Humans do not possess specialized “scent glands” like those found in other mammals used for marking territory or attracting mates. Instead, humans utilize adapted glands present across the skin for chemical signaling. These structures primarily serve other purposes but produce secretions that interact with the environment to create a unique chemical profile. This profile acts as a form of non-verbal communication, generated through a complex interaction between skin anatomy, glandular secretions, and surface microorganisms.
The Glands Responsible for Body Odor
The characteristic human scent originates from three primary types of glands embedded within the skin: apocrine, eccrine, and sebaceous glands. Apocrine glands are the structures most analogous to the scent glands of other animals, though they are much more localized in humans. These glands are concentrated in areas containing hair follicles, such as the armpits and groin, and become fully active after puberty, responding to emotional stimuli.
The secretion from apocrine glands is a thick, oily fluid rich in proteins, lipids, and fatty acids. This substance is initially odorless as it is discharged into the hair follicle, making apocrine glands the major source of body odor precursors.
Eccrine glands are the most numerous sweat glands, distributed over almost the entire body surface. Their primary function is thermoregulation, producing clear, watery sweat to cool the body through evaporation. Although eccrine sweat is generally odorless, its presence creates a moist environment that encourages microbial growth, indirectly influencing body odor.
Sebaceous glands also contribute to skin chemistry by secreting sebum, an oily substance. Sebum is a complex mixture of lipids whose main role is to lubricate and waterproof the skin and hair. The breakdown of sebum by bacteria, particularly on the scalp and face, adds to the compounds that form an individual’s total scent profile.
The Mechanism of Odor Production
Glandular secretions are not inherently odorous until processed externally by the skin’s natural microbial inhabitants, which form the skin microbiome. This community of bacteria and other microorganisms varies significantly across body regions and between individuals. The warm, moist environment of the armpit, rich in apocrine compounds, provides an ideal habitat for specific odor-producing bacteria.
Certain bacteria, notably Corynebacterium and Staphylococcus hominis, possess enzymes that break down the odorless precursors in apocrine sweat. These microbes metabolize fatty acids and proteins into smaller, highly volatile compounds (VOCs). These VOCs readily evaporate into the air and are detected as scent.
Key odorants contributing to characteristic human body odor include short-chain fatty acids like 3-methyl-2-hexenoic acid (3M2H), which has a goat-like smell. Thioalcohols, such as 3-methyl-3-sulfanylhexan-1-ol (3M3SH), contribute an onion or rotten meat-like note, even in trace amounts. These compounds result from bacteria enzymatically cleaving sulfur-containing precursors secreted by the apocrine glands. The composition of these VOCs, and the resulting body odor, is influenced by factors including diet, age, sex, and health.
Chemical Communication and Human Pheromones
The chemical profile generated by the glands and microbiome serves a biological purpose in communication, though it is not as overt as in other animals. Pheromones are secreted chemical signals that trigger a specific behavioral or physiological response in the same species. Many mammals use the vomeronasal organ (VNO) to detect pheromones, but the human VNO is considered non-functional in adults, lacking the necessary sensory neurons.
Humans still respond to certain airborne chemical signals, which are detected through the main olfactory system. Steroids found in apocrine secretions, such as axillary steroids, are considered putative human pheromones. They can activate brain areas associated with social and emotional processing, such as the hypothalamus. Human scent can communicate non-conscious information related to fear, emotional state, or attraction.
Research has explored how chemical cues may influence phenomena like menstrual cycle synchronization. However, the exact mechanism and conscious influence of these human chemical signals remain a subject of ongoing investigation. Humans emit a complex array of volatile compounds that convey information, but this signaling is more subtle and less direct than the specialized scent marking seen in other species.