How Hot is Hot?


How Hot is Hot?


It was interesting to hear the Australian Open tennis tournament last month used the “wet bulb globe temperature” and not the normal dry bulb temperature to decide if it was too hot to continue play.

So what is the “wet bulb globe temperature” and why is it better to use it to measure the risk of heat stress?

Traditionally, businesses have used the (dry bulb) thermometer to measure ambient temperatures and as an indication of heat likely to cause adverse health effects, particularly for those involved in heavy manual work or exposed to direct sun light.

Essentially, you and I only suffer from excessive heat when our core body temperature rises. This condition is generally referred to as “heat strain”. Personal health and fitness and level of exertion also have an effect on how quickly our body temperature rises.

Most people can remember a time when they felt “drained” and fatigued even in moderate temperatures when there was high humidity, as often experienced in the tropics. Clearly, humidity has an effect on how we react to ambient temperature.

Wet Bulb Globe Temperature (WBGT) combines three key factors that impact on our ability to cope with the heat. These three factors include:

  • Temperature (dry & wet bulb);
  • Relative Humidity; and
  • Barometric (Atmospheric) Pressure.

The formula to calculate the WBGT index is complex and some websites have ready calculators that only require you to insert your local readings e.g. °C, humidity & barometric pressure.

Guidance on WBGT Heat Stress Index

Guidance on the WBGT values that are considered acceptable* is shown in the table below:

* Per reference values of WBGT heat stress index from ISO 7243 related to a maximum rectal temperature of 38°C.

Warning on Over-Hydration

Whilst drinking sufficient water is important to cope with hot weather, health authorities also warn that harm can occur if too much water is consumed.

The general guidance on fluid intake is that people consume about 1 litre per hour, and not exceed 1.5 L/hr, or 12 litres over a day.

Instruments to Measure WBGT

There are also instruments available that can automatically combine these factors to immediately give the WBGT index.


When determining the parameters for safe working in hot conditions, use the WBGT where possible. As dry bulb thermometers only measure one factor and people could suffer ill-health affects at relatively low (dry bulb) temperatures – particularly if the humidity and barometric pressure are high.

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