Dogs are not the only mammals that can smell fear — humans can too!

Humans produce two main types of sweat. Eccrine glands cover most of the body and release watery sweat to cool us down. This sweat is mostly a saltwater mix and carries little odor. Apocrine glands sit in the armpits and groin and respond to emotional arousal like fear, stress, pain, and excitement. They release a thicker secretion filled with proteins, lipids, and steroid compounds.

The apocrine secretion does not smell at first. Skin bacteria break it down into volatile molecules that create the familiar “sweat” odor. Scientists have identified sulfur-containing compounds, which produce the onion-like, musky, or pungent scent. This explains why underarm sweat from anxiety smells different from sweat on the forehead during a run.

Researchers suspect that this difference exists for a reason. A 2015 study discovered that smelling sweat collected from people experiencing fear changed how others behaved. Participants who were exposed to the fear-related sweat responded faster to threatening events, scanned their surroundings more carefully, and entered into a state of vigilance.

Another large study deepened this picture. Jan Havlíček, Jasper de Groot, and colleagues measured how much underarm sweat people produced while watching fear-inducing, happy, or neutral videos. Fear did not just change sweat chemistry. It changed sweat volume. Men, in particular, secreted significantly more apocrine sweat when they reported feeling intense fear. Happiness and rest did not trigger the same effect. Quantity matters for airborne signals. More sweat releases more volatile compounds into the air, which increases the chance that someone nearby detects them. This study added to the evidence that fear appears to be a chemical broadcast to others.

A 2020 study also connected this process to the fast-acting fight-or-flight mechanism. When participants anticipated giving a stressful speech, their heart rate rose quickly and their apocrine glands produced more sweat, causing others to display fearful facial expressions and increased vigilance. The slower hormonal stress response did not produce the same social effect. Findings from a 2015 chemosensory study found that fear sweat moved through the air as a part of an alarm system, creating a “contagious” effect.

Together, these findings suggest how this transfer may have functioned in early human groups. A person experiencing fear would release more apocrine sweat, and then the resulting volatile compounds would enter the shared air. Others nearby would shift into a state of alertness without needing to speak or gesture. This signal would travel quietly and unconsciously prepare others to respond.

The same biology applies today, but the difference lies in how modern habits treat that signal. What was once an evolutionary necessity is now interpreted as an unpleasant odor. No one wants to smell B.O. during a final exam or on their wedding day, so it’s masked with deodorant and antiperspirant. In doing so, a subtle chemical signal that once connected humans through shared awareness of danger is blocked.

But never forget, fear still has a scent. We just cover it up.