Of all the animals on Earth, humans are the best at sweating. We’ve got not one, but two sets of sweat glands: one to cool us off, and one that kicks in when we’re anxious.
Our bodies need to stick right around 98.6°F (37°C), so when warm weather or exercise makes us overheat, eccrine glands just under the surface of our skin secrete water and some salts, sometimes more than a liter per day. As this water evaporates, it cools us down. That’s the sweating you’re probably most familiar with.
But we also sometimes sweat when we’re not overheated. That happens when heightened anxiety and nerves trigger the flight-or-fight response response of our sympathetic nervous system. That speeds up our breathing and heart rates, as the body tries to direct more blood away from the extremities and toward its vital organs (which is why you may get literal cold feet). It also turns on our second set of sweat glands, called apocrine glands, which are in our hairiest regions, including the armpits, groin, chest, back, and head. (We have eccrine sweat glands there, too.)
This anxiety sweat is physically different than heat sweat.”It’s a thick, milky liquid,” says Ivan Ong, the head of research and development at Microban, an anti-microbial company based in North Carolina. “It’s not just water—it’s fatty acids, steroids, proteins, and that comes from the glands that are associated with hair follicles.” Heat sweat, on the other hand, is basically just water with a little salt.
The chemical mixture of sweat from apocrine glands happens to make a fantastic meal for bacteria living in and around our hair follicles and pores. As the bacteria break down these chemicals, they release a notorious odor—far worse than that produced by regular sweat, although sweaty clothes can also become smelly microbial feeding grounds.
Heat sweat has an obvious evolutionary purpose that’s still relevant today. But nervous sweat just seems like an inconvenience. It can even cause more anxiety, putting you into a nervous-sweat feedback loop.
Most sweating disorders, like hyperhydrosis, are caused by overactive eccrine glands. Because excessive nervous sweating is less common,scientists haven’t looked too hard into why human ever evolved to sweat nervously—although there is one gross theory.
It could be that we used to need this fatty sweat to retain the watery sweat produced in bouts of prolonged physical stress, like running for hours while hunting (or being hunted). “An unstable sweat drop that falls to the ground is a sweat drop wasted,” wrote Alan Porter in a 2001 article published in the Journal of the Royal Society of Medicine. His theory is that apocrine sweat would have been produced as a result of the adrenaline rush of physical sweat, and could then mix with eccrine sweat and other skin oils to form a long-lasting cooling goop.
Extreme cold may also have been a physical stressor that humans dealt with by releasing apocrine sweat, Porter wrote, in this case to act as a sort of insulation. “Bearing in mind the caveman was pretty hairy all over, that makes sense,” Ong says. Adding oils to these hairs would make them stick together like a thin coat. Now, we are mostly hairless and most things that stress us out don’t require a physical response.
There’s no lab test to verify this theory, so it’s impossible to prove. But it sure is comforting to know that flopsweat may have been useful once, even if it’s now just a pain.