Washing the hair is a lot like washing skin. Water can remove most visible dirt and debris, but might not eliminate odors or oily deposits. Shampoo helps water remove dirt, debris, and odors, such as smoke or sweat effectively. Shampoos can also remove oil. The hair gets its oil from sebaceous glands that secrete oil called sebum, which keeps the hair moisturized.
Moisturized hair is less likely to break or look dry and frizzy. But too much moisture can make the hair look greasy, limp, and dirty. After several days without shampooing, oil tends to build up closest to the scalp, making any hair around the face look dirty. Most shampoos are designed to strip excess oil, which helps the hair look cleaner for longer.
Shampoo cleans the hair with chemicals called surfactants. These are soaps that remove surface debris from the scalp and hair. Many shampoos also contain compounds called sulfates, which produce a rich lather that removes oil from the hair. This can help the hair look cleaner, but it can also damage the hair.
Keeping at least some oil on the hair is important to protect the hair from damageTrusted Source. Some people opt to use sulfate-free or moisturizing shampoos to preserve hair health, although there is little evidenceTrusted Source that sulfate-free shampoos are less aggressive than regular products.
In summary, There is no correct amount of times that a person should wash their hair per week. Expectations about how hair should look and how often a person should wash their hair also vary with culture, age, and from decade to decade. How often someone washes their hair is a personal decision and not one that is likely to affect overall health.