How much heat can humans survive? Scientists reveal

Scientists reveal the maximum heat and humidity humans can survive and warn of the rising threat of climate change on human health.
Rizwan Choudhury
A senior man avoiding extreme heat.
A senior man avoiding extreme heat.

Credit: Liudmila Chernetska/iStock 

A new study has revealed the maximum combination of heat and humidity that a human body can tolerate before it starts to overheat and shut down.

According to the researchers, even a healthy young person would die after six hours of exposure to 35 degrees Celsius (95 Fahrenheit) with 100 percent humidity. However, this limit could be lower for some people, depending on various factors.

This is because, at such high levels of heat and humidity, the body’s main cooling mechanism – sweating – becomes ineffective, as the sweat cannot evaporate from the skin. This leads to a rise in the core body temperature, which can cause heatstroke, organ failure, and death.

Wet bulb temperature

This extreme condition is measured by what is known as “wet bulb temperature”, which takes into account both the dry heat and the humidity in the air. The wet bulb temperature is calculated by wrapping a wet cloth around a thermometer and exposing it to the air.

The researchers said that the wet bulb temperature of 35 degrees Celsius is the critical limit for human survival, and it has only been breached around a dozen times, mostly in South Asia and the Persian Gulf. However, none of those events lasted more than two hours, so there have been no mass casualties linked to this limit.

But the researchers warned that as global temperatures rise due to human-caused climate change, such dangerous wet bulb events will also become more frequent and widespread.

As Science Alert reported on a study, led by Colin Raymond of NASA’s Jet Propulsion Laboratory, projected that wet bulb temperatures will “regularly exceed” 35 degrees Celsius at several locations around the world in the coming decades if the world warms 2.5 degrees Celsius above pre-industrial levels.

To test the human survival limit, another team of researchers at Pennsylvania State University in the United States conducted an experiment with young, healthy volunteers inside a heat chamber.

They found that the participants reached their “critical environmental limit” – when their body could not stop their core temperature from rising – at 30.6 degrees Celsius wet bulb temperature, well below the previously assumed 35 degrees Celsius.

The team calculated that such conditions would take five to seven hours to reach core temperatures that are extremely dangerous.

However, not everyone has the same tolerance for heat and humidity, and some people are more vulnerable than others.

A related study on South Asia

Another study by Joy Monteiro in Nature examined wet bulb temperatures in South Asia. Wet bulb temperatures measure the combined effect of heat and humidity on human health and survival. 

Monteiro found that the most deadly heatwaves in the region were below the lethal threshold of 35 degrees Celsius. However, he also said that human endurance depends on many factors, such as age, health, and social and economic conditions. For instance, people who lack access to toilets may drink less water and become dehydrated.

Effect on different groups

The article also highlighted the different groups of people who are more vulnerable to heat and humidity, such as children, older people, and outdoor workers. Ayesha Kadir, a pediatrician and health advisor at Save the Children, said that children have more difficulty regulating their body temperature and are affected by their environment. 

Older people have fewer sweat glands and are more likely to suffer from heat-related illnesses. In fact, nearly 90 percent of the heat-related deaths in Europe last summer were among people over 65. The ability to cool down occasionally, such as in air-conditioned spaces, is also crucial for survival. 

Raymond said that climate change impacts those who are least able to protect themselves the most. He also linked wet bulb temperatures to El Nino events and ocean surface temperatures, which hit a record high last month, according to the European Union’s climate observatory.