The document discusses factors that contribute to heat stress, including air temperature, humidity, clothing, physical exertion, and individual characteristics. It then compares several common heat stress indices - predicted heat strain, thermal work limit, and wet bulb globe temperature - under different meteorological conditions such as temperature, humidity, wind speed. Finally, it recommends a risk-based approach to managing heat stress that involves preparation, monitoring, early warning systems, control measures, and review.

1. HEAT
STRESS
Sensitivity analysis of Heat Stress indices
to input meteorological variables
Christine Killip

2. Weather
Air temperature, humidity, air
speed, radiant surfaces
Clothing
PPE
Work Rate
Strenuous work, when and how
often breaks scheduled
The Individual
Acclimatisation, age, medical
conditions
MAJOR FACTORS CONTRIBUTING TO HEAT STRESS
+ HIDDEN FACTORS
Isolation
Working alone and/or
remote locations
Culture
Working without breaks, drinking
energy drinks & alcohol
Monitoring & Controls
Reactive controls, poor monitoring,
wrong choice of heat stress
variables / indices to monitor
Education
Lack of knowledge on signs of
heat stress & dehydration,
amount of water to drink

3. Complexity of model and correlation to heat stress
HEAT STRESS INDICES – to help you assess the situation
Apparent
Temperature
(AT)
Effective
Temperature
(ET)
Basic Effective
Temperature
(BET)
Corrected
Effective
Temperature
(CET)
Required
Sweat Rate
Predicted
Heat Strain
(PHS)
Wet Bulb
Globe
Temperature
(WBGT)
Heat Stress
Index (HSI)
Predicted
4-hour Sweat
Rate (P4SR)
Thermal Work
Limit
(TWL)
Ease of use and most often used

4. Heat Stress Index Comparison for Hot, Dry and Windy conditions
Relative humidity = 15%
Temperature PHS (minutes) 1 TWL (W/m2) WBGT(°C)
WBGT "look up
table" (°C) 2
30 Not exceeded 326 21.4 23.5
35 Not exceeded 284 25.0 27.1
40 32 231 28.6 31.0
45 19 175 32.2
35.6
Notes:
All calculations assume WS = 3 m/s, clothing factor of 0.5 (light short sleeve
shirt and long pants), acclimatized, hydrated, work rate of 150 W/m2.
1 PHS gives the estimated minutes a person can work until their rectal
temperature exceeds 38°C
2 WBGT "look up table" is an estimate based on T and RH
(http://www.bom.gov.au/info/thermal_stress/index.shtml)
TWL PHS WBGT
Acclimatisation - No restrictions
for acclimatised workers
TWL > 140 W/m2
Rectal temperature not
predicted to exceed 38°C
WBGT less than 28°C - no
work rest cycles required
Buffer - Work-rest cycling
applies
TWL 115 - 140 W/m2
Rectal temperature predicted to
exceed 38°C between 30 and
480 minutes
WBGT between 28 and 30°C -
work rest cycles required
Withdrawal - 20 minutes of
work, 40 min rest
TWL <115 W/m2
Rectal temperature predicted to
exceed 38°C in less than 30
minutes
WBGT greater than 31°C -
work rest cycles of 0-25%
required
Heat Stress Index Comparison for Hot, Dry and Windy conditions
Relative humidity = 25%
Not exceeded 305 23.0 25.1
Not exceeded 254 26.9 29.3
33 191 30.8 33.9
19 120 34.7 38.9
Heat Stress Index Comparison for Hot, Dry and Windy conditions
Relative humidity = 35%
Not exceeded 283 24.5 26.8
Not exceeded 218 28.6 31.5
32 147 32.8 36.8
19 114 36.9 42.7
RH = 15%RH = 25%RH = 35%

5. Heat Stress Index Comparison for Hot, Humid and Calm conditions
Wind Speed = 2 m/s
Temperature PHS (minutes) 1 TWL (W/m2) WBGT(°C)
WBGT "look up
table" (°C) 2
25 Not exceeded 257 24.7 27.5
30 Not exceeded 174 29.5 33.5
35 83 114 34.2 40.4
Heat Stress Index Comparison for Hot, Humid and Calm conditions
Wind Speed = 1 m/s
Not exceeded 231 24.7 27.5
Not exceeded 160 29.5 33.5
83 <114 34.2 40.4
Notes:
All calculations assume RH= 75%, clothing factor of 0.5 (light short sleeve shirt
and long pants), acclimatized, hydrated, work rate of 150 W/m2.
1 PHS gives the estimated minutes a person can work until their rectal
temperature exceeds 38°C
2 WBGT "look up table" is an estimate based on T and RH
(http://www.bom.gov.au/info/thermal_stress/index.shtml)
TWL PHS WBGT
Acclimatisation - No restrictions
for acclimatised workers
TWL > 140 W/m2
Rectal temperature not
predicted to exceed 38°C
WBGT less than 28°C - no
work rest cycles required
Buffer - Work-rest cycling
applies
TWL 115 - 140 W/m2
Rectal temperature predicted to
exceed 38°C between 30 and
480 minutes
WBGT between 28 and 30°C -
work rest cycles required
Withdrawal - 20 minutes of
work, 40 min rest
TWL <115 W/m2
Rectal temperature predicted to
exceed 38°C in less than 30
minutes
WBGT greater than 31°C -
work rest cycles of 0-25%
required
Heat Stress Index Comparison for Hot, Humid and Calm conditions
Wind Speed = 0.5 m/s
Not exceeded 231 24.7 27.5
Not exceeded 138 29.5 33.5
36 <114 34.2 40.4
Heat Stress Index Comparison for Hot, Humid and Calm conditions
Wind Speed = 0.25 m/s
Not exceeded 174 24.7 27.5
210 117 29.5 33.5
32 <114 34.2 40.4
WS = 2 m/sWS = 1 m/sWS = 0.5 m/sWS = 0.25 m/s

6. Reduce your uncertainty to variations in meteorological conditions
and heat stress indices by using a risk based approach
Heat Stress is complex - don’t leave it in the too hard basket
1 Pre-Summer Preparation – Risk Profiling and establishing triggers
2 Daily Monitoring – monitoring your triggers
3 Early Warning System – targeted alerts to managers
4 Mitigation/Control – proactive measures when required
5 Post-Summer Review – update triggers
Visit: www.heatmanager.com.au