As people age, the chemistry of their skin changes, leading to the production of a specific odor compound that regular washing cannot fully remove. This change becomes noticeable after the age of 40 and is caused by a molecule called 2-nonenal. Identified in 2001 by researchers at the Shiseido Research Center in Yokohama, 2-nonenal is an unsaturated aldehyde that appears on the skin after this age and remains there. This phenomenon is not due to poor hygiene but is a natural result of the aging process. By the age of 60, this compound tends to concentrate in five specific areas of the body. The skin's outer layer, or epidermis, produces more omega-7 fatty acids as people age. These lipids are prone to oxidation when exposed to air, which leads to the formation of 2-nonenal. This compound has a rancid or waxy smell and becomes more concentrated as people enter their 40s. According to research published in the Journal of Investigative Dermatology, the team studied skin secretions from volunteers aged 26 to 75 and found that the levels of this aldehyde increase steadily with age. Unlike many other sweat components, 2-nonenal does not dissolve in water, meaning that regular cleansers cannot fully remove it. It also sticks to fabric, which means freshly washed clothes may still carry the scent. Wearing the same bedding for several nights can leave a lasting odor. As the body’s antioxidant defenses weaken with age, the oxidation of surface lipids accelerates, increasing the concentration of 2-nonenal. This phenomenon is universal but has a specific name in Japan: kareishū, and it is addressed through specialized skincare products. The five areas most affected by 2-nonenal are the tongue, nostrils, under the armpits, the feet, and the navel. These regions tend to accumulate the compound due to factors such as microbial growth, dryness, and lack of proper cleaning. For example, the tongue can trap food particles and bacteria between its papillae, contributing to bad breath. The nostrils may collect secretions, especially during dry winter months. The feet, constantly exposed to moisture and pressure, are prone to fungal infections if not properly dried. The navel, often neglected, can trap sweat and dead skin cells. To manage the effects of aging on body odor, special attention should be given to these areas during showering. Using warm water and a gentle cleanser, along with specific tools like a tongue scraper or a soft brush, can help. The feet and navel should be thoroughly dried, especially in creases. A deodorant can help under the armpits but should not replace proper hygiene. For the nostrils, a rinse with physiological serum can soothe dry mucous membranes. Products with natural antioxidants, such as those found in persimmons, may help reduce the presence of 2-nonenal on the skin. If an odor is accompanied by redness, pain, or skin lesions, it is advisable to consult a dermatologist.