What does the WBGT index represent?
WBGT stands for Wet Bulb Globe Temperature. It summarizes environmental components that influence heat exchange between a person and their surroundings. It is used as a screening method to assess exposure conditions and guide the need for further measurement or more detailed analysis.
It is part of the assessment of heat stress. The environmental result alone does not describe the full extent of the demands placed on the body. The same condition can have different consequences during light activity or intense physical work, and when wearing clothing that facilitates or hinders heat loss.
Variables and measurement
The method uses temperatures measured with appropriate instruments, including natural wet-bulb temperature and globe temperature. The calculation settings depend on the presence of solar radiation. A site-specific measurement must not be replaced by the reported temperature for a location, which may represent a different environment from the actual workplace.
The instrumentation requires testing and stabilization time. Setting up equipment and reading it immediately can produce unrepresentative data. Location, conditions, and procedures must be recorded, including any changes during the observation period. The competent person must be familiar with the characteristics of the sensors and the limitations of the devices they intend to use.
Select representative conditions
The assessment should identify tasks, locations, and times that may result in greater exposure. A reading taken first thing in the morning does not allow us to conclude that the entire workday is safe if radiation or the industrial process increases later. Nor does a measurement taken in the shade automatically represent an activity performed in the sun.
If environments and workloads alternate, the methodological rules for characterizing these periods must be applied. Indiscriminately averaging periods of intense exposure with periods of calm is not advisable, as this can obscure relevant factors. The report must explain which scenario it describes and under what conditions it would no longer be valid.
Effort, clothing and acclimatization
The comparison requires characterizing metabolic activity and the clothing used. Clothing adjustments must be derived from the methodology and applicable to the actual group. It is not appropriate to introduce a generic value for any protection, especially when it significantly limits evaporation or falls outside the expected conditions.
Heat acclimatization also influences the reference criteria. It should not be assumed solely based on seniority or residence in a warm climate. Recent exposure and return after absences should be verified. The technical assessment should also consider individual needs through appropriate preventive and health channels.
Interpret and decide
A result must be accompanied by the comparison criteria, assumptions, and resulting preventive measures. WBGT does not provide a universal maximum temperature valid for any activity. When screening indicates problems or does not fit the scenario, it may be necessary to apply the predicted heat strain (PHS) model or other relevant techniques.
The assessment should not delay obvious measures: reducing heat sources, facilitating recovery in a cool place, and reorganizing tasks. Data helps design and verify these measures, but it does not justify maintaining exposure while a report is being completed. If symptoms appear or controls fail, the planned response should be implemented even if a previous assessment was favorable.
Practical example
Two people work in the same yard. One operates equipment, and the other moves materials manually while wearing additional protective clothing. Using the temperature from the weather app for both and drawing a single conclusion ignores differences in exertion, clothing, and position relative to heat-radiating surfaces.
The assessment characterizes each activity, measures relevant conditions, and applies appropriate criteria. The plan may combine schedule changes, reduced effort, and sufficient recovery. It is then verified that the measures remain adequate when the weather or workload changes. The example illustrates why the same location does not imply identical exposure.
Integration into the heat plan
The results must be linked to operational decisions: who reviews conditions, which tasks are modified, and when they are interrupted. Weather forecasts help anticipate problems, while assessment characterizes the work. Both resources are complementary and must be coordinated with first aid preparation and heatstroke detection.
In Spain, the obligations applicable to outdoor work and areas that cannot be enclosed must be considered. The assessment must take into account weather phenomena, task characteristics, and relevant personal circumstances. WBGT’s technical criteria do not replace these obligations, nor do they equate the absence of a weather warning with a guarantee of safety.
Common mistakes
Common mistakes include confusing WBGT with dry-bulb temperature, using a device without checking its range, or applying a limit without considering activity and clothing. It is also a mistake to declare the entire workforce acclimatized or to retain an assessment after significant changes in processes, protective equipment, or work schedules.
A useful application maintains traceability between measurement, interpretation, and action. The goal is to control exposure, not to produce a reassuring figure. It must be possible to explain what conditions were assessed, what uncertainties exist, and how it is verified that people can work with adequate protection during the periods actually planned.
