Wet Bulb Globe Temperature (WBGT) Measurement
When discussing heat-related concerns with the HSE (Health, Safety, and Environment) officer at an industrial or construction site, it is critical to reference heat measurement using a wet bulb globe thermometer, which is not a standard instrument. This device considers humidity and thermal radiation, as HSE assessments are based on the WBGT index rather than air temperature. The WBGT index evaluates an individual's ability to work safely in a given environment over an 8-hour period.
The WBGT index is standardized by ISO 7243:2017: Ergonomics of the Thermal Environment – Assessment of Heat Stress Using the WBGT Index (Wet Bulb Globe Temperature).
The formulas for calculating WBGT are:
- Without solar radiation: WBGT = 0.7 × thn + 0.3 × tg
- With solar radiation: WBGT = 0.7 × thn + 0.2 × tg + 0.1 × ta
Where:
- thn: Natural wet bulb temperature
- tg: Black globe temperature
- ta: Air temperature
The black globe temperature (tg) is measured using a thermometer housed in a 15 cm matte black copper sphere, designed to capture radiative heat. The thermometer bulb is placed at the sphere's center.
For example, at an air temperature of 35°C, the WBGT varies with relative humidity:
- 35% humidity: WBGT = 26.8
- 55% humidity: WBGT = 29.8
- 80% humidity: WBGT = 32.9
Natural wet bulb temperature and WBGT values can also be estimated using specialized websites or specific tables available on the SPF Emploi website. WBGT measurement devices are commercially available, including models on platforms like Amazon.
Impact on Work
In Belgium, heatwaves have led to multiple incidents and severe clinical cases in industrial and infrastructure sectors. Rising temperatures increase workplace risks such as fatigue, dehydration, and heatstroke, which can result in workplace accidents.
When temperature thresholds are exceeded, employers must act promptly based on occupational health physician recommendations, implementing measures such as protection against direct radiation and ensuring effective ventilation within 48 hours.
Regular breaks become mandatory depending on the WBGT index and physical workload:
- Evaluate WBGT: Determine the WBGT index for an 8-hour work period.
- Assess Physical Workload: Refer to examples below:
- Light work: Office tasks, car driving.
- Moderately heavy work: Carpentry, tractor operation.
- Heavy work: Digging, manual sawing, pushing/pulling wheelbarrows.
- Very heavy work: Deep digging, excavation, climbing ladders, stair ascent.
- Match WBGT to Limits: If WBGT values exceed permissible limits, introduce rest periods based on ISO standards such as NBN EN ISO 7243, NBN EN ISO 7933, or NBN EN ISO 9886, or follow the occupational health physician's guidance.
If no prior arrangements exist, use predefined rest-work schedules.
For example, with 60% relative humidity and 30°C air temperature (WBGT = 26), rest periods are required only for heavy or very heavy work.
Ozone Peaks
During heatwaves, ozone levels pose additional risks, especially for outdoor workers. To mitigate these:
- Plan physical tasks for early morning when ozone levels are lower.
- Limit overtime.
- Ensure regular indoor breaks and reduce exposure to irritants.
These precautions are especially critical for engineers with respiratory or cardiovascular sensitivities.
If conditions remain unsafe despite measures, employees may be placed on temporary unemployment to prioritize health and safety.
