As skin matures, it produces more of a compound called 2-nonenal. It's made when fatty acids on the skin's surface break down, and the process speeds up in your forties and fifties. The tricky part: 2-nonenal is oil-based. It clings to skin, fabric, and bedding in a way that surface sweat doesn't. And most soap is designed for surface dirt and sweat, not for the residue that 2-nonenal leaves behind. Which is why a lot of women shower twice a day and still feel like something isn't quite right. It's not a hygiene problem. It's chemistry.