Sometimes you really do not have to wash your hair. Here is why.
Shampoos and conditioners fulfill very specific functions. Most balanced shampoos have a pH of +/- 5.5. This will ensure that the cuticle of the hair is opened to thoroughly clean the hair. The higher the pH, the more the cuticle will open.
Good conditioners have a pH around 4.5 to securely close the cuticle after shampooing. It hydrates the hair (i.e. replaces some of the oil that has been removed by the shampoo) and some will film the hair to smooth the cuticle and to prevent tangles. Conditioners are VERY important and should not be skipped, as you do not want to leave the cuticle open. That will lead to colour and moisture loss. In other words: Dry, dull hair.
Now water has a pH of +/- 7 – which is higher than that of a shampoo. Hence, it would be sufficient to “wash” your hair using only water and conditioning afterwards – i.e. Co-washing.
However, co-washing will not be sufficient if there is sebum on the scalp and roots or if you use any products that could possibly leave a residue that will lead to product build-up (i.e. hairspray, dry shampoo, etc.). Then you would need to use a shampoo that is both balanced and efficient.
Sebum is the oil that is produced by the sebaceous gland in the scalp. This oil is necessary to prevent hair from drying out, but dirt and dust accumulates on the oil, making it a little bit more difficult to remove to dirt and dust. Hence why shampoos are formulated to contain surfactants. A surfactant lowers the surface tension between the sebum and the water, making it possible to wash away the oil. Water alone (irrespective of its pH) cannot do this.
But, if your hair is never oily, or if you use products that are residue-free, co-washing could be a great way to save time and money. But remember – Always, always finish with a good quality conditioner! And remember – conditioner goes on the lengths and ends, not the scalp!