Temperatures have been soaring around the world as current heatwaves prompt health warnings and raise alarm among climate scientists. In fact, July 4 was the hottest day on modern record worldwide.
According to new research from the University of Roehampton in England, the human body may lose the ability to rid of excessive heat and stop functioning optimally when outside temperatures reach beyond 40 degrees Celsius (104 Fahrenheit).
The thermoneutral zone is a range of temperatures in which the body doesn’t have to increase its metabolic rate or exert more energy to maintain its ideal core temperature of 37°Celsius or 98.6 Fahrenheit.
Studies show that the zone’s lower limit is 28°C or 82.4F. Below this, the body expends more energy to maintain its ideal temperature. One of the key ways it does this is by shivering — when key muscle groups involuntarily contract to produce heat.
At higher temperatures, the body uses other mechanisms to cool down, such as sweating and vasodilation of blood vessels at the skin surface to increase heat loss.
However, while the thermoneutral zone’s lower range has been established, its upper limit is still uncertain.
One study suggests that the upper limit may stand at around 32°C (89.6F) as this is when humans start to sweat. Another study, however, noted that the metabolic rate starts to increase at 40°C (104F).
Further research into the upper limit of the thermoneutral zone could inform policies on working conditions, sports, medication, and international travel.
As a follow-up study of a 2021 investigation, researchers at the University of Roehampton in England conducted a second set of experiments to investigate the upper limit of the thermoneutral zone. They found that the thermoneutral zone’s upper limit likely lies between 40℃ (104F) and 50℃ (122F).
How temperature and humidity affect the human body
For the study, the researchers recruited 13 healthy volunteers aged between 23 and 58 years old. Seven of the participants were female. Each participant was exposed to five temperature conditions (Temperature and RAH) for an hour while resting. Throughout each condition and at baseline, the researchers recorded several 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 researchers found that participants’ metabolic rate increased by 35% when exposed to 40°C (104F) and 25% RAH, and by 48% at 40°C (104F) and 50% RAH. Although the 50°C and 25% RAH condition did not increase metabolic rate compared to 40°C (104F) and 25% RAH, metabolic rate was 56% higher than baseline in the 50°C (122F) and 50% RAH condition.
The increased metabolic rate at the 40°C-25% RAH condition was not accompanied by an increase in core temperature. However, participants in the 50°C-50% RAH condition experienced a rise in core temperature of 1°C, or 1.8 Fahrenheit.
The researchers noted that these findings suggest that the body is able to dissipate heat at 40°C (104F), but not at 50°C(122F). The researchers also noted that drinking water in each of the conditions did not cool the body.