Study Reveals Critical Threshold: The Maximum Tolerable Heat for the Human Body.
A recent study conducted by researchers at the University of Roehampton in England examined the point at which the body begins to expend more energy to regulate its temperature in high heat conditions. The findings indicate that this upper-temperature limit falls between 40℃ (104F) and 50℃ (122F), beyond which the human body ceases to function optimally.
As heatwaves and unusually warm temperatures continue to impact regions worldwide, it becomes crucial to comprehend the mechanisms behind this phenomenon. Further research is needed to gain insights into how the body’s ability to cope with excessive heat diminishes beyond the identified limit.
The ongoing surge in temperatures globally, leading to health warnings and concern among climate scientists, has prompted an urgent need for understanding how the human body responds to extreme heat. July 4th, for instance, has been recorded as the hottest day in modern history across the world.
The thermoneutral zone is the temperature range within which the body does not need to increase its metabolic rate or expend extra energy to maintain its optimal core temperature of 37℃elsius or 98.6 Fahrenheit.
Research has indicated that the lower limit of the thermoneutral zone, where the body does not need to exert additional energy to maintain its ideal temperature, is at 28℃ or 82.4F. Below this threshold, the body expends more energy by shivering, as key muscle groups involuntarily contract to generate heat.
Conversely, at higher temperatures, the body utilizes different mechanisms to cool down, such as sweating and vasodilation of blood vessels in the skin to enhance heat dissipation.
Although the lower range of the thermoneutral zone has been well-established, the upper limit remains uncertain. One study proposes that the upper limit could be around 32℃ (89.6F), as this is when humans typically start to sweat. However, another study suggests that the metabolic rate begins to rise at 40℃ (104F).
Further research is needed to better understand and determine the precise upper limit of the thermoneutral zone, especially in the context of increasing concerns about heatwaves and rising temperatures due to climate change.
Additional investigation into the upper boundary of the thermoneutral zone could provide valuable insights for shaping policies concerning working conditions, sports activities, medication practices, and international travel guidelines.
Following a previous investigation in 2021, researchers at the University of Roehampton in England conducted a subsequent series of experiments to explore the upper limit of the thermoneutral zone.
Their findings indicate that the upper limit of the thermoneutral zone likely falls within the range of 40℃ (104F) and 50℃ (122F).
Dr. J. Wes Ulm, a bioinformatic scientific resource analyst, and biomedical data specialist at the National Institutes of Health, who was not involved in the study, commented to Medical News Today that these new findings offer a more precise understanding of how the body responds to prolonged heat and humidity. They also shed light on the nature and mechanisms of increased metabolic rate that occur in such conditions.
The researchers recently presented these fresh insights at the annual conference of the Society for Experimental Biology in Edinburgh, Scotland.
The impact of temperature and humidity on the human body
The study enlisted 13 healthy volunteers, with ages ranging from 23 to 58 years, including seven female participants.
During the experiment, each participant underwent one-hour exposure to five different temperature conditions while at rest. These conditions were as follows:
- 28℃ (82.4F) and 50% relative air humidity (RAH)
- 40℃ (104F) and 25% RAH
- 40℃ (104F) and 50% RAH
- 50℃ (122F) and 25% RAH
- 50℃ (122F) and 50% RAH
Throughout each condition and at the baseline, the research team measured various metrics, including:
- Core and skin temperatures
- Blood pressure
- Sweating rate
- Heart rate
- Breathing rate
- Volume of air inhaled and exhaled per minute
- Movement levels
Ultimately, the results showed that participants’ metabolic rate increased by 35% when exposed to 40℃ (104F) and 25% RAH, and by 48% at 40℃ (104F) and 50% RAH.
Although the 50℃ (122F) and 25% RAH condition did not result in a higher metabolic rate compared to the 40℃ (104F) and 25% RAH condition, the metabolic rate was 56% higher than baseline in the 50℃ (122F) and 50% RAH condition.
The increased metabolic rate observed in the 40℃-25% RAH condition did not coincide with a rise in core temperature. However, in the 50℃-50% RAH condition, participants experienced a 1℃ increase in core temperature, equivalent to 1.8 Fahrenheit.
These findings led the researchers to conclude that the body is capable of dissipating heat effectively at 40℃ (104F), but not as efficiently at 50℃ (122F).
Dr. Mark Guido, an endocrinologist with Novant Health Forsyth Endocrine Consultants in Winston Salem, North Carolina, who was not involved in the study, commented to Medical News Today that the results seem to vary based on humidity levels. The study provided evidence that resting metabolic rate was higher at higher humidities, even at the same temperature, indicating that humidity also plays a significant role in the metabolic rate.
Furthermore, the researchers highlighted that participants in the 50℃-50% RAH condition exhibited a 74% increase in sweating and a 64% rise in heart rate compared to baseline.
In addition to the earlier findings, the researchers observed that participants in the 50℃-50% RAH group experienced an elevated myocardial workload, indicating that their hearts needed more oxygen to sustain optimal performance compared to the baseline.
Furthermore, their breathing rate increased by 23%, and their minute ventilation (the volume of air they could inhale and exhale per minute) rose by 78%.
Interestingly, the researchers also observed that drinking water during each of the conditions did not effectively cool the body.
