By Eugene le Roux, FSAIRAC, and Eamonn Ryan
In Part 1, we looked at why dry-bulb temperature alone does not provide a complete picture of workplace conditions. The Wet Bulb Globe Temperature (WBGT) approach considers the combined effects of dry-bulb temperature, wet-bulb temperature and radiant heat.
Workers can also become acclimatised to hot conditions, but acclimatisation should not be interpreted as immunity to heat stress.
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But there is another side to the equation: the worker. Two people exposed to exactly the same environmental conditions may experience very different levels of thermal strain. Factors such as physical activity, clothing, exposure time and acclimatisation all influence how effectively the body can gain or lose heat.
Workload makes a difference
One of the most important factors is metabolic activity. A person sitting at a desk generates relatively little internal heat, while someone performing heavy manual work can generate considerably more.
Consequently, a temperature that may be acceptable in an office cannot automatically be regarded as suitable for workers undertaking strenuous activity in a factory, warehouse or workshop.
For HVAC professionals, this is an important consideration. Maintaining a specified dry-bulb temperature does not necessarily mean that workers performing physically demanding tasks are comfortable or protected from excessive heat stress.
Clothing and humidity
Clothing also affects the body’s ability to lose heat. Protective clothing may be essential for safety, but heavy or restrictive garments can make it more difficult for heat to escape. This becomes particularly important when combined with high humidity.
As discussed in Part 1, the body relies heavily on evaporation of sweat for cooling. As humidity rises, evaporation becomes less effective. Air movement can assist this process, which is one reason why ventilation and air movement can be important elements in managing hot working environments.
The solution, therefore, is not always simply to lower the dry-bulb temperature. Depending on the circumstances, controlling humidity and increasing air movement may also improve conditions.
Exposure matters
The length of time a worker spends in a hot environment is another consideration. A short exposure may produce a very different response from several hours of continuous work. Work/rest cycles, hydration and opportunities to recover in a cooler environment can therefore form part of an overall heat-management strategy.
Workers can also become acclimatised to hot conditions, but acclimatisation should not be interpreted as immunity to heat stress.
The key message is that there is no universal workplace temperature that is automatically suitable for every worker and every activity. Environmental conditions need to be considered alongside the work being performed and the characteristics of the people exposed to them.
For HVAC designers and building operators, this means looking beyond the thermostat. Temperature, humidity, radiant heat and air movement all influence the environment, while metabolic activity, clothing and exposure time influence how workers respond.
The objective is therefore not simply to achieve a particular temperature reading, but to create working conditions that are safe, appropriate and sustainable for the people using the space. The dry-bulb thermometer remains an important measurement, but it is only one part of the thermal equation.
