Science Snippets: The Heat Is On

Draft script:

The 2 September 2024 headline at ABC News in Australia reads When heat turns deadly. The subhead: “A world-first study challenges our understanding of how humans cope with extreme heat.”

An engaging overview of the study is then described in several paragraphs:

“Owen Dillon’s heart is pounding. Sweat is dripping down his neck, and he’s feeling tired and weak.

Inside the climate changer where he’s sitting, it’s unbearably hot.

It’s been set to 54 degrees Celsius, with 26 per cent humidity – a combination believed to be lethal after six hours.

After just a short period of time, he understands why.

Owen has been put into the climate chamber by Jem Cheng, a research fellow at the Heat and Health Research Centre at the University of Sydney.

It’s part of a world-first study all about finding out at what point heat becomes deadly.

Fifteen years ago, scientists proposed an environmental threshold at which no person would be able to survive for six hours.

But these conditions have never been tested on humans.

Until now.”

The Research Fellow at the University of Sydney is then quoted in the article: “This study is all about human survivability.

We are the first to actually put people in these environments to actually see, physiologically, what is happening to their core temperature or to their heart rate.”

As I reported on 17 March 2022 in this space, the 35 C wet-bulb temperature commonly accepted as lethal is too high. Not surprisingly, the researchers focused on lethal wet-bulb temperatures in the ABC News article are not familiar with the peer-reviewed literature I cited more than two-and-a-half years ago. I’m not sure how they missed what I found, considering that this topic is their forte, not mine. However, they do conduct an important, empirical test of high temperatures and high humidity on young, healthy human subjects.

Wet-bulb temperatures and their lethality are evaluated based on temperature and humidity. Specifically, the name comes from the temperature a thermometer would read if its bulb was wrapped in a wet cloth. This wet cloth around the bulb would cool the thermometer the same way perspiration cools a human. A wet-bulb temperature of 35 C translates to a temperature of 35 C and a relative humidity of 100%. For those of you tuning in from the land of Fahrenheit, that’s 95 degrees and 100% relative humidity. In a word, it’s unbearable.

Imagine exercising in a very hot sauna. Now imagine running and jumping to escape from being murdered in that very hot sauna. With 35 C and 100% humidity, there is no escape. Either the murderer catches and kills you or you die from the heat and humidity.

Speaking of an either-or situation, the article at ABC News quotes the researcher in the study in concluding that there are two factors that can interfere with the human body’s ability to cool down. First, on a humid day, the air is saturated. The sweat produced by the human body has nowhere to go. The body cannot cool if the perspiration cannot evaporate off the skin. Second is the inherent limit imposed by the human body. Humans are physiologically capable of producing a finite amount of perspiration.

At the typical body temperature of 37 C, the human body produces a lot of perspiration. When the body temperature increases, problems arise. I conducted field-based ecological research in southern Arizona for 20 years. During this time, I saw many examples of mild heat exhaustion. I witnessed a few examples of severe heat exhaustion, and even heat stroke. Along with students and field technicians, I experienced severe heat exhaustion on a few occasions. I learned that there is a balance to be struck between working diligently on a hot day and surviving to work another day. Even drinking abundant water will not ward off lethal wet-bulb conditions. Allowing the human body’s core temperature to rise to 43 C, or 109 F, virtually guarantees death.

I used to tell my graduate advisees and field technicians that the problem with being a martyr was you had to die for the cause. Ecological field research is important. It’s not important enough to die.

The global situation has worsened considerably since I left behind my life of ecological field research in 2009. Last year, the hottest year on record, more than 47,000 people in Europe died from heat, according to a study published in Nature Medicine, a peer-reviewed journal. These deaths occurred in conditions below the commonly accepted 35 C wet-bulb threshold.

A significant advance in knowledge resulting from the study published in ABC News is that even for healthy, young people, the wet-bulb threshold could be as low as 25.8 C. That’s a huge difference from 35 C.

For older people—and I qualify for that category—the wet-bulb threshold could be as low as 21.9 C. That’s slightly cooler than 71.5 degrees Fahrenheit, otherwise known as room temperature.

The article at ABC News concludes with comments from the young man subjected to the experiment. He said “I would say the first time running 80km felt pretty similar to doing 90 minutes in that room. [80 km is 50 miles]

It’s definitely made me a lot more aware of the balance between temperature and humidity, and also a lot more aware of how that’s going to impact your ability to perform.

Now I can look at a weather forecast and say for sure that I will not go running that day.”

I made that same decision about 35 years ago. At that point, my favorite expression regarding exercise changed from “no pain, no gain” to “no pain, no pain.”