How to Monitor Exertional Heat Stroke Risk by Region Using WBGT

Each summer, preventable deaths from exertional heat stroke occur despite the availability of reliable forecasting tools. The gap is not in data — it is in access and workflow integration. Fire and rescue teams, EHS managers and event medical directors still lack a simple, location-specific way to translate weather data into actionable risk levels before and during operations.

Why WBGT and not air temperature

Air temperature alone is a poor predictor of heat stroke risk. At 28°C under direct sun with 70% relative humidity, intense physical effort becomes dangerous long before the thermometer reads 35°C in the shade.

WBGT (Wet-Bulb Globe Temperature) captures the four determinants of thermal stress simultaneously:

Air temperature
Relative humidity
Solar radiation (direct and diffuse)
Wind speed

The Liljegren approximation allows estimation from standard weather data: WBGT ≈ 0.567 × T + 0.393 × Tw + 3.94, where Tw is the wet-bulb temperature derived from temperature and humidity.

This is the method retained by the ACSM in its 2023 Expert Consensus Statement as the decision basis for managing events in hot conditions — and by the IOC (Hosokawa et al., BJSM 2021) as the international reference for competitive sports.

The four ACSM risk levels

WBGT	Level	Action
< 26°C	Low	Standard monitoring
26–28°C	Moderate	Increased vigilance, enhanced hydration
28–32°C	High	Restrict intense effort, mandatory cold water access on site
> 32°C	Critical	Suspend high-risk activities

These thresholds are not theoretical. They are calibrated against documented exertional heat stroke case series and validated across military, industrial and sports contexts.

Why location-specific monitoring matters

Europe spans a wide climatic range. In June, WBGT can reach 30°C in Provence while remaining at 22°C in northern France or Scotland. A national average is useless for local decision-making.

For SDIS units, military medical teams, and medical directors for trail runs or triathlons, the relevant value is the one for the specific location of the event or intervention — ideally updated every hour as conditions evolve through the day.

Real-time WBGT monitoring in practice

The Kollder EHS Monitor calculates WBGT in real time for any location in France, using Open-Meteo data updated every 15 minutes. It displays:

Current WBGT with its ACSM risk level
Hourly forecast over 7 days
Emergency protocols by risk level
An EHS alert subscription form

In practice: before a mountain trail or an industrial site intervention, a medical director can check the 48-hour WBGT forecast, adapt cooling equipment deployment, and anticipate peak-risk windows during the event.

What monitoring does not replace

WBGT is a decision-support tool, not a substitute for treatment. Once an exertional heat stroke case is confirmed — confusion, core temperature above 40°C — the only validated intervention is immediate cold water immersion.

The "Cool First, Transport Second" protocol (ACSM 2023, Hutchins et al., Frontiers in Physiology 2022) establishes that lowering core temperature below 39°C takes absolute priority over transport. Survival rate reaches 100% on more than 401 documented cases when this threshold is achieved within 30 minutes (Korey Stringer Institute, Dr Douglas Casa, UConn).

The Kollder emergency cooling tub is built for this protocol: deployable in under 30 seconds by a single person, it enables immediate on-site immersion regardless of terrain or setting.