MOM's guidelines on WBGT monitoring for outdoor work, explained
MOM's WBGT monitoring guidelines in plain language: what the number is made of, how to measure it, and what to record.
The Ministry of Manpower publishes Guidelines on Wet Bulb Globe Temperature (WBGT) Monitoring for Outdoor Work. They set out how WBGT should be measured so that the rest breaks, water and other heat-stress measures are based on a reliable number.
This article explains what the guidelines cover and what they mean in practice.
What WBGT is made of
WBGT combines three readings into one heat-stress index:
- Natural wet bulb temperature (Tnwb): reflects humidity and how well sweat can evaporate.
- Globe temperature (Tg): reflects radiant heat from the sun and hot surfaces.
- Air (dry bulb) temperature (Ta): the ordinary air temperature.
For outdoor work in the sun, MOM's guidelines use the standard formula:
WBGT (outdoor) = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Ta
Humidity carries 70% of the weight. That is why a humid, overcast Singapore afternoon can still reach a high WBGT band even when the air temperature looks moderate.
For indoor or shaded areas without direct sun, the formula drops the air temperature: WBGT (indoor) = 0.7 × Tnwb + 0.3 × Tg.
Who must measure on site
Construction sites with a contract value of S$5 million or more, shipyards and the process industry must measure WBGT on site with a meter. Other workplaces may use the readings from the nearest NEA weather station in the myENV app.
Meter specifications
| Reading | Range | Resolution | Accuracy |
|---|---|---|---|
| WBGT (outdoor) | 10 – 50°C | 0.1°C | ±2°C |
| Globe temperature | 10 – 60°C | 0.1°C | ±2°C |
| Wet bulb and dry bulb | 10 – 50°C | 0.1°C | ±1°C |
How to measure
The guidelines set out a simple procedure:
- Place the meter about 1 metre above the ground.
- Put it in direct sun, not blocked by buildings or shade, close to where people work.
- Wait at least 10 minutes for the reading to stabilise.
- Measure every hour, especially during the hotter parts of the day.
- Record each hourly reading.
The guidelines also mention real-time, automated meters as an option, which remove the need for someone to take each reading by hand.
Calibration
Meters must be calibrated every year, or as the manufacturer specifies, by a supplier authorised by the manufacturer or by a laboratory accredited by the Singapore Accreditation Council (SAC). Keep the certificate on site.
Turning readings into action
A reading only protects workers when it changes what happens next. Link each band to a clear action for supervisors, as set out in our guide to what to do at each WBGT level. For a closer look at who needs a meter and common mistakes, see WBGT monitoring on site.
How HeatAware helps
HeatAware kits for sites that must measure on site include a WBGT meter that meets MOM's specification, with yearly calibration by an accredited lab in the renewal. The app records every reading and raises alerts when the band changes. Size your kit.
Frequently asked questions
Does the meter have to stay in one place?
Place it where the most exposed work is happening. If work areas differ a lot, measure where the heat risk is highest.
Can we skip readings on cloudy days?
No. Humidity drives most of the WBGT, so cloudy days can still be high. Keep the hourly routine.
Based on MOM's Guidelines on WBGT Monitoring for Outdoor Work and its Heat Stress Measures for Outdoor Work. Always refer to the latest version of the guidelines.