Heat danger limit may be lower than previously theorized

Time for a rethink?

- Jayashree Nandi

NEW DELHI: The widely accepted wet-bulb temperature survival threshold of 35°C is being questioned after recent physiological studies suggest the limit may actually be closer to 31°C, Harvard researchers revealed following a recent interdisciplinary conference with the Union environment ministry to discuss heat stress in India and adaptation interventions.

Wet-bulb temperature combines heat and humidity measurements to indicate how effectively the human body can cool itself through sweating when outdoors. Unlike regular air temperature readings, wet-bulb measurements reflect the body's cooling capacity in humid conditions—when the wet-bulb temperature reaches the same level as human skin temperature, sweat evaporation becomes impossible, preventing the body's primary cooling mechanism from working.

Temperature thresholds that should trigger emergency responses during extreme heat episodes are "far more complex than presently understood", the scientists said based on their research and discussions at the four-day Delhi conference last month.

The 35°C wet-bulb threshold was originally proposed by scientists Steven C Sherwood and Matthew Huber from the University of New South Wales in a 2010 study. They theoretically concluded that exceeding this temperature for extended periods should induce heat stroke within just six hours of exposure.