There is a violation of the ph balance under the influence of water. The pH index determines the ratio of acids. and alkalis in a particular environment In our case it is water and the upper layer of the epidermis. For skin, the optimum pH is from 5 to 6 units, i.e. acidic. If this level is outside the 5 to 6 range, you are dealing with dehydration and other problems, up to acne. Tap water has a pH in the range of 6.5 to 9.5.