Why is the 35°C threshold being revised
Health studies show lower limits: Experiments with healthy young adults have found that severe heat stress can occur at wet-bulb temperatures as low as 31.5°C.
Experimental findings: In controlled experiments, critical wet-bulb temperatures for healthy young adults ranged from 25°C to 28°C in hot-dry environments and 30 °C to 31 °C in warm-humid environments.
Lower limits for vulnerable groups: Research suggests that older adults have even lower survival limits, ranging from 21.9°C to 27.8°C
Uncompensable heat stress: The point at which the body can no longer cool itself through sweating is reached at a lower wet-bulb temperature than the theoretical 35°C.
Real-world events: Recent heatwaves have seen wet-bulb temperatures in the 31°C to 33.6°C range, demonstrating that these lower thresholds are being reached in the real world.
Britain is surprisingly humid. Anyone who suspects that the above Wet Bulb Global Temperature Chart is on the conservative side is wrong. UK humidity is regularly over 60% relative humidity, and often higher. A wet-bulb temperature of 31°C to 33.6 °C would be 30°C @ 55-75% humidity.
In the UK, the media loves a heatwave. It’s used to stimulate internal tourism.
The Tabloid approach to heatwaves in the UK is to show people in the sun, on the beach and eating ice creams. They spend hours in gridlock, stuck bumper to bumper in the sun, cooking in their cars. It’s only a matter of time before severe heat kills whole families on their way to the beach, and they are found in their vehicles after heatwaves.
Wet Bulb Global Temperature (WBGT) of 31.5°C and above is entering the lethal range. This was measured today, 22/06/2026
But…
Everyone has a story of how they survived something far hotter, or perhaps they cite the World Sauna Championships. The World Sauna Championships were banned when people died in a non-sporting event.
A Finn once lectured me about how Finns can withstand extremely hot saunas — they sit in there with no clothes on, drink as much water as they want, and then take a cold plunge to cool off.
Most people in the Global South currently on the WBGT frontline don’t have the luxury of a cold plunge pool.
Britain is now on the wet-bulb frontline
You can’t talk about Climate Risk on the Bushcraft UK forum; it triggers too many idiots. This is how people die from heat exhaustion on expeditions.
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