People accumulate heat primarily as a consequence of exposure to heat or heat generation by metabolism, without effective methods to dissipate heat from the body.
Since the primary mechanism of heat dissipation is by the evaporation of perspiration, it is clear that measures of heat load that do not take into account factors that may inhibit evaporative heat loss may underestimate the level of thermal discomfort experienced by workers.
In addition, air temperature in itself not the best guide to thermal comfort, as it does not take into account radiant energy sources nor the influences of humidity and air movement.
Calculating the thermal environment
Different measurements may be made of the thermal environment that take some or all of the contributing factors into account.
If humidity is in the range 30 - 60% ad in the absence of hot radiant sources (such as hot uninsulated roofs) a simple dry bulb temperature can be used as a first estimate of thermal comfort.
With light work and sedentary activity indoor air temperatures should be 18C - 30C (optimum 21C - 26C).
For sedentary office occupations discomfort becomes marked at 30C, when free moisture from perspiration causes clothing to stick to skin and furniture.
Although some air movement may ameliorate this somewhat, and the upper limits of acceptability may be increased slightly, the influence of these air movements as uncomfortable draughts should be avoided.
In humid conditions cooling by perspiration is inhibited. Wet bulb thermometers (usually in the form of sling psychrometers combining wet and dry bulb thermometers) enable a measure of humidity.
Similarly, radiant sources, such as hot uninsulated roofs, may directly increase heat load. Radiant temperatures are measured using a black globe thermometer.
These measurements can be combined to yield a single estimate, the WET BULB GLOBE TEMPERATURE INDEX. The WBGT is obtained as;
outdoors in sunshine - WBGT = 0.7 twb 0.2 tg 0.1 tdb
indoors with no sunshine - WBGT = 0.7 twb 0.3 tg
where twb is wet bulb temperature, tg is globe temperature, and tdb is dry bulb temperature.
As a guide, different work-rest regimes (per hour) are recommended for different estimated WBGT indices. These are the time weighted averages that can be tolerated for 8 hours each day for 40 hours each week by healthy acclimatised individuals.
Hotter conditions are tolerable for short periods. A further advantage of the WBGT index is its applicability to outdoor workers also.
Recommended maximum WBGT indices
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75% wrk/25% rst
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50% wrk/50% rst
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25% wrk/75% rst
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- For office based work, if dry bulb temperature exceeds 30C for a significant part of the day (and relative humidity is in the range 30 - 60%) onditions are generally considered uncomfortable and further investigation should be initiated.
- This further investigation will be coordinated and managed by the OH&S Unit with costs for external expertise met by the relevant Cost Centre.
- The objective definition of "uncomfortable" for work purposes should be the WBGT index as shown in Table 1. The WBGT should be measured if dry bulb temperatures suggest a thermal problem exists, or for outdoor workers where heavy or energetic work is the norm.
- If conditions are found to be uncomfortable or unacceptable using the WBGT index, area supervisors should adopt measures to reduce heat accumulation. These may include:
- alter work regimes to permit regular rest periods;
- alter work distribution so that more energetic or heavy work is performed in cooler times of day or on cooler days;
- provide mechanical assistance for strenuous activity or heavy lifting;
- provide engineering controls to reduce heat accumulation (for example, install air conditioning units in rooms, insulate roofs, etc).
- Review of work conditions should be initiated by area supervisors, in consultation with staff who work in the area, and their elected Health and Safety Representative(s) to ensure modifications or management is successful.
The basis for this background information was provided by Dr John Edwards, Occupational Hygienist. School of Medicine - Faculty of Medicine, Nursing and Health Sciences