By Eugene le Roux, FSAIRAC, and Eamonn Ryan
Temperature in the workplace is about far more than setting a thermostat to a predetermined value. For employers, engineers and facilities managers, understanding the thermal environment is important not only for compliance with occupational health and safety requirements, but also for worker comfort, wellbeing and productivity.
The key message is that workplace temperature should be considered as part of a wider thermal environment rather than as an isolated number.
DC Studio | Magnific.com
Yet one of the most common ways of describing a workplace environment remains the simplest: the dry-bulb temperature. A thermometer might tell us that a space is at 25°C, for example, but that figure alone does not necessarily tell us whether the conditions are comfortable or whether workers could be exposed to excessive heat stress. Other environmental factors – particularly humidity and radiant heat – can significantly influence how the human body responds to its surroundings.
This is where Wet Bulb Globe Temperature (WBGT) becomes important.
What is WBGT?
WBGT is an environmental heat-stress index that combines several temperature measurements into a single value. It is intended to provide a more meaningful indication of the thermal conditions experienced by a person than dry-bulb temperature alone.
The basic relationship is: WBGT = 0.2 × dry-bulb temperature + 0.7 × wet-bulb temperature + 0.1 × globe temperature
The weighting immediately highlights an important point. The wet-bulb temperature accounts for 70% of the calculation, compared with 20% for dry-bulb temperature and 10% for globe temperature.
This is significant because wet-bulb temperature provides an indication of the combined effect of temperature and moisture on the body’s ability to lose heat through evaporation.
In simple terms, the hotter and more humid the environment becomes, the more difficult it is for sweat to evaporate efficiently. The body’s natural cooling mechanism is therefore progressively compromised. This is why two workplaces with the same dry-bulb temperature can potentially produce very different thermal experiences.
Why dry-bulb temperature can be misleading
Dry-bulb temperature is the familiar temperature measured by an ordinary thermometer. It is an important parameter for HVAC systems and remains fundamental when determining cooling and heating requirements.
However, it does not describe the complete thermal environment. Consider two workplaces, both measured at 28°C dry-bulb temperature. In one, the air is relatively dry and there is good air movement. In the other, humidity is high and workers are exposed to additional radiant heat from equipment or processes.
The thermometer may show the same temperature in both spaces, but the physiological response of the workers could be very different. This illustrates why a single indoor temperature target should not automatically be regarded as a universal measure of acceptable workplace conditions.
The relationship between dry-bulb and wet-bulb temperature is particularly important. Because the wet-bulb component has such a substantial weighting in the WBGT calculation, changes in humidity can have a significant effect on the resulting index.
It is also possible to have different combinations of dry-bulb and wet-bulb temperatures while arriving at a similar WBGT value. The thermal environment therefore cannot always be adequately described by one temperature measurement.
The importance of radiant heat
The third component of the calculation is globe temperature, which accounts for the effect of radiant heat.
This becomes particularly relevant in workplaces where employees may be exposed to hot surfaces, machinery, furnaces, direct sunlight or other sources of radiant energy.
A worker standing close to a hot piece of equipment may therefore experience a significantly greater heat load than someone working only a few metres away, even though the measured air temperature is similar.
For HVAC professionals, this reinforces the importance of looking beyond the nominal room temperature when assessing workplace conditions. An air-conditioning system may successfully maintain a specified dry-bulb temperature, but that does not necessarily mean that all sources of thermal stress have been eliminated.
Comfort is not the same as heat stress
There is another important distinction to make. Thermal comfort and occupational heat stress are related, but they are not the same thing.
Comfort is concerned largely with whether people perceive their environment as acceptable. Heat-stress assessment, on the other hand, considers whether environmental and personal factors could place excessive thermal strain on the body.
This distinction becomes particularly important when moving from offices and other sedentary environments into industrial workplaces.
An office worker sitting at a desk, for example, has a very different heat load from a worker undertaking physically demanding activity in a factory, warehouse or workshop. Applying the same temperature target to both situations may therefore be inappropriate. The HVAC industry consequently has an important role to play in understanding what a temperature reading actually represents – and what it does not.
A broader way of looking at workplace temperature
The key message is that workplace temperature should be considered as part of a wider thermal environment rather than as an isolated number.
Dry-bulb temperature remains an essential measurement, but humidity, radiant heat and other environmental factors can substantially alter the conditions experienced by workers.
The WBGT approach provides one way of bringing these environmental influences together into a single index. It also demonstrates why a more sophisticated assessment may be necessary where workers are exposed to elevated temperatures or heat-producing processes.
But even WBGT does not tell the whole story. The environment is only one side of the equation. The other is the person working within it.
In Part 2, we will look at the human side of workplace temperature and examine why factors such as metabolic activity, clothing and workload need to be considered alongside environmental measurements when assessing acceptable working conditions.
