfessional multi-parameter weather monitoring
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Questions
WBGT (Heat Stress) FAQ
Why not just use the air temperature or heat index?
Air temperature ignores humidity, radiant heat, and air movement, all of which strongly affect heat stress; the consumer "heat index" accounts for humidity but not solar radiant load or workload. WBGT combines wet-bulb, globe, and dry-bulb temperatures into the index that occupational and sports heat-safety standards are actually written around.
What WBGT level is dangerous?
There is no single number: the safe limit depends on how hard people are working and whether they are acclimatized. The ACGIH Heat-Stress TLV sets lower WBGT thresholds for heavier workloads. A WBGT that is safe for light seated work can be hazardous for heavy manual labor, which is why the index is always read against the workload.
What is the difference between indoor and outdoor WBGT?
Outdoor (in-sun) WBGT includes the black-globe radiant term with the standard 0.7/0.2/0.1 weighting; indoor or shaded WBGT omits the direct solar load and uses a different weighting. Using the wrong formula for the setting gives a misleading result, so WBGT meters distinguish the two.
How do I monitor heat stress on site?
Pair a Thiamis gateway with a WBGT heat-stress monitor to log the index continuously at the work location. Use the configurator to combine WBGT with air temperature and humidity, and with dust or noise if the same site has other exposure hazards.
Monitor WBGT (Heat Stress) on the Record
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