Just as facial blotting papers are used to control facial sebum, dry shampoos are used to control scalp sebum. Dry shampoo is an alcohol aerosol spray that contains starch to absorb sebum. It must be sprayed at least six inches away from the scalp to prevent hair clumping and a white residue. The shampoo is applied by lifting up the hair and spraying the product on the roots to hide the white residue. The hair is then brushed to remove the starch, which has absorbed the sebum.
The dry shampoo adds texture and increases apparent hair volume by increasing inter-hair shaft friction as some of the starch particulates remain on the hair.
The most important reason consumers use dry shampoo is to prolong the life of an expensive hair dye treatment. Water encourages diffusion of the hair dye out of the hair shaft leading to fading. This can be avoided by minimizing water contact.
While dry shampoo does absorb sebum, it does not replace a traditional shampoo and water cleansing. Dry shampoo is not hygienic and the residual starch resembles dandruff. The frequent use of dry shampoo can encourage seborrheic dermatitis resulting from fungal overgrowth on the scalp. Increased bacterial colonization can also lead to folliculitis. Patients with scalp disease should not use dry shampoo and patients without scalp issues should shampoo traditionally at least once weekly perhaps with the intervening use of dry shampoo.