🌿 Hot thermal environment — the WBGT index
In short
Screening assessment of the heat stress on a worker in a hot environment: measurement of the natural wet bulb temperature, of the temperature of the blackened globe and of the air temperature, calculation of the WBGT index and comparison with the reference values depending on the metabolic rate and on acclimatisation. The test is performed according to PN-EN ISO 7243:2018-01; Range and accuracy of the sensors: tnw: 5–40 °C (±0.5 °C); tg: 20–50 °C (±0.5 °C) and 50–120 °C (±1 °C) for a globe of Ø 150 mm; ta: 10–60 °C (±0.5 °C). The procedure comprises 7 steps; it is used for: Measurements of the hot thermal environment at workplaces required by the health and safety regulations — comparison with the NDN laid down in the regulation on the maximum admissible concentrations and intensities, Foundries, steelworks, rolling mills, boiler houses, bakeries, drying plants, halls with thermal processes, Work outdoors in summer (building, road building, agriculture) — the variant of the formula with solar radiation.
At a glance
- Standard: PN-EN ISO 7243:2018-01
- Category: Environment
- Procedure steps: 7
- Formula without solar radiation: WBGT = 0.7·tnw + 0.3·tg
- Formula with solar radiation: WBGT = 0.7·tnw + 0.2·tg + 0.1·ta
- Range and accuracy of the sensors: tnw: 5–40 °C (±0.5 °C); tg: 20–50 °C (±0.5 °C) and 50–120 °C (±1 °C) for a globe of Ø 150 mm; ta: 10–60 °C (±0.5 °C)
Overview
The WBGT index (wet bulb globe temperature) is an index of heat stress, and its name comes from the sensors used for the measurement: the natural wet bulb thermometer and the blackened globe. The standard gives a screening method for assessing the heat stress to which a person is exposed and for establishing whether heat stress occurs. It applies to the total exposure during a working day (up to 8 h), indoors and outdoors, for adults fit for work; it does not concern very short exposures.
The degree of heat stress depends on three factors: the parameters of the environment deciding the exchange of heat between the surroundings and the body, the heat produced in the organism as a result of physical activity, and the clothing, which changes the exchange of heat with the surroundings. A full analysis requires a knowledge of the air temperature, the mean radiant temperature, the air velocity and the absolute humidity (ISO 7726); WBGT gives a first approximation on the basis of derived parameters, measured directly.
In Poland the WBGT index was previously described in PN-EN 27243:2005; it was replaced by PN-EN ISO 7243:2018-01, which introduces EN ISO 7243:2017 and ISO 7243:2017 [IDT] — according to the PKN cards the English version was published on 15 January 2018 and the Polish version on 30 July 2019 (read on 23.09.2026). The 2017 edition introduced the taking into account of the clothing (the correction value CAV and the index WBGTeff), a description of the errors of globe sensors of non-standard diameter and a method of predicting the natural wet bulb temperature. The reference values of WBGT for the individual classes of metabolic rate are laid down in the regulation on the maximum admissible concentrations (NDS) and the maximum admissible intensities (NDN).
Method principle
The natural wet bulb temperature (tnw), the temperature of the blackened globe (tg) and — where there is solar radiation — the air temperature (ta) are measured. Without a solar load WBGT = 0.7·tnw + 0.3·tg; with a solar load WBGT = 0.7·tnw + 0.2·tg + 0.1·ta, because the second formula corrects the overestimation of the direct radiation from the sun. The globe temperature takes in the total load of thermal radiation from the sun and from other sources. The reference values were worked out for cotton working clothing (0.6 clo, im = 0.38); for other clothing the correction value CAV is added to the measured WBGT, giving WBGTeff = WBGT + CAV. The metabolic rate is estimated in accordance with annex E of the standard or ISO 8996 and classified in one of five classes; the result is compared with the reference value proper to the class and to the state of acclimatisation.
Applications
- Measurements of the hot thermal environment at workplaces required by the health and safety regulations — comparison with the NDN laid down in the regulation on the maximum admissible concentrations and intensities
- Foundries, steelworks, rolling mills, boiler houses, bakeries, drying plants, halls with thermal processes
- Work outdoors in summer (building, road building, agriculture) — the variant of the formula with solar radiation
- Screening before a detailed analysis of the heat stress in accordance with ISO 7933, where the WBGT exceeds the reference value
- Planning of the work-rest regime and of the selection of clothing in a hot environment
Key parameters
| Parameter | Value |
|---|---|
| Formula without solar radiation | WBGT = 0.7·tnw + 0.3·tg |
| Formula with solar radiation | WBGT = 0.7·tnw + 0.2·tg + 0.1·ta |
| Range and accuracy of the sensors | tnw: 5–40 °C (±0.5 °C); tg: 20–50 °C (±0.5 °C) and 50–120 °C (±1 °C) for a globe of Ø 150 mm; ta: 10–60 °C (±0.5 °C) |
| Representative period | about 1 h; the values averaged with weighting by time |
| Reference clothing | cotton working clothing 0.6 clo, im = 0.38 (CAV = 0) |
| Reference values of WBGT (acclimatised / non-acclimatised) | class 0 (rest): 33 / 32 °C; class 1: 30 / 29 °C; class 2: 28 / 26 °C; class 3: 26 / 23 °C; class 4: 25 / 20 °C |
Standard
- Standard number
- PN-EN ISO 7243:2018-01
- Title (PL)
- Ergonomia środowiska termicznego — Ocena obciążenia cieplnego za pomocą wskaźnika WBGT (temperatura wilgotnego termometru i poczernionej kuli)
- Title (EN)
- Ergonomics of the thermal environment — Assessment of heat stress using the WBGT (wet bulb globe temperature) index
Step-by-step procedure
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Choice of the time and of the place
Carry out the measurements at the season of the year and at the time of day when the heat stress is the most probable (summer, the middle of the day). Set the sensors at the height of the abdomen of the person exposed; in a non-uniform environment — where the heat stress is the greatest. If it is not possible to set the sensors at the normal place of work, set them where they will be under the same influence of the environment.
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Division of the work
If the working day is divided into distinctly different kinds of work (e.g. light in the morning, heavy in the afternoon), or if the WBGT differs significantly over periods longer than an hour, make separate measurements and assessments for each kind.
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Measurement in the steady state
Before you record a reading, wait for the readings of all the sensors to settle — especially the globe temperature, the response time of which may be considerable. Record the parameters at a high frequency over a representative period of about 1 h; the total time of the measurement may exceed an hour.
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Calculation of the WBGT
Without solar radiation: WBGT = 0.7·tnw + 0.3·tg. With solar radiation: WBGT = 0.7·tnw + 0.2·tg + 0.1·ta. Where there is variation in time, calculate the mean weighted by the duration of the individual levels, with a time base of 1 h.
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Metabolic rate
Estimate the heat produced by the organism in accordance with annex E of the standard or ISO 8996 and assign the work to a class: 0 — rest (about 115 W), 1 — light work (about 180 W), 2 — moderate (about 300 W), 3 — heavy (about 415 W), 4 — very heavy (about 520 W). With intermittent work use the mean weighted by time; in case of doubt assume the higher class.
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Correction for the clothing
For clothing other than cotton working clothing add the correction value CAV from annex F of the standard: WBGTeff = WBGT + CAV. If the CAV for the given ensemble cannot be established, do not apply this method — carry out a detailed analysis in accordance with ISO 7933.
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Assessment
Compare the WBGT (or the WBGTeff) with the reference value for the class of metabolic rate and for the state of acclimatisation. A person is considered acclimatised who has been exposed to similar or more extreme hot conditions for at least a working week immediately before the assessment. An exceeding means an increase of the risk of heat disorders: limit the load or carry out a detailed analysis in accordance with ISO 7933.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Thermal environment meter (WBGT meter) with a set of sensors | natural wet bulb thermometer, blackened globe of a diameter of 150 mm, air temperature sensor — in accordance with annex B of the standard | — |
| Stand allowing the sensors to be set at the height of the abdomen | 1.1 m for the standing position, 0.6 m for the sitting one (in accordance with PN-EN ISO 7726) | — |
| Data logger | recording of the readings at a high frequency, e.g. every second or every minute | — |
| Tables of metabolic rate and of the CAV values | annexes E and F of the standard, ISO 8996, ISO 9920 | — |
Health and safety (OHS)
- The person making the measurements in a hot environment is himself subject to heat stress — plan breaks and the replenishment of fluids
- In the vicinity of furnaces, ladles and heating equipment keep to the safety zones; set the sensors without entering the dangerous zones
- The heated parts of the stand and the shield of the globe are liable to cause burns — after the measurement wait before dismantling
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7243:2018-01 — “Ergonomics of the thermal environment — Assessment of heat stress using the WBGT (wet bulb globe temperature) index”.
How does this method work?
The natural wet bulb temperature (tnw), the temperature of the blackened globe (tg) and — where there is solar radiation — the air temperature (ta) are measured. Without a solar load WBGT = 0.
What is the measuring range and accuracy?
Formula without solar radiation: WBGT = 0.7·tnw + 0.3·tg; Formula with solar radiation: WBGT = 0.7·tnw + 0.2·tg + 0.1·ta; Range and accuracy of the sensors: tnw: 5–40 °C (±0.5 °C); tg: 20–50 °C (±0.5 °C) and 50–120 °C (±1 °C) for a globe of Ø 150 mm; ta: 10–60 °C (±0.5 °C); Representative period: about 1 h; the values averaged with weighting by time.
How long does the test take?
The procedure comprises 7 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Thermal environment meter (WBGT meter) with a set of sensors, Stand allowing the sensors to be set at the height of the abdomen, Data logger, Tables of metabolic rate and of the CAV values.
Where is this test used?
Measurements of the hot thermal environment at workplaces required by the health and safety regulations — comparison with the NDN laid down in the regulation on the maximum admissible concentrations and intensities; Foundries, steelworks, rolling mills, boiler houses, bakeries, drying plants, halls with thermal processes; Work outdoors in summer (building, road building, agriculture) — the variant of the formula with solar radiation; Screening before a detailed analysis of the heat stress in accordance with ISO 7933, where the WBGT exceeds the reference value; Planning of the work-rest regime and of the selection of clothing in a hot environment.
What safety precautions apply?
The person making the measurements in a hot environment is himself subject to heat stress — plan breaks and the replenishment of fluids; In the vicinity of furnaces, ladles and heating equipment keep to the safety zones; set the sensors without entering the dangerous zones; The heated parts of the stand and the shield of the globe are liable to cause burns — after the measurement wait before dismantling.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 7243.