Heat stress is the physiological and health risk that can arise when workers are exposed to high-temperature, humid and radiant environments. As temperatures rise with climate change, particularly during the summer months, work environments such as iron and steel, foundries, glass, textile dyehouses, bakeries, kitchens and open construction sites are placed under critical risk. The ISO 7243 standard and the WBGT (Wet Bulb Globe Temperature) index form the internationally recognised methodology for measuring and managing this risk.
WBGT Calculation Formula
The WBGT index is a combination of three separate temperature measurements:
- Tw (natural wet-bulb temperature): Measures the cooling effect of evaporation.
- Tg (black globe temperature): Measures heat transfer through radiation.
- Ta (dry-bulb air temperature): Measures convective heat transfer.
Indoor formula: WBGT = 0.7 × Tw + 0.3 × Tg
Outdoor (in sun) formula: WBGT = 0.7 × Tw + 0.2 × Tg + 0.1 × Ta
WBGT Limit Values by Workload Class
| Workload | WBGT Limit (Acclimatised worker) | WBGT Limit (Unacclimatised) |
|---|---|---|
| Light (seated, light arm work) | 30°C | 29°C |
| Moderate (walking) | 27.5°C | 26°C |
| Heavy (fast walking, digging) | 25.5°C | 24°C |
| Very Heavy (continuous heavy lifting) | 23°C | 22°C |
Heat Stress Symptoms and Risk
- Heat cramps: Muscle spasms caused by salt and fluid loss.
- Heat exhaustion: Fatigue, dizziness, nausea and sweating.
- Heat stroke: Body temperature above 40°C; life-threatening.
- Vulnerable groups: High risk for children, the elderly, and people with cardiovascular conditions.
Control Measures
- Engineering controls: Ventilation, air conditioning, radiation shielding and thermal insulation.
- Administrative controls: Shorter working hours, frequent rest breaks, shift rotation and avoiding the hottest hours.
- Personal measures: Access to cold beverages (1-1.5 L/hour), breathable workwear and cooling vests.
- Health monitoring: Pre-employment health checks for workers, with additional checks during summer.
- Acclimatisation: A 5-7 day gradual adaptation programme for new workers.
- Emergency plan: A rapid-response procedure for heat stroke symptoms.
Work-Rest Ratio
The ratio between a worker's working and resting time is determined according to the WBGT level:
- WBGT ≤ 25°C: 100% work (unlimited working)
- WBGT 25-27.5°C: 75% work, 25% rest
- WBGT 27.5-30°C: 50% work, 50% rest
- WBGT 30-32.5°C: 25% work, 75% rest
- WBGT > 32.5°C: Work is stopped
Heat stress cannot be dismissed with the argument of being "acclimatised". A high WBGT carries a vital risk even for experienced workers. Without accurate measurement and management, a single hot hour can result in a workplace accident.
Frequently Asked Questions
- How often should WBGT measurement be carried out?
In hot work environments, 3-4 times a day during the summer. On an annual basis, it is covered within OHS measurements spanning all hot and cold seasons.
- Is investing in air conditioning essential?
Not in every case. Engineering controls (increased ventilation, radiation shielding) are generally more economical. Air conditioning is suitable for areas such as seated control rooms.
- Are cooling vests effective?
Yes; gel or phase-change material (PCM) based cooling vests can lower body temperature by 5-10°C for hours. They are widely used in construction sites and foundry operations.