Human skins pH value is somewhere between 4.5 and 6. Newborn babies’ skin pH is closer to neutral (pH7), but it quickly turns acidic. This is probably to protect young children’s skin, since acidity kills bacteria. As people become older, the skin becomes more and more neutral, and won’t kill as many bacteria as before. This is why the skin gets weak and starts having problems. The pH value goes beyond 6 when a person actually has a skin problem or skin disease. This shows that it is necessary to choose cosmetics that have a pH value close to that of your own skin.
 
The skins pH value is one of the major contributors to acne and other skin problems. P.acnes is a normal bacteria found in all persons regardless of the presence or absence of acne. However, in individuals prone to acne, the number of P.acnes is greatly increased. It has been found that the growth of P.acnes is very much dependent on the pH value of the skin. Its growth is at its minimum at the normal skin pH of 5.5. A slight shift towards the alkaline pH would provide a better environment for P.acnes to thrive.

Ordinary soap is one of the worse pH offenders in that most commercially available skin care soaps are very alkaline (pH range 9-11) which changes the skin’s pH to be more alkaline, if not adjusted with a toner or other product, allowing for the growth of bacteria.