By Eugene le Roux, FSAIRAC, and Eamonn Ryan
How do activity levels and body mass affect the temperature a worker can safely tolerate?
The activity level also represents the amount of heat that the person is contributing to the surrounding environment.
Magnific.com
The amount of heat a person contributes to their environment depends significantly on their level of physical activity. This is expressed as metabolic rate, or M, measured in watts.
As a general indication, office work has a metabolic rate of approximately 120W, while heavy physical work can be rated at around 400W. This becomes particularly important when determining the WBGT Action Limit (AL) for workers who are not acclimatised to heat. The relationship is expressed as: AL = 59.9 – 14.1 × log M.
For example, where a worker has a metabolic rate of 300W, the allowable WBGT for a non-acclimatised worker is approximately 25°C.
For acclimatised workers, the WBGT Threshold Limit Value (TLV) is given by: TLV = 56.7 – 11.5 × log M.
At a metabolic rate of 300W, this gives an allowable WBGT of approximately 28°C for an acclimatised worker.
It is important to emphasise that these figures represent maximum allowable WBGT levels rather than comfort temperatures. A worker may experience discomfort well before reaching these limits.
The activity level also represents the amount of heat that the person is contributing to the surrounding environment. The harder someone works, therefore, the greater their internal heat production and the more demanding the environmental conditions become.
Body mass
Body mass also needs to be considered when applying metabolic rate to an individual. The standard body mass used for these calculations is 70kg. The allowable metabolic rate of a worker should be adjusted in proportion to their body mass before it is entered into the formula.
For example, consider a worker weighing 90kg. Their metabolic rate should be increased by a factor of 90/70 = 1.29.
The resulting metabolic rate is then used in the WBGT calculations. This means that the same type of physical activity does not necessarily represent the same heat load for workers of different body masses. Body mass and metabolic activity therefore need to be considered together when assessing acceptable workplace temperatures.
But metabolic rate and body mass are only part of the picture. What a worker is wearing can also have a significant effect on their ability to lose heat.
This brings us to the third factor: clothing insulation and its Clothing Adjustment Factor (CAF).
In part two, we look at how clothing affects acceptable workplace temperatures and how metabolic rate, body mass and clothing can ultimately be combined into a single perceived WBGT value.
