New Research Questions Heat Survival Limits
Recent studies are challenging the widely accepted limit for wet-bulb temperature, which is critical for understanding heat survival. Originally set at 35°C, new findings suggest that the real survival threshold may actually be closer to 31°C. This was revealed by researchers from Harvard University following an important conference with India’s Ministry of Environment, where they discussed the impacts of heat stress and ways to adapt to it.
Wet-bulb temperature measures both heat and humidity, providing an insight into how well a person can cool down through sweating. Unlike standard temperature readings, wet-bulb temperatures reflect how effectively our bodies can manage heat in humid conditions. When the wet-bulb temperature reaches skin temperature, sweating becomes ineffective, which can lead to overheating.
According to scientists at the conference, the complexity of temperature thresholds that should prompt emergency responses during extreme heat is much greater than previously understood. The 35°C limit was first suggested in a study by Steven C. Sherwood and Matthew Huber in 2010, which indicated that prolonged exposure at this temperature could result in heat stroke within just six hours.
However, more recent research has indicated that the survival limit could be lower. A study from the Center for Healthy Aging at Pennsylvania State University examined healthy young individuals in environments of 40°C and 50% humidity. They found that after prolonged exposure of over six hours, core body temperatures began to rise continuously, a condition known as uncompensable heat stress, even before reaching levels of heat exhaustion or stroke.
In their investigations, Penn State researchers also pointed out that the old 35°C threshold might actually overestimate dangerous heat conditions. Their earlier work found that critical wet-bulb temperatures ranged from 25°C to 28°C in dry heat and between 30°C and 31°C in humid conditions.
Another study from the University of Ottawa found that during even short periods of low activity within heat waves, a person’s resting body temperature can exceed 38°C, which is often regarded as the safe upper limit in work environments.
Insight Gaps About Global South Conditions
Dr. Satchit Balsari, an associate professor at Harvard Medical School and chair of the conference’s scientific committee, highlighted a major gap in understanding how prolonged exposure to high temperatures affects vulnerable populations, particularly in the Global South, where data is limited. He mentioned, "Most existing studies are conducted on Caucasians, primarily men, with little focus on the living conditions of poorer communities.”
These studies often do not account for the high daytime and nighttime temperatures experienced in poorly insulated homes, commonly seen in many regions of India. Dr. Balsari’s team is currently monitoring hundreds of workers in their everyday environments to create what they call a “personalized heat exposure index.” This index aims to assess individual exposure to heat and the body’s responses to it over time.
Dr. Robert Meade from Harvard’s TH Chan School of Public Health further explained that the 35°C wet-bulb temperature is a theoretical limit. He emphasized that if skin temperature also reaches 35°C, sweating cannot cool the body effectively, causing dangerous body temperatures to rise quickly. He warned that focusing solely on the survival temperature does not capture the broader health risks posed by heat waves, which can lead to serious conditions like heart attacks, even if body temperatures remain stable.
Dr. Balsari believes the goal should not only be survival but also ensuring "human flourishing." He shared a relatable story from a street vendor who pointed out that while 45°C might be the threshold for health issues, her vegetables start spoiling at temperatures of 40-42°C. He noted that extreme heat affects productivity, rest, and quality of life, making it crucial to address these issues holistically.
The Pennsylvania State study also warned that regions like the Middle East and the Indus River Valley could potentially experience dangerous heat levels with even a small rise in global temperatures. Dr. Balsari cautioned about balancing acclimatization with potential complacency, referring to the high death rates during the COVID-19 pandemic, despite thoughts that South Asians might have better immunity. He concluded by questioning to what extent people can adapt to heat and where the limits of that adaptability might be.
