A complete and supportable heat stress index must account for all of the contributing parameters. Currently, the most complete and supportable index of heat stress is the concept of air-cooling power, which was developed by the South African Chamber of Mines. This index was however specific to South African mining practices (in particular, it assumed personnel were essentially nude and fully wetted by perspiration).
There have been a number of updates and modifications to the original scales and it is important to state which air cooling power scale is being used. McPherson (1992)8 describes
how to calculate air-cooling power, referred to as the “M” (McPherson’s) Scale Air Cooling Power.
The concept of air-cooling power relies on the quantification of the ambient environment’s ability to remove metabolic heat from the human body. The scale used to determine the rate of generation of heat by the human
body and also air-cooling power is W/m2 of body (skin) surface area9.
By definition, provided the air cooling power is equal to, or greater than the metabolic rate, then there will be less than a one in one million chance of the healthy, acclimatised and “self pacing” individual developing dangerous body core temperatures (>40°C), potentially leading to heat stroke. This is held to be an “acceptable” risk.
A widely used thermal acceptance criterion is a minimum air cooling power of 115 W/m2 (i.e. if the air
cooling power is less than 115 W/m2, then an individual could not
even sustain a “moderate” work rate without incurring an unacceptable (greater than one in one million) chance of a healthy acclimatised individual suffering from heat stroke.
Whilst air-cooling power is the most complete heat stress index, it requires fairly complex calculations (ideally requiring a computer) to solve. The calculations require inputs including; amount and type 8
“The Generalisation of Air Cooling Power” M.J. McPherson. Fifth International Mine Ventilation Congress The Mine Ventilation Society of South Africa, Johannesburg, 1992.
9
For reference, the average skin surface area of a 1.7m tall, 60.5 kg South African miner has been determined to be 1.8 m2. The modified DuBois formula, relating skin surface area (As, m2) to body mass (m, kg) and height (h, m) is As = 0.217m0.425h0.725 (From “The Mine Ventilation Practicioner’s Data Book” Second Edition, Andrew Patterson et. al. 1999 The Mine Ventilation Society of South Africa, Johannesburg.
18 20 22 24 26 28 30 32 34 36 100 200 300 400 5 m /s 4 m /s 3 m /s 2 m /s 1.5 m /s 1 m/s 0.5 m/s 0 m/s 5 m/s 3 m/s 2 m/s 1 m/s 0.5 m/s 0 m/s 5 m/s 3 m/s 1 m/s 0 m/s A ir C o o li n g p o w e r ( M s c al e) o r M e ta b o lic H ea t W /m
Wet Bulb Temperature t C
Radiant Temperature = Dry Bulb Temperature Dry Bulb Temperature = Wet Bulb Temperature + 5°C
(Note that the graph may be used without undue error for differences between wet bulb and dry bulb temperatures of between 2 and 8 °C)
Figure From “The Generalisation of Air Cooling Power” by M.J. McPherson
Fifth International Mine Ventilation Congress The Mine Ventilation Society of South Africa, Johannesburg, 1992.
Heavy Clothing (Long Sleeved Overalls and Long Sleeved Shirt)
115 W/m Line 2
(Light Work, 5m/s Air Velocity & Light Clothing)
Light Clothing (Thin Trousers, Short Sleeved Shirt)
Unclothed
w
of clothing worn, wet bulb temperature, dry bulb temperature, radiant temperature of surroundings, air velocity, barometric pressure and the work rate of the individual. The large number of required inputs and requirement for a computer to calculate air-cooling power has limited its use to date as a practical tool for determining on the spot whether the thermal conditions in an underground environment are “acceptable”. Recently this problem has to a large extent been overcome with the development of a robust, portable heat stress meter designed for use underground. The meter uses algorithms based on air-cooling power and is based on an instrument that was originally developed for use by the US military in “Operation Desert Storm” in Kuwait.
For mines which do not have access to the heat stress meter, it should be noted that normal ranges of some of the factors mentioned above in the underground mining context have a relatively weak effect on air cooling power. A generalised Air Cooling Power Chart that assists in the rapid manual assessment of the acceptability of the thermal environment can be produced on this basis. (See McPherson’s “M” Scale Air Cooling Power Chart opposite) The following assumptions were made in order to produce such a chart:
Typical Metabolic Work-rate Classifications for Healthy Adults
Light Work Moderate Work Hard Work Very Hard Work
< 115 W/m2 115 to 180 W/m2 180 to 240 W/m2 >240 W/m2 Sleeping 40 Seated 60 Standing 70 Walking 5 km/h Trades people Jumbo drilling Diesel Operator Walking 6.5 km/h Building brick walls Scaling
Hand-held drilling
Shovelling Timbering
Barometric pressure – Assume P = 100 kPa. Air cooling power is largely unaffected by normal range of pressures found within underground mines.
Radiant temperature of surroundings is equal to the dry-bulb temperature (This is usually the case, except near hot surfaces such as diesel radiators etc).
Dry-bulb temperature is equal to the wet-bulb temperature + 5°C (Note that the graph may be used without undue error for differences between wet bulb and dry bulb temperatures of between 2 and 8°C.) The majority of underground temperatures except near diesel radiators etc will fall within this range.
Clothing - Assumptions were also made about the thermal resistance and area factor of different clothing specifications and also regarding the body posture factor.
The above assumptions allow an air-cooling power graph to be produced, which only considers wet-bulb temperature, air velocity (air speed over the individual), and clothing type.
The protective clothing worn underground in Australian operations would fall somewhere between the “Light” and “Heavy” categories.
An example has been outlined with the heavy dashed line in the “M” scale chart. It assumes “light” clothing, “light” work rate (115 W/m2) and air velocity of 0.5 m/s. This line projects
down to a wet-bulb temperature of 29.5°C. In other words, at the conditions and work rate outlined, a wet bulb temperature of less than 29.5°C will ensure that there is less than a one in one million chance of heat stroke occurring in a healthy, acclimatised individual.
In any mining operation, there will be variation in many of the variables listed earlier . It can however be concluded that based on all the assumptions implied in McPhersons ‘M’ Scale, an acceptable air cooling power can not be provided for even a “light” work rate at wet bulb temperatures above 32 °C.
BASIC MINE VENTILATION OCCUPATIONAL HEALTH AND SAFETY
It is also important to note that for a range of circumstances (e.g. heavier work rates), conditions which could lead to the development of heat stroke could occur at wet bulb temperatures below 32°C. For the purposes of providing a simple, practical measurement to determine the thermal “acceptability” of an underground environment, a “stop work” cut-off of 32°C wet
bulb is supported. This temperature cut-off is combined with a requirement that the air velocity must be greater than 0.5 m/s at wet bulb temperatures over 25°C (e.g. refer to W.A. Regulations).