How hot is too hot for the human body? – A recent study provides fresh insights
New research conducted by the University of Roehampton in England indicates that when outside temperatures exceed 40 degrees Celsius (104 degrees Fahrenheit), the human body may lose its ability to effectively regulate excessive heat, resulting in suboptimal functioning.
The thermoneutral zone refers to the range of temperatures where the body can maintain its ideal core temperature of 37 degrees Celsius (98.6 degrees Fahrenheit) without needing to increase metabolic rate or expend additional energy.
Previous studies have established the lower limit of the thermoneutral zone at 28 degrees Celsius (82.4 degrees Fahrenheit), below which the body expends more energy to maintain its temperature through mechanisms such as shivering. As temperatures rise, the body utilizes other cooling mechanisms like sweating and vasodilation of blood vessels in the skin to facilitate heat loss.
While the lower range of the thermoneutral zone has been determined, the upper limit remains uncertain. One study suggests that sweating begins at around 32 degrees Celsius (89.6 degrees Fahrenheit), while another study indicates that metabolic rate starts to increase at 40 degrees Celsius (104 degrees Fahrenheit).
To explore the upper limit further, researchers at the University of Roehampton conducted a follow-up study. They discovered that the upper limit of the thermoneutral zone likely falls between 40 degrees Celsius (104 degrees Fahrenheit) and 50 degrees Celsius (122 degrees Fahrenheit). This finding provides more precise insights into the body’s responses to prolonged heat and humidity, as well as the nature and mechanisms behind the enhanced metabolic rate that occurs under such conditions.
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, stated that these findings shed light on the body’s reactions to sustained heat and humidity, as well as the mechanisms involved in the increased metabolic rate. Further research into the upper limit of the thermoneutral zone could have implications for various domains such as setting guidelines for working conditions, sports activities, medication administration, and international travel.
The researchers presented their latest findings at the Society for Experimental Biology’s annual conference held in Edinburgh, Scotland.
In the study, the researchers enlisted 13 healthy volunteers, ranging in age from 23 to 58 years old, with seven of them being female participants.
Each participant was exposed to five temperature conditions for an hour while resting. The conditions included:
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 baseline, the researchers recorded several metrics, including:
While the condition of 50 degrees Celsius (122 degrees Fahrenheit) and 25% relative air humidity did not result in an increased metabolic rate compared to the 40 degrees Celsius (104 degrees Fahrenheit) and 25% relative air humidity condition, the metabolic rate was 56% higher than the baseline in the 50 degrees Celsius (122 degrees Fahrenheit) and 50% relative air humidity condition.
The increased metabolic rate observed in the 40 degrees Celsius (104 degrees Fahrenheit) and 25% relative air humidity condition did not coincide with a rise in core temperature. However, participants in the 50 degrees Celsius (122 degrees Fahrenheit) and 50% relative air humidity condition experienced an increase in core temperature of 1 degree Celsius (1.8 degrees Fahrenheit).
Based on these findings, the researchers concluded that while the body is capable of dissipating heat at 40 degrees Celsius (104 degrees Fahrenheit), it struggles to do so effectively at 50 degrees Celsius (122 degrees Fahrenheit).
“The findings do seem likely to vary by […] humidity,”Dr. Mark Guido, an endocrinologist with Novant Health Forsyth Endocrine Consultants in Winston Salem, North Carolina, not involved in the study, told Medical News Today.
“In the study there was some evidence that resting metabolic rate was higher at higher humidities, even at the same temperature. It seems like humidity also plays a large role in the metabolic rate,” he added.
The researchers also observed that participants in the 50 degrees Celsius (122 degrees Fahrenheit) and 50% relative air humidity condition exhibited a 74% increase in sweating and a 64% increase in heart rate compared to the baseline measurements.
Additionally, the participants in the 50 degrees Celsius (122 degrees Fahrenheit) and 50% relative air humidity condition experienced an increased workload on their hearts, indicating that their hearts required more oxygen to maintain optimal functioning, when compared to the baseline measurements.
Furthermore, the breathing rate of the participants increased by 23%, and the volume of air they could inhale and exhale per minute increased by 78% compared to the baseline measurements.
The researchers also noted that drinking water during the different conditions did not effectively cool down the body.
How does climate affect metabolic rate and health?
MNT asked Dr. John P. Higgins, Sports Cardiologist at McGovern Medical School at The University of Texas Health Science Center at Houston (UTHealth), not involved in the study, how living in different climates may affect metabolic rate and the thermoneutral zone.
“People living in warm climates tend to acclimatize and not increase their body temperature and in turn their metabolic rate as much. Likewise, people living in cool-freezing temperatures may get more of a response to heat exposure as they are not acclimatized to the heat as much,” Dr. Higgins noted.
MNT also spoke with Dr. Ulm: “The body, in general, will find ways to activate the various feedback loops needed to achieve homeostasis i.e., the painstaking regulation of physiological processes that allow for the complex biochemistry of organs and tissues to be carried out efficiently and properly.”
“Body temperature and metabolic rate are integral components of this delicate dance, and for those who are resident in hotter climates year-round, it may be more likely for such countervailing feedback loops to be active and functioning. This may be attributable both to heritable factors- for communities present in such conditions longer-term- and to short-term adaptations more generally.”
“It’s similar to the way permanent residents of high-altitude regions will acclimate with compensatory mechanisms, for example, in their red blood cell physiology and other aspects of oxygen-carrying capacity, both acutely- as through iron turnover rates- and chronically,” he said.
What are the study limitations and takeaways?
MNT spoke with Dr. Ulm about its limitations.
“As always with such studies, there is the question of how representative the cohort sample of subjects is of both the general and specific populations being surveyed, in regard to the physiological characteristics and responses being measured.”
“The studies, in this case, were also particularly challenging given the ambient conditions, and there is also the perennial issues of the applicability of the experimental environment to real-world correlates,” he added.
Dr. Guido noted: “It is hard to draw real world conclusions from a small laboratory study, but my main takeaway is that higher heat stress does seem to increase the resting metabolic rate by increasing how hard the body has to work to try to stay cool, particularly by causing a significant increase in the heart rate. If this holds true in real world conditions, it very well could lead to an uptick in cardiovascular disease by putting more strain on the heart,” he noted.
Dr. Higgins added: “Also, might it be beneficial for weight management to perform exercise in warmer temperatures indoors or outdoors to boost metabolic rate and thus burn more calories – further research needs to be done.”
