The WBGT is the workhorse of heat stress assessment worldwide. Unlike a simple thermometer reading, it is an ISO-standardised index that integrates four environmental factors at once: air temperature, humidity, wind speed, and radiation, covering both shortwave radiation from the sun and longwave radiation emitted by surrounding surfaces. Because it blends these variables into a single number, WBGT is widely used to trigger heat management plans at sporting events, military training exercises and industrial sites. When the index crosses agreed thresholds, organisers can activate shade stations, adjust pacing requirements, or postpone starts. The problem, until now, has been that WBGT is usually measured at a handful of fixed weather stations, which tells organisers almost nothing about what a walker experiences deep in a sun-baked forest clearing or on a shaded stretch of canal path two hours into the course.

That gap matters because heat stress at the human scale is intensely local. A runner moving through alternating sun and shade, over asphalt and through woodland, encounters rapidly shifting microclimates that a regional weather model simply cannot resolve. To capture this variability, the research team—Kobe Vandelanotte, Bram Du Moulin, Siebe Puynen, Thomas Vergauwen, Sara Top and Steven Caluwaerts—deployed a mobile measurement campaign during the 2024 Dodentocht. Instruments were carried along the route to collect spatio-temporal WBGT data as the event unfolded, producing a moving picture of heat stress conditions that matched the actual experience of participants rather than the conditions at a distant airport weather station.

These field observations served a second, crucial purpose: they allowed the team to evaluate a modelling system designed to simulate heat stress at metre-scale resolution. The system couples a numerical weather prediction model, which handles the broader atmospheric state, with a micro-scale radiation model that resolves how buildings, trees and terrain shape the radiation environment street by street and field by field. When the modelled WBGT values were compared against the mobile measurements along the 100-kilometre route, the system delivered what the authors describe as a reasonable approximation of observed conditions. Importantly, it was able to capture some local shade-induced cooling effects—the dips in heat stress that occur when a route passes under tree cover—demonstrating that the micro-scale radiation component was doing real work rather than smoothing over the fine-grained detail.

With the model validated, the researchers turned it from a diagnostic instrument into a decision-making tool. They simulated a series of alternative event-management scenarios for the 2024 Dodentocht, asking a question that every endurance organiser now confronts: what could we have done differently? The scenarios included shifting the start time, rerouting the course, and adjusting assumed walking speeds, each of which changes when and where participants encounter the hottest part of the day and how much heat they accumulate over the event.

The headline finding is striking. For the 2024 edition, starting the event three hours earlier—at 18:00 local time instead of the official 21:00 start—would have reduced cumulative heat-stress exposure by 35 percent, measured as degree hours above a WBGT threshold of 20 degrees Celsius. That single scheduling decision, achievable with no change to the route and no additional infrastructure, would have removed roughly a third of the dangerous heat load carried by participants. In practical terms, an earlier start shifts more of the course into the cooler evening and overnight hours, so that walkers reach the most punishing midday stretch further along, or in some cases past, the peak of the day’s heat.

Route choice proved equally consequential. Under identical meteorological conditions and the official 21:00 start time, the 2023 route accumulated 16.5 percent more degree hours above 20 degrees Celsius WBGT than the 2024 route. The difference illustrates how sensitive endurance events are to seemingly small course design choices: the amount of shade, the exposure of open stretches, and the timing with which the route delivers participants into sun-exposed sections all feed directly into cumulative exposure. A course that is a few kilometres longer but better shaded, or one that reorders its segments so that open terrain is crossed at night, can meaningfully lower the health risk for every participant on the field.

The metric the team uses—degree hours above a WBGT threshold—deserves attention because it captures what a single snapshot cannot: the accumulation of heat stress over time. Physiologically, heat illness risk depends not just on how hot it is, but on how long the body is working hard while hot, and whether it gets a chance to recover. A walker crossing a hot, sunlit section at kilometre 60 after twenty hours of exertion faces a very different risk profile from one crossing the same section fresh. Cumulative exposure metrics of this kind therefore align better with the way heat accumulates in the human body than peak-temperature warnings alone, and they give organisers a quantifiable target to optimise against rather than a binary go/no-go decision.

The broader context makes tools like this increasingly urgent. Climate change is driving more frequent and more intense extreme heat episodes across Europe and beyond, and participants in outdoor endurance events are among the most exposed populations, combining prolonged outdoor duration with high metabolic heat production from sustained physical exertion. Mass-participation events—from marathons and ultramarathons to long-distance walking events and cycling sportives—have already seen cancellations, medical surges and, in tragic cases, fatalities linked to heat. Organisers traditionally rely on historical averages and fixed station data to plan, but a warming climate is eroding the reliability of the past as a guide. High-resolution heat stress modelling offers a way to plan for the conditions that will actually occur, and to stress-test contingency plans before race day rather than improvising during a medical emergency.

What this study demonstrates, ultimately, is a workflow: measure, validate, simulate, decide. Mobile observations ground-truth the model; the validated model becomes a virtual laboratory in which organisers can compare start times, routes and pacing scenarios; and the resulting exposure estimates feed directly into heat management plans, from shade and hydration placement to medical staffing and cancellation thresholds. The authors present the system as a reliable and actionable tool for developing and implementing effective heat management strategies for outdoor endurance events, and the Dodentocht results show that the payoff can be large—an earlier start alone cutting exposure by more than a third. As summers grow hotter, the events that adapt fastest will be those that treat heat stress not as an act of nature to be endured, but as a design variable to be engineered, modelled and minimised long before the first participant takes a step.

Subject of Research: High-resolution wet bulb globe temperature modelling to assess and reduce heat stress exposure in ultra-endurance events

Article Title: Too hot to race? Demonstrating heat stress modelling as an adaptation tool for endurance events

Article References: Too hot to race? Demonstrating heat stress modelling as an adaptation tool for endurance events. (n.d.). https://doi.org/10.1371/journal.pclm.0001058

Image Credits: AI Generated

DOI: 10.1371/journal.pclm.0001058

Keywords: heat stress, wet bulb globe temperature, endurance events, numerical weather prediction, micro-scale radiation modelling, climate change adaptation, ultra-endurance, Dodentocht, heat management, WBGT, event planning, Belgium