WBGT vs PHS: methods compared for assessing heat stress in the workplace
Climate change and the increase in global temperatures are making the monitoring of heat stress in workplaces increasingly urgent, especially in industrial environments, construction sites, and non-air-conditioned indoor spaces. Prolonged exposure to critical climate conditions can compromise workers’ health, reduce productivity and increase the risk of accidents and heat stroke.
In this context, the WBGT (Wet Bulb Globe Temperature) and PHS (Predicted Heat Strain) methods represent two fundamental approaches to assess and prevent hazardous situations.
WBGT and PHS: Two approaches to the same goal
Both indices are recognized by international standards (ISO 7243 for WBGT and ISO 7933 for PHS) as methods for estimating occupational heat risk. However, they present substantial differences in terms of complexity, accuracy and scope of application.
- WBGT is a rapid, simple and intuitive method, ideal for preliminary investigations;
- PHS is an advanced analytical method, more suitable for in-depth analyses in severe and complex environments.
Using them in combination is the most effective strategy for a complete assessment: WBGT can serve as a first filter to identify critical situations, while PHS can come into play for the detailed analysis of the most at-risk conditions.
How WBGT and PHS indices are calculated
1) WBGT – simplicity and immediacy
The calculation of the WBGT index is based on three fundamental environmental quantities:
- Black globe temperature (Tg);
- Wet bulb temperature (Tnw);
- Air temperature (Ta).
Depending on the presence or absence of direct solar radiation, different formulas are used:
- WBGT outdoor: 0.7×Tnw + 0.2×Tg + 0.1×Ta
- WBGT indoor (or without sun exposure): 0.7×Tnw + 0.3×Tg
The WBGT index is expressed in degrees Celsius (°C) and provides an immediate indication of the level of risk. It is possible to customize the result with clothing values (CAV) and metabolic rate, thus obtaining the actual WBGT and the reference WBGT for acclimatized or non-acclimatized subjects.
2) PHS – Advanced Heat Balance Analysis
The PHS method provides a more detailed thermo-physiological model and requires both environmental and individual parameters:
- Air temperature (Ta);
- Relative humidity;
- Air speed;
- Average radiant temperature;
- Activity performed;
- Clothing insulation;
- Worker weight and height.
The PHS calculates, among other things, the temporal evolution of:
- Core body temperature;
- Fluid loss due to sweating;
- Maximum safe exposure duration (Dlim).
This makes it an irreplaceable tool for designing breaks, staff rotation, hydration and other prevention strategies.
Advantages and limitations of the WBGT index
The WBGT index is a widely used tool for a preliminary assessment of heat stress risk. Its main strength is its simplicity of application: it requires a limited set of environmental data and returns a result expressed in °C, easily interpretable even by non-specialized personnel. The calculation is rapid and allows for the rapid identification of potentially dangerous situations. Furthermore, it offers useful indications for immediate environmental interventions, such as shielding direct heat sources.
However, the method has limitations, especially in complex or high-risk contexts. The WBGT is based on a general estimate of the heat balance and uses a standard value for clothing insulation (0.6 Clo), with the possibility of adaptations for specific technical garments. It does not consider individual physiological variables such as weight and height. For this reason, it is often advisable to use it as a first screening tool, to be integrated with more in-depth methods, such as the PHS, if the detected values exceed the reference thresholds.
Advantages and limitations of the PHS index
The PHS method is the most detailed and reliable tool for risk assessment in hot and severe thermal environments. It is based on an in-depth analysis of the individual’s thermal balance, allowing for the monitoring of key variables for risk assessment over time, such as total fluid loss and core body temperature. This allows for the limitation of internal temperature increases by introducing exposure time limits. In addition, the method takes into account the specific insulation of clothing and some individual characteristics, such as weight and height, providing precise indications for optimising the organisation of the working day and limiting exposure times, also providing strategic breaks.
However, the PHS method is more complex, requiring a greater commitment in terms of time for data collection, analysis and synthesis. Furthermore, it does not provide immediate or simple indications on how to intervene to shield or reduce the heat source.
The synergy of the two methods in practice with LSI LASTEM
The combined application of WBGT and PHS, as also suggested by ISO standards, represents the best practice for managing thermal stress risk.
Thanks to solutions such as the Heat Shield system, portable microclimatic stations (data sheet) and ISTSA stations by LSI LASTEM, companies can monitor both indices in real time, manage alarms, send data via SCADA or use dedicated software for analysis and reporting (such as GIDAS-TEA) and prevent critical situations before they become dangerous.
Choosing the right tool to protect workers
In the current scenario, where record temperatures are increasingly frequent, it is essential to equip oneself with reliable and versatile technological tools. The integrated use of WBGT and PHS indices, supported by advanced environmental monitoring systems, allows to safeguard the health of workers, optimize shifts and prevent the effects of extreme heat.
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FAQ: Frequently Asked Questions
WBGT or PHS: which index should be used to assess heat stress?
It depends on the objective. WBGT is ideal for rapid heat stress risk screening, while PHS is better suited for more in-depth assessments in severe or complex conditions. In many cases, the most effective approach is to use WBGT first and then PHS for a more detailed analysis.
When is WBGT not sufficient on its own?
WBGT may not be enough when a more precise estimate of the worker’s physiological response is needed, or when exposure and recovery times must be defined. In these cases, PHS is more useful because it considers a wider range of environmental and personal variables and allows for a more complete evaluation of the heat balance.
What is the main advantage of WBGT?
The main advantage of WBGT is speed. It is an index that is simple to apply in the field, easy to interpret, and very useful for quickly identifying critical situations in the workplace. For this reason, it is often the first step in heat stress assessment.
Why is PHS considered a more advanced method?
Because PHS does not simply provide a synthetic index, but estimates the impact of heat on the human body in greater detail and over time. It can help assess body temperature, fluid loss, and maximum exposure duration, making it particularly useful in high-risk contexts.
How does LSI LASTEM instrumentation support both WBGT and PHS?
LSI LASTEM instrumentation makes it possible to directly measure the environmental parameters needed for a reliable heat stress assessment, such as Tg, Tnw, Ta, RH, and air velocity. This strengthens WBGT calculation in the field and also provides a consistent data basis for more in-depth analysis using the PHS model, which can be easily calculated with LSI LASTEM software.