How Soap Cleans Your Hands

Most of us wash our hands several times a day without thinking about why it works. Yet the chemistry behind a bar of soap is surprisingly clever. The problem begins with the skin itself. It produces a thin layer of natural oil, and dirt, bacteria and viruses tend to stick to it. Because oil and water do not mix, rinsing with water alone removes much less than washing with soap. The water simply flows over the oily film and leaves most of it behind.

Soap solves this problem because of the shape of its molecules. Each molecule has two very different ends. The head is hydrophilic, which means it is attracted to water. The tail is hydrophobic: it avoids water but mixes easily with fats and oils. In other words, a soap molecule can hold on to water with one end and to grease with the other, which makes it a kind of go-between for two substances that normally refuse to combine.

When you rub your hands together with soap and water, the tails push into the oily film and the particles trapped in it. As more molecules join in, they form tiny spheres called micelles, with the tails pointing inward and the heads facing outward. A micelle therefore wraps a droplet of grease inside a shell that water accepts. Once the dirt is enclosed in this way, it is no longer attached to your skin, and the running water carries it down the drain. Soap can also damage some viruses directly. Flu viruses and coronaviruses are wrapped in a fatty outer layer, and soap molecules can break it apart, which stops the virus from working.

This explains why the way you wash matters as much as the soap you choose. Scrubbing creates friction, which loosens particles from the skin's surface, and the soap needs time to do its work. That is why health experts recommend at least twenty seconds, and why the backs of the hands, the spaces between fingers and the areas under the nails should not be skipped. Hot water is not necessary; warm or cold water cleans just as well, and very hot water can irritate and dry the skin. Hand sanitiser, by contrast, works by a different method: its alcohol damages germs but does not lift away visible dirt, and it works less well on greasy hands. Plain soap, used properly, remains hard to beat.