Working in
Hot Conditions Technical Information
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.
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% and 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
| Work/rest regime
(each hour) |
light work |
Moderate work |
Heavy work |
| Continuous work |
30.0 |
26.7 |
25.0 |
| 75% work/25% rest |
30.6 |
28.0 |
25.9 |
| 50% work/50% rest |
31.4 |
29.4 |
27.9 |
| 25% work/75% rest |
32.2 |
31.1 |
30.0 |
After NHMRC Occupational Health Guide, Heat Stress, Commonwealth
Dept Health, 1980
Recommendations
- 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%) conditions 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 Health Sciences
Last revised: 24 October 2006
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