Professor Byung Mook Weon of the School of Advanced Materials Science and Engineering led pioneer research that for the first time identified the incomplete evaporation of sweat droplets as the physical source of the 'Heat Index' that determines body temperature.

As global warming has led to more hot and humid days, the deaths associated with heat waves have been rising in Europe and the United States. The body's feel temperature, which varies with air temperature and humidity, is determined by the ‘Heat Index’, used in weather forecasting all around the world. It is known that people experience higher temperatures on humid days compared to dry days. For example, a day with 32°C and 70% humidity has a ‘Heat Index’ of 41°C, meaning that the feeling temperature in the human body is much higher than the actual air temperature. Therefore, very moist and hot days can be life-threatening because of the high ‘Heat Index’ values, leading to difficulty in maintaining body temperature. However, little is known about the exact physical roots of the ‘Heat Index’. All we know is that hot and humid days are hard to endure.

Our bodies maintain body temperature by releasing sweat and transferring heat as these sweat droplets evaporate, a phenomenon understood as evaporative cooling. Maintaining body temperature is a vital life-sustaining process. It is directly related to our health, whether it's in our daily lives or when we exercise. It is well known that sweating reduces body temperature. Still, there is a need to clarify how exactly the mechanism of thermoregulation by sweat relates to the ‘Heat Index’ depending on temperature and humidity conditions.

Based on the fact that sweat droplets and pure water droplets have a compositional difference in salts of around 1%, Prof. Weon’s research team including Dr. Beigtan and Dr. Gonçalves, closely observed the evaporation of sweat droplets as a function of temperature and humidity. Their results report that, unlike pure water droplets, sweat droplets do not completely evaporate in hot and humid conditions, but instead undergo "incomplete evaporation" in which some water is retained. By measuring the heat transfer while sweat droplets undergo evaporation in different conditions, this team was the first to show that incomplete evaporation of sweat droplets is the physical source of high ‘Heat Index’ in humid environments.

At low humidity, sweat droplets evaporate completely, leaving only sweat deposits, while at high humidity, both sweat deposits and moisture are retained. This is because at high humidity, sweat residue continuously absorbs moisture from the surrounding air. This incomplete evaporation reduces the efficiency of heat transfer by sweat evaporation, increasing the ‘Heat Index’ and reducing the body's ability to regulate its temperature via evaporative cooling.

The understanding of how sweat droplets maintain body temperature, as revealed by this research, is expected to have far-reaching implications. It is crucial for human health, sports science, weather forecasting, and the development of functional materials, particularly in the context of the current climate crisis characterized by wet and hot weather, most recently called “global boiling”.

The findings were published on April 16 as a cover story in Environmental Science & Technology, a prestigious journal in the field of environmental science by the American Chemical Society. This research was supported by Amorepacific's research grant.

※ Journal: Environmental Science & Technology (2024), Impact factor 11.4, JCR Top 6.7% in Environmental Science.

※ Paper title: Heat transfer by sweat droplet evaporation

※ DOI 10.1021/acs.est.4c00850

https://pubs.acs.org/doi/10.1021/acs.est.4c00850

※ Co-first authors: Dr. Mohadese Beigtan and Dr. Marta Gonçalves

※ Corresponding author: Prof. Byung Mook Weon (Sungkyunkwan University - Department of Advanced Materials Science & Engineering)

Research shows how incomplete evaporation of sweat droplets contributes to ‘Heat Index’ values.

Cover paper, Published on April 16th