1. How Is Cooling Sweat Different From Palm Sweat?
Most of the sweat that cools you comes from eccrine glands. Cleveland Clinic describes them as tiny structures in the skin, with millions spread across the body from the face to the feet, that make clear, watery sweat. When the sweat evaporates, it carries heat away and lowers your body temperature [1]. A second type, apocrine glands, also makes sweat, but the eccrine glands do most of the cooling work [1].
Palms and soles are a special case. A 1975 study notes that people sweat on their palms and the soles of their feet in response to stress and exercise, but not in response to heat [2]. So the damp palms you get when you are nervous are not your body trying to cool down. The rest of this article asks what that sweat might be for.
2. Why Do Running Animals Sweat on Their Paws?
People are not the only animals with sweaty feet. In 1975, Sue Adelman, C. Richard Taylor and Norman Heglund studied four mammals that have sweat glands on their footpads: the rat, the tenrec, the hyrax and the dog. All of them sweated on their paws while running [2]. The team asked whether this moisture helps the paws grip the ground. They blocked the sweating with a drug called atropine sulfate and measured the friction between paws and surface. According to the Japanese-language article this piece is based on, friction dropped sharply, and the researchers concluded that the main job of paw sweat is to stop slipping when running or climbing [2]. I could not read that part of the abstract myself. This was an experiment on animals, so it does not prove the same thing for people.
The same paper lists other ideas for palm and sole sweat in people: making the skin tougher and improving the sense of touch [2]. Grip is the idea the experiment was designed to test, which is why the next section looks at people.
3. Does Sweaty Skin Really Help People Hold Things?
In 1997, Allan Smith and colleagues at the University of Montreal tested a version of this idea on eight adults, five men and three women. They used scopolamine, a drug that reduces palm sweating, and asked the volunteers to lift and hold objects with different surface textures between thumb and index finger. The paper's title reports the result: less palm sweat meant lower skin friction, and the volunteers pressed harder to keep hold [3]. That is a small group, and I could not read how the result differed between surfaces.
This does not mean that more sweat always gives a better grip. Anyone who has tried to open a jar with soaking wet hands knows that too much water makes skin slippery. The result suggests that a little moisture can help and that having none at all can be worse. How much is ideal is a question the summaries I read do not answer.
4. Why Would Being Nervous Switch On Grip Sweat?
A situation that makes you tense is the kind in which an animal might need to run, jump or climb. If palm sweat prepares the paws and hands for that, then sweating when startled could be a leftover from this old job. This is an explanation, not something these experiments proved. Adelman and colleagues only showed that paws sweat while running and that blocking the sweat changes friction [2]. The papers I read do not test whether people grip better when they are nervous.
Today the same reaction often shows up when you are about to speak in front of a class, which is why it can feel like a nuisance. If your hands are often very wet and it bothers you, you are not alone. Talk with a parent or a doctor, who can check whether anything needs help.
5. Did Sweating Help Humans Become Good Long-Distance Runners?
The cooling kind of sweat may have a story of its own. In a 2004 paper in the journal Nature, Dennis Bramble and Daniel Lieberman argued that humans are poor sprinters compared with most four-legged mammals, yet can be very good at sustained long-distance running [4]. Lieberman said in a 2007 talk at Harvard that animals like antelopes and cheetahs are built for speed rather than endurance, and that humans are "the tortoises of the animal kingdom": slow and steady [5]. The Harvard Gazette report of the talk links this to humans being hairless and sweaty [5]. In the part of the abstract I could read, the paper itself does not mention sweat, so the link comes from the news report of the talk. Treat it as one research group's proposal, not a settled fact.
6. How Can You Notice Your Own Palm Sweat Safely?
Sit comfortably in a room that is not hot and look at the skin of your palm in good light. Then silently do a hard mental math problem, such as counting backward from 100 by 7, and look again. Some people see a faint shine appear, and some see nothing, which is also normal. To read more, the 1975 and 1997 studies listed below have short summaries online, and Lieberman's talk is reported on the Harvard Gazette site.