Heat Stress Index Gets a Climate-Smart Makeover With Fuzzy Weighting

For more than six decades, one deceptively simple number has governed how employers, militaries, and sports organizations decide when it is too hot to work: the wet-bulb globe temperature, or WBGT. The index blends air temperature, humidity, radiant heat, and airflow into a single value that sits at the heart of the international occupational standard ISO 7243. Now a team of Iranian researchers argues that the index carries a hidden flaw, one baked into its very formula, and they have proposed a way to fix it using a decision-science technique borrowed from engineering and risk analysis. Their study, published in Theoretical and Applied Climatology, suggests that recalibrating WBGT for local climates could change real-world work-rest recommendations for outdoor laborers.

The problem, according to Seyed Mahdi Mousavi of Isfahan University of Medical Sciences and his colleagues Habibollah Dehghan and Saeid Yazdanirad, lies in the fixed empirical weights at the core of the classical WBGT formulation. When the index was first developed in the United States military context of the 1950s, its components were combined using coefficients that reflected the conditions in which it was validated. Those coefficients have remained essentially unchanged even as the index has been exported to deserts, tropics, and temperate zones around the world. Previous reviews, including the authors’ own systematic narrative assessment of WBGT performance across Iran’s diverse climatic regions, have reported that the index does not perform equally well everywhere, hinting at a systematic bias when a single weighting scheme is applied to radically different environments.

To test that idea rigorously, the team designed a field campaign spanning March to September 2025 in three Iranian cities chosen to represent sharply contrasting climates. Abadan, in the country’s scorching southwest near the Persian Gulf, provided a hot-humid environment where moisture-laden air impedes the evaporation of sweat. Isfahan, in the central plateau, offered a hot-dry setting dominated by intense solar radiation and large day-night temperature swings. Gorgan, near the Caspian Sea in the north, contributed a temperate-humid profile. At each site the researchers measured the four environmental parameters that drive heat stress: air temperature, relative humidity, wind speed, and solar radiation. Baseline WBGT values were computed strictly according to ISO 7243, giving them a standard reference point against which any alternative weighting could be judged.

The innovation came in how the new weights were derived. Rather than assuming the standard coefficients are universally correct, the team employed a Fuzzy Analytical Hierarchy Process, or FAHP, a multi-criteria decision-making framework designed for situations where expert judgment is inherently uncertain. In a conventional Analytical Hierarchy Process, experts compare criteria pairwise and the resulting judgments are crunched into priority weights. FAHP replaces the crisp pairwise values with triangular fuzzy numbers, which encode a range of possible values with a most-likely estimate at their peak. This fuzziness acknowledges that an expert who says humidity matters ‘somewhat more’ than wind speed is expressing an imprecise relationship, and the mathematics of fuzzy sets propagates that imprecision through the entire weighting calculation instead of pretending it does not exist. The approach has gained traction in fields from drought risk assessment to renewable energy planning, but applying it to the calibration of a physiological heat index is a novel step.

The results confirmed that the relative importance of environmental parameters shifts dramatically with climate. In Abadan, air temperature and relative humidity emerged as the dominant drivers of heat strain, which makes physical sense: in humid heat, the evaporative cooling pathway that normally dissipates human body heat is throttled by moisture-saturated air, so both the dry-bulb temperature and the humidity itself become critical. In Isfahan’s dry heat, air temperature alone rose to the top of the hierarchy, since sweat evaporates readily there and the main burden is simply the sheer thermal load of scorching air and sun. In temperate-humid Gorgan, relative humidity was the single most influential parameter. Wind speed, by contrast, never claimed the leading role in any of the three settings, though its weight still varied by location.

When the researchers rebuilt the WBGT calculation using these climate-specific fuzzy weights, the numbers moved in a consistent direction. Weighted WBGT values increased by 0.4 to 0.8 degrees Celsius compared with the standard index. That may sound like a rounding error, but in occupational health it can be decisive. ISO 7243 defines threshold limit values that trigger mandatory work-rest cycles, and exposure levels that fall close to those limits are precisely where a half-degree shift flips a site from ‘continuous work permitted’ to ‘scheduled rest required.’ For safety managers operating near the boundary, the difference between the classical and climate-adjusted index could determine whether crews get additional shade breaks or keep working through the afternoon.

Crucially, the revised index did not just shift numbers; it tracked human physiology better. To validate their weighting scheme, the team drew on heart rate and thermal sensation data from 150 outdoor workers. Heart rate is a well-established proxy for physiological heat strain, because cardiovascular system load rises as the body struggles to shed heat, while thermal sensation captures the perceptual side of the experience. Under the classical WBGT, the correlation between the index and workers’ heart rates ranged from 0.55 to 0.62. With the climate-weighted version, those correlations climbed to between 0.66 and 0.71. At the same time, the root mean square error, a standard measure of prediction error, dropped by 10 to 18 percent. In biometeorological terms, the fuzzy-weighted index explained more of the variance in real bodily strain and made smaller mistakes when it did err.

The study also subjected its own conclusions to scrutiny through a sensitivity analysis, perturbing each environmental parameter by plus or minus ten percent and observing how the WBGT estimate responded. This kind of stress test is essential for any model built on expert-derived weights, because it reveals whether the rankings are robust or artifacts of particular judgment calls. The finding that the parameter hierarchy held up under perturbation strengthens the case that the climate-related differences in weighting reflect genuine physical structure rather than noise in the expert elicitation process.

The broader context gives the work urgency. Heat is among the deadliest occupational hazards, and systematic reviews have linked occupational heat strain to kidney disease, reduced productivity, and elevated accident risk, with climate change steadily expanding the geographic reach and intensity of dangerous conditions. Global burden-of-heat studies estimate population-level exposure using WBGT derived from meteorological data, meaning that any systematic bias in the index propagates into climate-adaptation planning, labor regulations, and projections of future work capacity. If the standard formulation systematically underestimates strain in humid regions or misallocates importance among parameters in dry ones, then the protective thresholds built on it may be quietly miscalibrated for hundreds of millions of workers.

The authors are careful to frame their contribution as a theoretical evaluation rather than an immediate replacement for ISO 7243, and the study’s scope, three Iranian cities and one warm-season measurement window, leaves open questions about cold climates, indoor settings, and seasonal variation. Still, the message is striking: a sixty-year-old formula can be meaningfully improved not by adding new sensors or exotic physics, but by letting the weights themselves flex with the climate they are asked to describe. As heat waves lengthen and labor protections come under increasing scrutiny, tools that squeeze extra accuracy out of existing measurements may prove among the most practical adaptations available. For the outdoor workers of Abadan, Isfahan, and Gorgan, and for the safety officers who set their schedules, a few tenths of a degree could soon make all the difference.

Subject of Research: Climate-dependent performance and fuzzy-weighted recalibration of the wet-bulb globe temperature index for occupational heat stress assessment

Article Title: Climate‑related performance of the wet‑bulb globe temperature (WBGT) index: a fuzzy analytical hierarchy process approach

Article References: Climate‑related performance of the wet‑bulb globe temperature (WBGT) index: a fuzzy analytical hierarchy process approach. (n.d.). https://doi.org/10.1007/s00704-026-06597-x

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06597-x

Keywords: wet-bulb globe temperature, heat stress, ISO 7243, fuzzy analytical hierarchy process, occupational health, outdoor workers, climate, Iran, thermal physiology, biometeorology, work-rest cycles, heart rate