Heat Index & WBGT Meters — Heat Stress Monitoring for South Africa
Professional Wet Bulb Globe Temperature (WBGT) and heat stress instruments for occupational safety, construction, mining, agriculture, and sport. Protect your workforce and comply with OHS legislation.
What is WBGT and why does it matter?
Wet Bulb Globe Temperature (WBGT) is the internationally accepted standard index for measuring occupational heat stress. Unlike a simple air temperature reading, WBGT combines four environmental factors — air temperature, humidity, solar radiation, and air movement — into a single value that accurately represents the thermal burden placed on the human body during physical work.
It is the measurement required by ISO 7243, the ACGIH Threshold Limit Values (TLVs), OSHA, and South Africa's own Occupational Health and Safety Act for assessing and controlling heat stress risk in outdoor and industrial environments.
WBGT Formulas
NWB = Natural Wet Bulb temperature | GT = Globe Temperature | DB = Dry Bulb (air) temperature
Governing standard
ISO 7243International standard for hot environments — WBGT index for thermal stress evaluation
NWB weighting
70%Humidity has the greatest single influence on heat stress — the body cools through sweat evaporation
ACGIH TLV action level
25 – 33 °CVaries by work rate and acclimatisation status — heavy work has lower safe thresholds
Globe sphere diameter
150 mmStandard black globe for radiant heat measurement per ISO 7726
Featured product: AZ87786 Heat Stress WBGT Meter and Data Logger
The AZ87786 is a professional all-in-one heat stress instrument measuring every parameter required for WBGT assessment, with built-in data logging and PC connectivity for OHS record-keeping and compliance reporting.
AZ87786 Heat Stress WBGT Meter and Data Logger
A comprehensive professional heat stress meter that simultaneously measures and displays WBGT (indoor and outdoor), Wet Bulb, Globe Temperature, Dry Bulb, Heat Index, Dew Point, and Relative Humidity. The built-in data logger records all parameters at set intervals and transfers data to a PC via USB — providing the documentation trail required for OHS compliance.
- Simultaneous WBGT (indoor & outdoor), WB, GT, DB, Heat Index, Dew Point, %RH display
- 150 mm black globe temperature sensor — ISO 7726 compliant
- Data logging with PC download via USB and included software
- WBGT range: 0 to 60 °C | Humidity: 0–100 %RH
- Displays WBGT bar chart and trend for quick field assessment
- Max/Min hold and auto power-off functions
- Complies with ISO 7243 and ACGIH heat stress guidelines
- Suitable for outdoor, industrial, sports, and military applications
WBGT heat stress risk levels
The WBGT reading maps directly to risk levels and required controls. These thresholds apply to workers performing moderate physical work — thresholds are lower for heavy work and higher for light or sedentary tasks.
< 25 °C
Low risk
Normal work permitted. Maintain hydration. Monitor conditions.
25 – 27.9 °C
Moderate risk
Increase rest breaks. Ensure shade and water access. Watch for symptoms.
28 – 29.9 °C
High risk
Reduce work pace. Mandatory rest periods. Limit heavy exertion.
30 – 32.9 °C
Very high risk
Frequent rest in shade. Reschedule heavy work. Only acclimatised workers.
≥ 33 °C
Extreme — Stop work
Cease strenuous outdoor work. Immediate cooling and medical monitoring required.
Thresholds based on ACGIH TLVs for moderate work rate. Adjust for work rate, acclimatisation, and PPE load. Consult your OHS professional for site-specific action levels.
WBGT vs Heat Index — what's the difference?
Both WBGT and Heat Index measure thermal comfort, but they serve very different purposes. Choosing the wrong metric for an occupational safety application can seriously underestimate risk.
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| Feature | WBGT | Heat Index |
|---|---|---|
| Factors measured | Air temp, humidity, solar radiation, radiant heat, air movement | Air temperature and humidity only |
| Accounts for solar radiation | Yes — via globe temperature sensor | No |
| Accounts for air movement | Yes — via natural wet bulb | No |
| Primary use | Occupational heat stress — workplace safety | General weather perception / public weather warnings |
| Governing standard | ISO 7243, ACGIH TLVs, OSHA, SA OHS Act | National Weather Service (NOAA) — no OHS standard |
| Required for OHS compliance | Yes | No — insufficient for workplace assessments |
| Accuracy for outdoor workers | High — accounts for all major thermal stressors | Low — can significantly underestimate risk in sun |
| Sensor required | Black globe, natural wet bulb, dry bulb thermometer | Thermometer + relative humidity sensor |
What does the AZ87786 measure?
The AZ87786 provides a complete heat stress assessment in a single instrument — here is what each measurement means and why it matters.
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| Parameter | Abbreviation | What it measures | Why it matters |
|---|---|---|---|
| WBGT (outdoor) | WBGT-out | Combined thermal stress index in direct sun | The primary OHS compliance value for outdoor work |
| WBGT (indoor) | WBGT-in | Combined thermal stress index in shade or indoors | Required for factories, foundries, bakeries, boiler rooms |
| Globe Temperature | GT | Radiant + convective heat load via black globe sphere | Captures solar and radiant heat — the WBGT component most often underestimated |
| Natural Wet Bulb | NWB | Evaporative cooling potential of the air | The dominant term in WBGT — high humidity sharply reduces the body's ability to cool |
| Dry Bulb Temperature | DB | Ambient air temperature | Standard air temperature — reference value in the WBGT formula |
| Heat Index | HI | Apparent temperature based on air temp + humidity | Useful for public communication and general comfort assessment |
| Dew Point | DP | Temperature at which air becomes saturated | High dew point indicates oppressive humidity — direct indicator of poor evaporative cooling |
| Relative Humidity | %RH | Moisture content of air as % of saturation | Critical input to WBGT and indicator of heat illness risk |
Where heat stress meters are used
Heat stress is a hazard wherever people work or train in hot or humid conditions — across a wide range of South African industries and environments.
Mining
Deep-level mines generate intense radiant heat. WBGT monitoring is required under the Mine Health and Safety Act for all wet areas and high-temperature zones.
Construction
Outdoor construction workers on the Highveld and Northern Cape face WBGT levels above the safe threshold on summer afternoons. Regular monitoring drives rest-break scheduling.
Agriculture
Farm workers harvesting, pruning, or irrigating outdoors in direct sun are among the highest-risk groups. Seasonal monitoring and shade provision are legally required under OHS regulations.
Industry & foundries
Smelters, foundries, bakeries, laundries, and glass plants generate extreme radiant heat. Indoor WBGT monitoring identifies zones requiring engineering controls or work rotation.
Sport & events
Athletics, rugby, cycling, and outdoor events use WBGT to set participation rules. World Athletics and FIFA both publish event cancellation thresholds based on WBGT readings.
Military & emergency services
Training exercises and emergency operations under load in full kit generate very high metabolic heat. WBGT-based training load management is standard in most defence forces.
Occupational hygiene
Industrial hygienists and OHS practitioners use WBGT as the baseline measurement for formal heat stress risk assessments — required by the OHS Act and for SACPCMP and SAIOH compliance reporting.
Road & civil works
Roadworks, pipeline laying, and municipal civil teams work under direct sun with heavy PPE. WBGT monitoring drives mandatory cool-down breaks that prevent heat stroke incidents.
Heat stress in South Africa — why local conditions demand serious monitoring
South Africa's climate creates some of the most challenging conditions for outdoor workers globally. The Highveld plateau receives intense ultraviolet and solar radiation at altitude, while the KwaZulu-Natal coast combines extreme humidity with high temperatures — a combination that dramatically reduces the body's ability to cool by sweating.
The following table gives typical summer WBGT ranges for major South African locations to illustrate when monitoring and intervention become critical:
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| Location | Climate character | Typical summer WBGT (midday) | Key risk factor |
|---|---|---|---|
| Johannesburg / Highveld | Semi-arid, high altitude | 26 – 30 °C | High solar radiation at 1 750 m; afternoon thunderstorms spike humidity |
| Durban / KZN coast | Subtropical, humid | 28 – 34 °C | Very high humidity suppresses sweat evaporation — high heat illness risk even at moderate temperatures |
| Cape Town (summer) | Mediterranean | 24 – 29 °C | Dry summer heat; Berg wind events can push WBGT rapidly above safe thresholds |
| Limpopo / Lowveld | Bushveld, hot & humid | 30 – 37 °C | Extreme combination of high temperature and humidity — among the highest WBGT readings in SA |
| Northern Cape (Kalahari) | Arid, very hot | 28 – 35 °C | Extreme dry heat; low humidity offers some cooling benefit but temperatures regularly exceed 40 °C |
WBGT values are indicative summer midday estimates. Actual readings depend on cloud cover, wind speed, work rate, and PPE load. Always measure on site with a calibrated WBGT meter.
How to choose the right heat stress meter
Ask these questions to select the right WBGT instrument for your application:
Do you need WBGT or just Heat Index?
For OHS compliance, regulatory reporting, or any formal risk assessment, you need a full WBGT meter with a black globe sensor — heat index alone is not accepted by ISO 7243 or ACGIH. If you only need a general comfort estimate for public events or informal monitoring, a heat index meter is sufficient.
Do you need to store and prove readings over time?
For OHS audits, labour inspections, and incident investigation, a data logging instrument like the AZ87786 is essential. It records all parameters at set intervals and downloads a timestamped record to PC — providing defensible evidence that monitoring was conducted and action levels were observed.
Is the work environment indoors, outdoors, or both?
If workers move between shaded and direct-sun environments, choose a meter that calculates both indoor and outdoor WBGT simultaneously — the AZ87786 does this. Indoor-only work (foundries, kitchens, laundries) can use the simpler indoor formula, but the globe sensor is still required for radiant heat environments.
What work rate are your workers performing?
WBGT action levels depend on metabolic work rate. Confirm whether your workers are performing light, moderate, heavy, or very heavy work using the ACGIH or ISO 8996 metabolic rate tables, and set your WBGT alert thresholds accordingly. The AZ87786 displays a colour-coded bar chart that makes field interpretation immediate.
Frequently asked questions about WBGT and heat stress meters
Common questions from safety officers, site managers, and OHS practitioners — answered clearly.