New Research Reveals Upper Limit of Human Body’s Thermoneutral Zone
Researchers from the University of Roehampton in England have conducted a study to determine the upper limit of the thermoneutral zone of the human body. The thermoneutral zone is the range of temperatures in which the body does not need to increase its metabolic rate or exert more energy to maintain its ideal core temperature. The study found that the upper-temperature limit of the thermoneutral zone lies between 40℃ (104F) and 50℃ (122F), beyond which the body stops functioning optimally.
The thermoneutral zone is an essential concept in understanding how the human body regulates its temperature. Below the lower limit of the zone, which is 28℃ or 82.4F, the body expends more energy to maintain its ideal temperature. Shivering is one of the ways the body generates heat to cope with lower temperatures. On the other hand, at higher temperatures, the body uses mechanisms like sweating and vasodilation of blood vessels to cool down and dissipate heat.
Previous studies have provided conflicting information regarding the upper limit of the thermoneutral zone. Some studies suggest that the upper limit is around 32℃ (89.6F), the temperature at which sweating begins. However, a separate study indicated that the metabolic rate starts to increase at 40℃ (104F).
To investigate this further, researchers recruited 13 healthy volunteers aged between 23 and 58 years old for their study. The participants were exposed to five different temperature conditions, ranging from 28℃ (82.4F) to 50℃ (122F), for an hour while at rest. The researchers recorded various metrics such as core and skin temperatures, blood pressure, heart rate, sweating rate, breathing rate, and movement levels.
The study found that the metabolic rate of the participants increased by 35% at 40℃ (104F) and 25% relative air humidity (RAH), and by 48% at 40℃ (104F) and 50% RAH. In the 50℃ (122F) and 50% RAH condition, the metabolic rate was 56% higher than baseline. Additionally, participants in the 50℃-50% RAH condition experienced a rise in core temperature of 1℃ (1.8F).
Drinking water did not effectively cool the body in any of the conditions tested. The researchers noted that humidity also plays a significant role in the metabolic rate, as resting metabolic rate was higher at higher humidities, even at the same temperature.
The findings of this study are crucial for understanding how the body responds to sustained heat and humidity and provide insights into the nature and mechanisms of enhanced metabolic rate under such conditions. Further research on the upper limit of the thermoneutral zone could inform various fields including working conditions, sports, medication, and international travel.
The researchers presented their findings at the annual conference of the Society for Experimental Biology in Edinburgh, Scotland. However, it is important to note that the limitations of this study include the small sample size and the applicability of the experimental environment to real-world conditions. Nevertheless, the study offers important insights into how the human body regulates its temperature and copes with extreme heat.