Published 27 June 2026 • By Dr. Megan Tranter
Walk onto any hot worksite, and you will hear someone quote the air temperature: “It’s 35 degrees out there.” That number, on its own, tells you almost nothing about whether a worker is in danger. Thermal strain is driven by four environmental factors (air temperature, humidity, radiant heat, and air movement) as well as the worker’s metabolic rate and clothing. The wet-bulb globe temperature (WBGT) index was developed to capture the environmental side of that equation in a single value, and it remains the backbone of every credible occupational heat standard worldwide. This post explains how WBGT actually works, how it differs from the heat index you see on a weather app, and how to translate a WBGT reading into a defensible work-rest decision.
What You’ll Learn
- The exact formulas for outdoor and indoor WBGT, and what each sensor measures
- Why WBGT is not the same as the heat index or “feels-like” temperature
- How the ACGIH heat stress TLV uses WBGT, metabolic rate, and clothing adjustment factors
- What ISO 7243 and the 2016 NIOSH criteria document require for instruments and exposure limits
- How to estimate metabolic rate and apply clothing adjustment values correctly
Introduction
Heat stress assessment is an exposure assessment problem, and like any exposure assessment, it demands the right metric, the right instrument, and the right interpretation framework. The WBGT index, first developed by the United States Marine Corps in 1956 to curb heat casualties among recruits, integrates dry-bulb temperature, natural wet-bulb temperature, and globe temperature into a single number that correlates well with physiological heat strain. Standardized internationally as ISO 7243 and adopted by the ACGIH and NIOSH, WBGT is the index used to govern occupational decisions. Understanding what it does and does not measure is the first step toward protecting workers in a warming climate.
The WBGT Formula: Three Sensors, Two Equations
WBGT is computed from three separate temperature measurements. The natural wet-bulb temperature (Tnwb) is measured with a thermometer whose wetted wick is exposed to natural (not forced) air movement; it captures the combined effect of humidity and air velocity on evaporative cooling. The globe temperature (Tg) is measured with a thermometer inside a 150 mm matte-black copper sphere, which captures radiant heat from the sun or hot surfaces. The dry-bulb (air) temperature (Tdb) is measured with the sensor shielded from radiation.
For outdoor conditions with solar load, the equation is WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb. For indoor conditions or for outdoor conditions without solar load, WBGT = 0.7 Tnwb + 0.3 Tg. The dominant weighting on the natural wet-bulb term (0.7) reflects the physiological reality that evaporative sweat loss is the body’s primary defense against heat, so humidity and air movement matter more than raw air temperature. Note that the natural wet-bulb temperature is not the same as the psychrometric (aspirated) wet-bulb temperature; substituting one for the other is a common and consequential error.
WBGT Is Not the Heat Index
The heat index (the apparent temperature reported by the United States National Weather Service) and WBGT are frequently conflated, but they serve different purposes. The heat index combines only air temperature and relative humidity, assumes shade and a light wind, and is calibrated to how heat “feels” to a typical adult. It ignores radiant load entirely, which means a worker in direct sun on a reflective surface can experience far more strain than the heat index suggests. WBGT, by contrast, explicitly measures solar and radiant heat using the globe thermometer and does not assume shade. A 2024 GeoHealth analysis comparing heat metrics estimated from gridded weather data against ground-based observations across the United States found that the heat index and WBGT are distinct quantities whose estimates carried root-mean-square errors as large as roughly 9 to 10 degrees Celsius for some metrics and climates, underscoring that the two indices are not interchangeable. OSHA’s proposed heat rule allows employers to use either a heat index trigger or a WBGT-based program, but for radiant-heavy work (roofing, foundry, glass), WBGT is the technically superior choice.
The ACGIH Heat Stress TLV
The ACGIH Threshold Limit Value for heat stress is expressed as a WBGT screening criterion that varies with metabolic workload and work-rest allocation. The TLV table gives action limits and TLVs for four work-intensity categories (light, moderate, heavy, and very heavy work) crossed with four work-rest cycles (continuous work down to 25 percent work / 75 percent rest each hour). For an acclimatized worker performing moderate continuous work, the TLV is 28 degrees Celsius WBGT. Because of the physiological strain of heavy work, the screening table provides no continuous-work value for heavy work, instead giving 27.5 degrees Celsius at 50 to 75 percent work and 29.0 degrees Celsius at 25 to 50 percent work. Unacclimatized workers are protected by the lower “Action Limit” table. Crucially, the screening criteria assume a worker wearing a standard single-layer summer-weight garment, so any heavier or vapor-impermeable ensemble requires adding a clothing adjustment value (CAV) to the measured WBGT before comparison. A double-layer woven ensemble adds about 3 degrees Celsius; a vapor-barrier coverall adds roughly 11 degrees Celsius. Accurately estimating metabolic rate using the ACGIH category descriptions or tabulated task values is just as important as the environmental measurement itself, and pairing WBGT assessment with a structured exposure assessment strategy helps keep the program defensible.
ISO 7243 and the NIOSH Criteria Document
ISO 7243:2017 is the international standard that specifies how WBGT must be measured: sensor specifications, the 150 mm globe, measurement heights (ankle, abdomen, head for non-uniform environments with a weighted average), and reference limit values for acclimatized and unacclimatized people. The 2016 NIOSH “Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments” provides the United States Recommended Exposure Limits (RELs) and Recommended Alert Limits (RALs), also expressed as WBGT curves as a function of metabolic rate. The NIOSH ceiling limit and time-weighted-average curves are slightly more conservative than the ACGIH TLV for some categories, and NIOSH explicitly recommends physiological monitoring (core temperature, heart rate recovery) as a backstop when environmental limits are exceeded or when clothing complicates the assessment. Both frameworks converge on the same principle: WBGT screens the environment, but the final safeguard is the worker’s own physiology, which is why a robust heat illness prevention program never relies on a single number.
From Measurement to Decision
A practical assessment proceeds in steps. First, position a calibrated WBGT meter (or assemble the three sensors) at the work location, at the height of maximum exposure, and allow the globe thermometer the full equilibration time (typically 20 to 30 minutes). Second, classify the metabolic workload using task observation and the ACGIH or ISO metabolic tables. Third, add the appropriate clothing adjustment value to the measured WBGT. Fourth, compare the adjusted WBGT against the TLV or REL for that workload and acclimatization status, and set the work-rest cycle accordingly. Where measured values exceed the limit, controls follow the hierarchy: engineering controls (shade, ventilation, radiant shielding) first, then administrative controls (work-rest scheduling, hydration, acclimatization), with personal cooling and physiological monitoring as the last layer.
Summary
WBGT remains the single most defensible environmental index for occupational heat stress because it integrates humidity, radiant load, and air movement, the factors that actually govern heat strain. It is not the heat index; it must be paired with a metabolic rate estimate and a clothing adjustment, and it is interpreted using the ACGIH TLV, ISO 7243, or the NIOSH RELs. Measure it correctly, adjust it honestly, and treat the resulting number as a screening trigger for controls rather than a guarantee of safety.
Helpful Resources
- NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (2016)
- ISO 7243:2017 Ergonomics of the thermal environment: Assessment of heat stress using the WBGT index
- OSHA Heat Exposure: Hazards and Solutions
- Related reading on this site: Exposure to Heat, Building a Heat Illness Prevention Program, and Air Sampling Strategies.
Bibliography
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International Organization for Standardization. (2017). ISO 7243:2017 Ergonomics of the thermal environment: Assessment of heat stress using the WBGT (wet bulb globe temperature) index. ISO.
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