You’ve just changed your shirt for the second time today. You showered this morning. You used deodorant. And when you raise your arm, there’s still that smell: not unpleasant exactly, but definitely there. Distinctly onion-like.
This isn’t a hygiene failure. It’s a very specific biological reaction that happens on most people’s skin to some degree, but is more pronounced in some people than others. The science behind it is genuinely interesting, and the specific mechanism points toward more effective fixes than just spraying more deodorant.
The Bacterium Behind the Smell
In 2020, researchers at the University of York published a detailed study on the bacterium responsible for the characteristic onion smell of armpit sweat. The culprit: Staphylococcus hominis.
S. hominis is a normal resident of human skin, particularly concentrated in the armpit. It’s not pathogenic (it doesn’t cause disease under normal circumstances). It’s just doing what bacteria do: metabolizing available nutrients in its environment.
The armpit’s available nutrients, in this case, come largely from apocrine sweat gland secretions.
The Chemical Reaction
Apocrine glands secrete a complex mixture of compounds into the armpit, including sulfur-containing amino acid derivatives related to testosterone metabolism. These compounds are largely odorless as secreted.
S. hominis carries an enzyme called C-S lyase that cleaves the sulfur-containing side chains off these compounds, releasing free thioalcohols. The key product is 3-methyl-3-sulfanylhexan-1-ol, abbreviated 3M3SH.
3M3SH is a thioalcohol (a sulfur-containing alcohol). Thioalcohols are among the most potently smelly organic compounds known, detectable by the human nose at extremely low concentrations. The detection threshold for 3M3SH is in the parts-per-trillion range.
And 3M3SH smells like onions.
The conversion is efficient: S. hominis is particularly well-equipped for this specific reaction, more so than other common skin bacteria. The reason S. hominis evolved this enzymatic activity isn’t fully understood, but the result is a distinctive odor signature that the human nose associates clearly with alliums.
Why Some People Have More Onion-Smell Than Others
The dominant variable is the population of S. hominis relative to other bacteria on your skin.
Some people’s skin microbiome is dominated by S. hominis. Others carry more S. epidermidis or Corynebacterium species, which process apocrine sweat compounds into different aromatic products (Corynebacterium produces more of the “cheesy” and “rancid” odor compounds; S. hominis specifically produces the thioalcohol/onion profile).
What determines your skin microbiome composition?
Genetics play a role. Skin pH, sebum composition, moisture levels, and immune factors all affect which bacteria thrive on your skin, and these are partly heritable. You inherit a tendency toward a certain microbiome composition.
Hygiene practices matter, but not in the way you think. Over-washing with harsh soaps disrupts the skin microbiome and can actually encourage S. hominis by reducing competing bacterial populations. Under-washing allows the bacterial population to grow larger, producing more substrate conversion and more odor. There’s a middle ground.
Testosterone-related compounds drive the substrate. Apocrine gland secretions include compounds related to testosterone metabolism. Higher androgen activity (higher in men, variable in women by cycle phase and hormonal status) produces more substrate for S. hominis to convert. This is part of why male-pattern body odor often has a stronger onion character.
Temperature and moisture. S. hominis, like most bacteria, thrives in warm, moist, dark environments. The armpit is ideal. The same person will produce more odor in summer than winter, during stress (which increases sweating), and in tight, non-breathable clothing.
Why Deodorant Often Isn’t Enough
Standard deodorants work by masking odor with fragrance and by creating a slightly unfavorable environment for bacteria (through alcohol, certain preservatives, or pH adjustment). They don’t specifically target S. hominis or specifically reduce its enzymatic activity.
Antiperspirants reduce sweat production, which reduces the moisture and substrate available for bacterial activity. This helps indirectly, but doesn’t eliminate the bacterial population.
Antibacterial deodorants (particularly those with triclosan, benzalkonium chloride, or high alcohol content) reduce bacterial populations more broadly. These can reduce S. hominis colonies, producing a more meaningful reduction in the thioalcohol production.
But the bacteria return. The armpit is a favorable habitat, and bacteria repopulate within hours to days of any disruption.
What Actually Works
Antibacterial soap, specifically in the armpit, regularly. Washing with soap that has antibacterial properties (look for benzalkonium chloride or triclosan, or simply soaps with high alcohol/isopropanol content) reduces bacterial load more effectively than regular soap. Apply to armpit skin specifically, leave for 30 seconds before rinsing.
Alcohol application between showers. A cotton pad with isopropyl alcohol (70%) applied to the armpit directly disrupts the bacterial population. It doesn’t last all day but is more aggressive at population reduction than typical deodorant.
Antiperspirant for sweat reduction. Less sweat means less moisture and less substrate for bacterial conversion. Antiperspirant (applied at night to dry skin for best effect) doesn’t kill bacteria but makes their habitat less hospitable.
Fabric choice. Natural fibers (cotton, linen, wool) are less hospitable to bacteria than synthetic fabrics. Synthetics trap moisture and heat more effectively, creating a better bacterial environment. This is why workout clothes in synthetics often develop persistent odor that regular washing doesn’t fully remove.
Exfoliating the armpit area. Dead skin cells provide substrate for bacterial growth. Light exfoliation of armpit skin removes this substrate and can reduce bacterial population density. Not often discussed but genuinely useful.
Dietary awareness. Eating garlic and onions adds dietary sulfur compounds that are excreted through sweat and breath, amplifying the effect. This isn’t the primary cause of the persistent onion smell but contributes to it acutely. Reducing these foods before situations where the smell matters reduces the dietary contribution.
The Future: Microbiome Engineering
The research that identified S. hominis specifically also opened the door to a more targeted solution: replacing S. hominis with bacteria that process apocrine sweat differently.
Studies have transplanted Staphylococcus epidermidis (a low-odor competitor) onto the armpits of people with strong body odor. S. epidermidis produces different metabolites from the same apocrine substrate, with significantly different and less prominent odor profiles. The transplanted bacteria colonized and partially displaced S. hominis, producing measurable odor reduction.
This isn’t a consumer product yet, but it demonstrates the principle: the bacteria are the key variable, and the right intervention targets them specifically rather than masking the result.
For now, the practical toolkit is: antibacterial washing, antiperspirant, breathable fabrics, and accepting that the onion smell is a bacterial feature of human biology rather than a personal failing.
→ Why Does Sweat Smell? The Science of Body Odor → Body Odor Causes: Why You Smell and How to Fix It → Bromhidrosis: When Body Odor Is a Medical Condition
Sources
- Thioalcohols as key compounds in human axillary odor, NCBI PMC
- Microbiome of human axilla and body odor formation, NCBI PMC
- Bromhidrosis, DermNet NZ
- Body odor, MedlinePlus