How WBGT and WBGTeff Are Revolutionizing Heat Safety
Understanding heat stress assessment through the PN-EN ISO 7243:2018-01 standard
Imagine working in an environment where the very air around you becomes a threat to your survival—where your body's cooling systems begin to fail, and your core temperature rises dangerously. This isn't a scene from a climate disaster movie but a daily reality for millions of workers in industries ranging from glass manufacturing to construction.
Each year, over 1,200 people in the United States alone die from heat-related illnesses, a number that continues to climb as temperatures break records worldwide 1 .
annual deaths in US
Traditional temperature measurements fall dangerously short in capturing how humans experience heat. The common thermometer might read 90°F, but depending on humidity, solar radiation, and wind speed, it could feel like 116°F—a critical difference that could mean the difference between safe working conditions and potentially fatal heat stroke 2 .
This is where the Wet Bulb Globe Temperature (WBGT) index comes in—a sophisticated approach to measuring heat stress that accounts for all these factors and more. Recently updated in the PN-EN ISO 7243:2018-01 standard, this method now includes the WBGTeff index, which provides even more accurate assessments of heat stress by considering personal protective equipment and clothing 3 .
The Wet Bulb Globe Temperature (WBGT) index represents a fundamental shift in how we measure environmental heat stress. Unlike simple temperature readings or even the heat index (which only considers temperature and humidity), WBGT incorporates four environmental factors that determine how the human body experiences heat: temperature, humidity, wind speed, and solar radiation 2 .
Where:
When we exercise or work physically, our bodies generate internal heat. Normally, we cool ourselves through:
(evaporative cooling)
to cooler surroundings
from air movement
High humidity undermines evaporation, still air reduces convection, and hot surroundings or direct sunlight minimize radiative cooling. WBGT captures all these impediments in a single number, providing an accurate predictor of heat stress that simple temperature measurements cannot match 5 .
The PN-EN ISO 7243:2018-01 standard represents the international consensus on how to properly assess heat stress using the WBGT index. Titled "Ergonomics of the thermal environment - Assessment of heat stress using the WBGT (wet bulb globe temperature) index," this document provides detailed guidelines for measurement procedures, instrument specifications, and interpretation of results 3 .
The standard serves as a screening method for evaluating heat stress exposure over a working day (up to 8 hours), helping organizations determine whether additional protective measures are necessary. It applies to both indoor and outdoor occupational environments and is designed to protect healthy adults who are fit for work 3 .
The 2018 revision introduced several important updates:
These updates allow for calculating WBGTeff (effective WBGT) that factors in clothing considerations, providing a more realistic assessment of actual heat stress experienced by workers 3 .
To understand how scientists apply the WBGT index in real-world settings, let's examine a comprehensive study conducted in Portuguese glass manufacturing facilities—an industry known for extreme heat exposure 6 .
Researchers visited five industrial units and nineteen different workplaces, taking detailed measurements in accordance with ISO standards. They used specialized equipment including:
The findings revealed disturbing levels of heat stress throughout the glass industry:
| Metabolic Rate (W/m²) | WBGT Reference Values (°C) | |
|---|---|---|
| Non-acclimatized | Acclimatized | |
| ≤65 | 32 | 33 |
| 65-130 | 29 | 30 |
| 130-200 | 26 | 28 |
| 200-260 | 22-23* | 25-26* |
| >260 | 18-20* | 23-25* |
*With sensible air movement
| Workstation | WBGT (°C) | Metabolic Rate (W/m²) | Status Relative to Reference Value |
|---|---|---|---|
| Forming area 1 | 31.5 | 185 | Exceeded by 3.5°C |
| Annealing entrance | 28.7 | 165 | Exceeded by 0.7°C |
| Packaging | 25.3 | 140 | Within limits |
| Furnace operation | 34.2 | 210 | Exceeded by 8.2-11.2°C |
The study also implemented the Predicted Heat Strain (PHS) model, a more sophisticated approach that predicts physiological responses like sweat rate, core temperature, and heart rate. This allowed researchers to compare the simpler WBGT screening method with the more complex PHS model 6 .
The findings confirmed that while WBGT provides an excellent initial screening tool, in extreme environments like glass manufacturing, it should be supplemented with more detailed physiological assessment methods like PHS for complete accuracy 6 .
Accurate WBGT assessment requires specialized equipment designed to measure its component parts. Here are the key tools used by researchers and safety professionals:
| Equipment | Function | Special Considerations |
|---|---|---|
| Black Globe Thermometer | Measures radiant heat through temperature change inside a black copper sphere | Standard 150mm diameter; smaller versions (40mm) require adjustment factors |
| Natural Wet Bulb Thermometer | Measures evaporative cooling potential using a wick-wrapped sensor maintained with distilled water | Critical that wick is properly sized and kept clean to ensure accurate measurements |
| Data Logger | Records measurements over time for analysis of trends and peak exposures | Should be capable of simultaneously recording all WBGT components at specified intervals |
| Anemometer | Measures air velocity, which affects both convective cooling and evaporation | Should be capable of measuring low air speeds typical in indoor industrial settings |
| Portable WBGT Meter | Integrated device that calculates and displays WBGT index in real-time | Must be calibrated regularly; Kestrel 5400 is a commonly used model in research 4 |
While WBGT provides an invaluable tool for heat stress assessment, professionals must understand its limitations:
For extreme environments or vulnerable populations, professionals often combine WBGT with other assessment tools:
As climate change increases the frequency and intensity of heat waves, proper heat stress assessment has evolved from a niche industrial concern to a critical public health priority. The WBGT index, particularly as refined in the PN-EN ISO 7243:2018-01 standard, provides scientifically robust yet practical methodology for protecting vulnerable populations .
Recent research has demonstrated that heat stress is increasing faster than air temperature alone would suggest—in North Carolina, inland locations like Raleigh and Charlotte have seen some of the fastest-increasing WBGT values in the southeastern United States, adding roughly two more extreme heat days per decade . Perhaps more concerning, nighttime recovery periods are becoming hotter, with Asheville experiencing particularly rapid increases in nighttime WBGT values—making it harder for bodies to recover from daytime heat exposure .
These trends highlight the growing importance of implementing proper heat stress assessment across industries, sports, and public health planning. By moving beyond simple temperature readings to comprehensive heat stress evaluation using WBGT and WBGTeff, we can develop evidence-based protection strategies—from work-rest cycles to hydration protocols—that save lives in our increasingly hot world.
The challenge is significant, but with robust standards like PN-EN ISO 7243:2018-01 and proper measurement tools, we have the knowledge needed to protect vulnerable workers and athletes from the growing threat of extreme heat.
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