Scientists discover hidden physics limiting sweat's cooling power in hot, dry air

In 1775, an English physician named Charles Brian Blagden walked into a room heated to over 230 degrees Fahrenheit—hot enough that raw meat would cook—and stayed there with a handful of friends to see what would happen. His body did something remarkable: it sweated, the moisture evaporated, and his core temperature barely budged. The mystery of why perspiration worked was solved. Or so it seemed.

Two and a half centuries later, Konrad Rykaczewski, an engineer at Arizona State University, discovered that the science of sweating still had major gaps. In research published this month in Science Advances, he and his team identified a hidden physical process that can slash sweat's cooling power by more than half in certain conditions—specifically in hot, dry air with no wind. The finding challenges the mathematical models scientists have relied on for decades to predict how much heat stress a person will experience.

The mechanism is counterintuitive. When the air around you is hotter than your skin, that air cools down and becomes denser, sinking downward. But when sweat evaporates in dry conditions, it releases moisture that makes the air lighter, causing it to rise. In still, arid environments around 105 degrees Fahrenheit, these two opposing currents can cancel each other out entirely, creating a dead zone of stagnant air right at the skin's surface. Without any breeze to carry away the humid layer, sweat sits there unable to evaporate efficiently. The body keeps heating up.

Rykaczewski's team didn't test this on human volunteers in an oven. Instead, they used ANDI, a sophisticated manikin equipped with thousands of sensors and artificial pores that release simulated sweat. They ran the machine through dozens of experiments and built a computer model that simulated roughly 100 different sweating scenarios. The math was brutal—isolating each heat-transfer pathway, replicating it in code, then combining them all into one coherent model took what Rykaczewski described as tremendous effort. But the results were clear: existing heat-balance models, the ones used to guide safety recommendations for construction workers, soldiers, and athletes, were significantly underestimating how hot the human body would get. In a person at rest in still air for two hours, they were off by nearly two degrees Fahrenheit in core body temperature.

This matters most in places where air doesn't move. A tent. A partially enclosed work site. An unfinished building. An indoor gym on a humid day. These are the environments where the hidden physics Rykaczewski uncovered becomes dangerous. The research also builds on earlier work his team published, in which they discovered that sweat doesn't simply wet the skin uniformly. It pools around pores, forms thin films, and spreads differently depending on whether the skin has been sweated on before. Salt residue from earlier perspiration actually helps new sweat spread faster, which could make evaporation more efficient—but only if the air around it is moving.

The practical applications are already taking shape. Rykaczewski's team is now studying how sweat either clings to skin and evaporates or runs off before it can cool the body, and they're planning experiments on how different clothing textiles interact with sweat. They're also conducting field studies across Arizona, measuring how different populations respond to extreme heat using advanced sensors and their improved evaporation models. The goal is to design better cooling clothing, safer buildings, and more effective heat-management systems for people whose work or sport puts them in dangerous thermal conditions.

What strikes Rykaczewski about his own research is how universal the subject is. Sweating is not glamorous. It's, as he put it, kind of an icky topic. But everybody does it, which means the physics of perspiration touches nearly every human being on Earth. A career spent understanding something so fundamental, so ordinary, and so consequential—that's the opposite of obscure research that only twenty experts care about. It's the kind of work that could change how we keep people safe in heat.