Heat tolerance in the human body.

Heat tolerance in the human body.

The Body’s Limit in Extreme Heat: Understanding the Upper Critical Temperature

Record-breaking heat waves are sweeping the nation and the world, leaving many people questioning just how much heat the human body can withstand. A recent study conducted in the United Kingdom sheds light on the upper critical temperature that overwhelms the body’s defenses against heat. Researchers found that individuals exhibit an increase in metabolic rate when ambient temperature rises, generating more heat. However, this adaptive response varies among individuals, suggesting that some people may be less well-adapted to the heat than others.

The study, led by senior researcher Lewis Halsey from the University of Roehampton School of Life and Health Sciences in London, also discovered that humidity exacerbates the effects of high temperatures. When it’s hot and humid, the increase in metabolic rate is even greater, making it more challenging for the body to regulate its internal temperature.

Understanding the upper critical temperature is vital as climate change leads to more frequent and intense heatwaves worldwide. Dr. Christopher Lemon from Johns Hopkins University School of Medicine emphasizes the importance of studying the effects of extreme heat on the body. As we increasingly find ourselves in hotter conditions, knowing the point at which we push our limits becomes crucial for our overall well-being.

The researchers recruited 13 healthy individuals to participate in the study. The participants spent an hour in an environment chamber where they were exposed to temperatures ranging from 104 to 122 degrees Fahrenheit and humidity levels between 25% and 50%. The results revealed a significant increase in metabolic rate between the baseline temperature of 82 degrees and the higher heat of 104 degrees. Furthermore, another substantial increase in metabolic rate occurred at 122 degrees when humidity rose from 25% to 50%.

To cool off, the human body relies on sweating, which evaporates and cools the skin. However, in humid conditions, sweating becomes less effective due to the high-water vapor pressure in the air that inhibits evaporation. Consequently, participants experienced the most significant discomfort at 122 degrees Fahrenheit and 50% humidity.

Additionally, as temperatures and humidity rose, heart rate also increased, indicating the body’s attempt to pump more blood to the skin for cooling purposes. Compared to the baseline temperature of 82 degrees, participants had a 16% increase in heart rate at 104 degrees and a staggering 64% increase at 122 degrees with 50% humidity. Interestingly, this increase in heart rate was more pronounced in women than in men.

Extreme heat can harm the body in various ways. Dr. Howard Weintraub from NYU Langone Health explains that increases in heart rate and blood pressure exert tremendous strain on the heart. The body’s efforts to cool down by shifting blood to the skin can also pose challenges for organ performance. For example, kidneys may suffer from reduced blood flow. Additionally, dehydration resulting from excessive sweating further destabilizes the body. These adverse thermal environments disrupt numerous biological processes.

At some point, the body reaches its limits, and a cascade of physiological changes occurs. According to Dr. Lemon, this can lead to a system shutdown when the body can no longer compensate for the extreme conditions. Therefore, it is essential to understand how the body responds to high temperatures, particularly in scenarios where individuals are physically active or working outdoors.

While this study provides valuable insights, there is still much to learn about the body’s response to extreme heat. Future research should explore the effects of high temperatures on individuals engaged in physical activity. Factors such as working in shade or using fans to aid sweat evaporation and body cooling should also be considered. Halsey and his team recommend focusing on air temperatures between 90 and 104 degrees Fahrenheit to determine the average point at which the body begins expending more energy to cool itself.

Additionally, researchers should investigate which individuals are more susceptible to higher temperatures. Vulnerable populations, such as the elderly, young children, those with chronic diseases, and individuals with mental illness, are at higher risk during extreme heat events, according to the U.S. Centers for Disease Control and Prevention.

As heatwaves become more frequent and intense due to climate change, understanding the upper critical temperature and its effects on the human body is crucial for public health and safety. By gaining a deeper understanding of how our bodies react and adapt to extreme heat, we can develop better strategies to protect ourselves and mitigate the potential health risks associated with rising temperatures.


  • Article Image: HealthDay Reporter
  • Dennis Thompson. “How Much Heat Can the Human Body and Mind Endure?” HealthDay. Accessed August 19, 2021.
  • U.S. Centers for Disease Control and Prevention. “Coping with Extreme Heat.” Accessed August 19, 2021.