Why do underarms smell

Overview

Underarm odor, medically known as axillary bromhidrosis, has been a human concern for millennia, with ancient Egyptians using scented oils and Romans employing alum as early deodorants around 200 BCE. The scientific understanding of body odor advanced significantly in the 20th century, with researchers identifying the specific bacterial culprits and metabolic pathways. Historically, body odor was often attributed to poor hygiene or moral failings, but modern science reveals it as a natural biological process. The social significance of underarm odor has varied across cultures, with some societies embracing natural scents while others developed elaborate grooming rituals. In Western cultures, the commercial deodorant industry emerged in the late 19th century and grew into a multi-billion dollar market, with Americans spending approximately $18 billion annually on deodorants and antiperspirants by 2020. The perception of underarm odor as socially unacceptable became widespread in the early 20th century, coinciding with advertising campaigns that created what marketers called "the armpit offensive."

How It Works

Underarm odor develops through a specific biological process involving specialized sweat glands and skin bacteria. Humans have two main types of sweat glands: eccrine glands distributed over most of the body that produce watery sweat for temperature regulation, and apocrine glands concentrated in underarms, groin, and around nipples that secrete a thicker, milky fluid. Apocrine glands become active during puberty when stimulated by hormones, producing secretions containing proteins, lipids, and steroids. When this secretion reaches the skin surface, resident bacteria—primarily Staphylococcus hominis, Corynebacterium species, and Cutibacterium acnes—begin breaking down these compounds. The bacteria produce enzymes that convert odorless precursors into volatile organic compounds, particularly 3-methyl-2-hexenoic acid and thioalcohols like 3-methyl-3-sulfanylhexan-1-ol. These compounds are detectable at extremely low concentrations, with some thioalcohols noticeable at just 0.006 parts per billion. The warm, moist environment of the underarm provides ideal conditions for bacterial growth, allowing populations to reach 10^6 to 10^7 bacteria per square centimeter of skin.

Why It Matters

Understanding underarm odor has significant implications beyond personal hygiene. Scientifically, it provides insights into human-microbe interactions and chemical communication, as body odor contains pheromones that may influence social and sexual behavior. Medically, excessive or sudden changes in body odor can indicate health conditions like diabetes, liver disease, or metabolic disorders. The deodorant and antiperspirant industry represents a major economic sector, with global sales exceeding $70 billion annually, driving innovation in antimicrobial compounds and delivery systems. Socially, cultural norms around body odor affect daily routines, workplace expectations, and interpersonal relationships across societies. Research into natural odor variation has even contributed to forensic science, with studies showing individual odor profiles can help distinguish people with about 85% accuracy. Additionally, the genetic basis of body odor differences has implications for understanding human evolution and migration patterns.