Normal tap water contains calcium and magnesium. When ordinary soap is used with tap water, calcium and magnesium salts of fatty acids are formed. These are “sticky,” not readily soluble salts. The salts remain on the skin surface and may lead to skin irritation.
Another reason regular soap may cause skin irritation is that it has a high pH. The pH of regular soap lies between 9 and 10 (and sometimes higher than 10)—much higher than the normal skin pH (which is between 4 and 6.5). Consequently, it raises the skin’s pH. However, healthy skin has mechanisms for adjusting its pH, so that shortly after it has been exposed to regular soap, its level of acidity returns to normal. (The pH returns to normal any time from half an hour to two hours after soap has been used.) Nevertheless, in some people, abrupt changes in pH can cause significant skin irritation. Therefore, the current trend in the cosmetics industry is to adapt the pH of cleansing agents and other cosmetic preparations to that of normal skin