Researchers at Arizona State University have discovered that still air in extreme heat can trap a stagnant layer over human skin, blocking sweat evaporation.
The study found that at 40.5 degrees Celsius in dry, windless conditions, two opposing air currents around the body can cancel each other out. This phenomenon reduces sweat evaporation by more than 50 percent and causes the body to accumulate heat much faster.
Standard models used to calculate heat stress risk may be overestimating how much sweat actually evaporates from skin. As a result, environments previously considered manageable, such as unfinished buildings or partially enclosed workplaces, could be significantly more dangerous than current guidelines suggest.
Mechanics of sweat evaporation
The cooling effect of perspiration has been documented for centuries. In 1775, English physician Charles Brian Blagden and several colleagues conducted experiments by remaining inside rooms heated to over 110 degrees Celsius. They maintained relatively stable body temperatures by sweating heavily, demonstrating that sweat cools the skin as it absorbs thermal energy to transform from liquid into vapor.
Cooling slows down when sweat remains pooled on the skin or drips onto the ground. While dry ambient air normally accelerates evaporation, stagnant air prevents that moisture from dissipating efficiently.
A team led by Konrad Rykaczewski, an engineering professor at Arizona State University, identified the precise mechanism behind this stall. Two natural air movement forces occur simultaneously around human skin when hot air meets the body.
First, hot ambient air touching relatively cooler skin becomes denser and sinks downward. Second, moisture evaporating from sweat makes the surrounding air humid. Because humid air is lighter than dry air, it rises upward.
When environmental temperatures reach 40.5 degrees Celsius with low humidity and no wind, these upward and downward forces balance each other out. The result is a pocket of stagnant air wrapping around the skin, drastically lowering the rate of sweat evaporation.
According to the researchers, failing to account for this stagnant zone can lead heat safety models to underestimate the core body temperature rise of a resting person by nearly one degree Celsius after two hours of exposure.
Thermal mannequin testing and computer modeling
To investigate these extreme environmental conditions without putting human volunteers at risk, the research team used ANDI, a thermal mannequin engineered to replicate human body heat exchange. The mannequin features sensors across its surface and microscopic pores that release a liquid formulation designed to imitate human sweat.
The scientists conducted dozens of physical experiments using ANDI inside controlled environmental chambers. Following the physical tests, the team ran approximately 100 computer simulations using specialized software to model variations in temperature, humidity, and airflow.
The findings highlight a critical gap in traditional human thermal balance models. Although buoyancy forces are well understood in engineering disciplines and widely studied in electronic cooling systems, they have rarely been applied to human physiological heat exchange.
Safety guidelines and fabric innovation
The new study builds upon earlier research by the same team regarding how sweat spreads across human skin. Previous observations showed that sweat initially gathers around individual pores before forming a thin, continuous liquid film. When the water evaporates, residual salt left on the skin allows subsequent sweat droplets to spread even faster across the body. A continuous liquid film increases the surface area exposed to air, which helps promote evaporation.
Rykaczewski's team noted that integrating these fluid dynamics into human heat models could yield major practical benefits. Revised models will allow public health officials and industrial managers to establish more accurate safety guidelines for outdoor laborers and industrial workers exposed to high temperatures.
The findings could also aid engineers in designing more effective cooling systems and developing advanced synthetic fabrics. Next-generation garments could be engineered to hold sweat close to the skin while spreading it over a larger surface area, enhancing natural cooling even in stagnant air.
