By Eugene le Roux, FSAIRAC, and Eamonn Ryan
Why the same workplace temperature can feel very different depending on what a worker is wearing.
The activity level also represents the amount of heat that the person is contributing to the surrounding environment. Drazen Zigic | Magnific.com
In part one, we considered how metabolic rate and body mass affect the amount of heat a worker generates and, consequently, the maximum allowable Wet Bulb Globe Temperature (WBGT). There is one further factor that needs to be incorporated: clothing insulation.
Clothing affects the body’s ability to transfer heat to the surrounding environment. The greater the insulation, the more difficult it becomes for a person to lose heat.
Clothing adjustment factor
The degree of clothing insulation can be represented by a Clothing Adjustment Factor (CAF). A single layer of clothing covering the whole body from the neck downward is assigned a CAF of zero. Clothing approximately twice this thickness has a CAF of 3, while clothing incorporating a vapour barrier, such as plastic, has a CAF of 11.
The effect can be significant, particularly in workplaces where protective clothing or specialised garments are required. A worker performing the same task at the same environmental temperature can therefore experience a very different heat load depending on the clothing being worn.
What about office workers?
For comparison, a typical office environment might have a dry-bulb temperature of approximately 22°C and a relative humidity (RH) of 55%. Using the psychrometric chart, this corresponds to a WBGT value of approximately 15.25°C.
For office workers wearing clothing with a CAF of zero, this would generally represent a typical comfortable environment. This also illustrates an important distinction made in the previous article: acceptable temperature, heat-stress limits and thermal comfort are not necessarily the same thing.
The WBGT limits calculated from metabolic rate are intended to establish maximum allowable environmental conditions rather than to define what people will necessarily find comfortable.
Bringing the factors together
The question is now: How can activity and clothing be integrated with the environmental WBGT to produce a single encompassing figure?
The relationship can be expressed as: WBGT (perceived) = WBGT (environment) + CAF.
In practical terms, the harder a person works, the greater their metabolic heat production. If they also have a greater body mass, the effective metabolic rate increases. And if they are wearing more heavily insulated clothing, their ability to dissipate that heat is further reduced.
The result is that the acceptable environmental temperature becomes lower as the worker’s heat production and clothing insulation increase. This is why a single workplace temperature cannot necessarily be regarded as suitable for everyone. An office worker sitting quietly in light clothing and a worker performing heavy physical work in protective clothing may occupy the same building but have very different heat-stress limits.
Assessing workplace temperature therefore requires more than simply looking at the dry-bulb temperature. Activity, body mass, humidity, environmental conditions and clothing all contribute to the thermal load experienced by the worker.
Understanding these factors allows WBGT to be used not merely as a temperature reading, but as a more meaningful indicator of the conditions under which people actually work.
