Why Your Skin Starts Smelling Different After 30 (Meet Nonenal)

Researchers at Shiseido's skin science lab in Yokohama identified a previously overlooked molecule rising from the skin of volunteers over 40. They called it 2-nonenal—a greasy, grassy-smelling aldehyde that younger skin barely produces. The discovery had a name in Japanese culture already: kareishu, or "old-person smell." Westerners recognize it too, though we're less direct about it. It's the faint, musty-oily scent that clings to pillowcases and shirt collars, especially after a long day. And it starts, reliably, around your third decade.

This isn't about hygiene. You can shower twice daily and still produce it. The culprit is a shift happening in your skin's lipid chemistry—and the microbial community that feeds on it.

What nonenal actually is

Nonenal is an unsaturated aldehyde, a breakdown product of omega-7 fatty acids. These fatty acids—palmitoleic acid, in particular—are secreted by your sebaceous glands as part of the skin's natural oil barrier. In younger skin, antioxidant defenses keep these lipids stable. But after about 30, two things happen: your skin produces more palmitoleic acid, and its antioxidant capacity starts to decline. The result is oxidative breakdown—lipid peroxidation, to be precise—which releases nonenal into the air around you.

The 2001 Shiseido study confirmed that nonenal levels spike in the skin surface lipids of people over 40, regardless of gender. The smell is subtle at first, but persistent, because nonenal doesn't evaporate quickly. It binds to fabrics and accumulates in enclosed spaces.

The microbes that make it worse (or better)

Your skin microbiome metabolizes the lipids your glands secrete. Certain bacteria—particularly Staphylococcus and Corynebacterium species—break down triglycerides and fatty acids, creating volatile compounds. Some of these smell fine. Some don't. As your skin's oil composition shifts with age, so does the substrate available to these microbes, and so does the odor profile they generate.

Interestingly, Staphylococcus epidermidis, one of the skin's dominant residents, produces enzymes that can further oxidize fatty acids. This can amplify nonenal production—or, in some cases, convert it into less odorous metabolites, depending on the strain and the local environment. The microbial balance matters. (For more on how bacterial populations respond to changing sebum, see our overview of skin pH and the microbiome.)

Age also brings hormonal shifts that alter sebum flow and composition. During menopause, skin microbiome changes are especially pronounced, as estrogen decline reduces lipid production in some areas and increases it in others—reshaping both the chemical and microbial landscape.

Why this matters for your skin

Nonenal isn't harmful, but its presence signals a real shift in your skin's surface chemistry—one that affects not just scent, but also barrier function, hydration, and microbial stability. Understanding that the smell isn't a hygiene failure but a lipid-microbe interaction can reframe how you think about aging skin care.

References

• Haze S, Gozu Y, Nakamura S, et al. 2-Nonenal newly found in human body odor tends to increase with aging. J Invest Dermatol. 2001;116(4):520-524.
• Byrd AL, Belkaid Y, Segre JA. The human skin microbiome. Nat Rev Microbiol. 2018.
• Grice EA, Segre JA. The skin microbiome. Nat Rev Microbiol. 2011.
• Nakatsuji T, Chen TH, Narala S, et al. Antimicrobials from human skin commensal bacteria protect against Staphylococcus aureus and are deficient in atopic dermatitis. Sci Transl Med. 2017.