Wet bulb globe temperature index

The wet bulb globe temperature (WBGT) index is the most widely used and accepted index for the assessment of heat stress in industry.  It has been published as British Standard BS EN 27243.

The WBGT index is an empirical index.  It represents the heat stress to which an individual is exposed.  The index was developed specifically for use in industrial settings.  The practicalities of an industrial application necessitated a compromise between the requirement for a precise index and the need to be able to easily take controlled measurements.  The WBGT should be used to estimate whether or not a problem exists, by identifying if the reference values are exceeded by the measured values.  When the reference values are exceeded, a more advanced index (BS EN 12515 - Required Sweat Rate) should be used to provide a more accurate estimation of heat stress.

The WBGT-index combines three measurements:

• Natural wet-bulb temperature (t_nw)
• Globe temperature (t_g)
• Air temperature (t_a)   

The following illustrates the relationship between the different measurements.

Inside buildings and outside buildings without experiencing the effects of radiation from the sun (solar load):

• WBGT = 0.7tnw + 0.3tg

Outside buildings with solar load, or where a radiant heat source is present indoors:

• WBGT = 0.7tnw +0.2tg +0.1ta

The measurements are entered into the above the equations to obtain a WBGT value.  The WBGT value is then compared to the reference values provided in the standard for the appropriate metabolic rate and state of acclimation of the worker.  A worked example has been provided to show how the WBGT reference values may be used.  They refer to conditions where 95% of the working population can be repeatedly exposed to heat stress with no adverse health effects.  It is important to note that these reference values assume an employee is physically fit, in good health, “normally clothed, with adequate salt and water intake and, if conditions stay within limits, are able to work effectively without exceeding a body core temperature of 38°C.

The following example shows how the WBGT may be used. 

• As part of a risk assessment in a factory, the thermal environment parameters are measured.
• Air temperature (ta) = 26°C,
• Globe temperature (tg) is 26.5°C and;
• Natural wet-bulb temperature (tnw) = 18°C (which gives a relative humidity of 55%, and a partial vapour pressure of 1.662 kPa.).
• There is little air movement. 
• The workers, who are acclimatised, are estimated to be working at 250 W/m²
• Using the WBGT equation, a WBGT value of 20.5°C is obtained.  This is then compared to the highlighted WBGT reference value
• Using the reference value of 25°C for “Metabolic Rate Class 3” when the “person acclimatised to heat”, and “no sensible air movement”.
• The measured value of 20.5°C, is below the WBGT reference value of 25°C (for the metabolic rate, air movement and acclimation of workers) for that environment.  Therefore, since the WBGT reference value of 25°C is not exceeded, it could be concluded that heat stress was not a risk in that environment.

However, during a hot summer the employees complain of feeling too hot.  Another assessment is carried out and measures are taken again:

• Air temperature (ta) = 32°C,
• Globe temperature (tg) is 33°C and;
• Natural wet-bulb temperature (tnw) = 28°C (which gives a relative humidity of 55%, and a partial vapour pressure of 1.662 kPa.). 

The result is that the WBGT value is 29.5°C thßis exceeds the WBGT reference value of 25°C and indicates that under these circumstances there may be a heat stress risk.

Reference values of WBGT heat stress index from ISO 7243 related to a maximum rectal temperature of 38°C

	Metabolic rate, M		WBGT Reference value
Metabolic Rate class	Related to a unit skin surface area W/m-2	Total (for a mean skin surface area of 1.8m2) W	Person acclimatised to heat °C			Person not acclimatised to heat °C
0 (resting)	M≤65	M≤117	33			32
1	65<M≤130	117<M≤234	30			29
2	130<M≤200	234<M≤360	28			26
3	200<M≤260	360<M≤468	No sensible air movement 25		Sensible Air movement 26	No sensible air movement 22	Sensible air movement 23
4	M>260	M>468	23		25	18	20

Limitations of WBGT

A limitation of this index is that the reference values are representative of the mean effect of heat, over a long period of work.  It does not provide a reference for those instances where workers are exposed to heat for very short periods of time (eg a few minutes).  These exposures could be as a result either to exposure to a very hot environment or a short period of intense physical activity.  In these cases the reference values may not be exceeded even though the heat stress may exceed the permissible value.  The highest metabolic rate value is used as the reference value, when there is uncertainty about the metabolic rate that is to be adopted.

The estimation of metabolic rate causes a high variability in reference values.  There may be difficulty in interpreting the results when small deviations in the reference values are observed;

Under conditions of light work in humid environments, the WBGT correlates well with the skin temperature, but poorly with other physiological variables of heart rate, rectal temperature, and sweat loss;

The standard BS EN ISO 27423 does not provide corrections to WBGT values for different types of PPE.  The American Conference of Governmental Hygienists (ACGIH) Threshold Limit Values (TLVs) (which are based on the WBGT index) do provide corrections for some PPE.

Some points to consider:

• The reference values are modified when clothing being worn by an employee is non-standard (permeable to air and steam, I_cl = 0.6 Clo) taking into account the special properties of the garment and the work environment.  When clothing differs significantly from the specified reference clothing, it is recommended that a specialist is consulted.
• Reference values for WBGT established for various work/rest schedules are provided.  They are based on the hypothesis that the WBGT values for both the place of resting and the work place are the same or very similar.
• A gradual increase in heat stress over a 7 day period, can result in acclimation of the worker.  Those workers who have not been exposed gradually to daily heat during the preceding working week are not considered acclimatised.

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