Skin pH - what does it mean?
I’ve always thought it weird when people say that skin has a certain pH. This seems a complete misunderstanding of the term pH as pH refers to the concentration of H+ ions in a solution. Since skin is a solid, it makes no sense that it has a pH. Solids do not have pH’s. 
Skin mantle
Although my initial reaction is correct, that’s not going to stop people from talking about the skin pH and the importance of making cosmetics that are “pH balanced.” So, if skin does not have a pH what could people be talking about when they say skin pH?
It turns out the pH of skin really refers to the pH of the acid mantle on the outer layer of the skin. In fact, the way that the pH of skin is measured is by using a flat pH electrode, which is placed directly on the skin after putting a drop of deionized water on it. The acid mantle is a thin layer coating on skin comprised of sweat and sebum. It has a pH range from 4.5 - 6.2 and is helpful in providing a suitable environment for good bacteria to grow while killing other microorganisms.
The skin pH is affected by a number of things including your genetic makeup, age, skin moisture, the amount you sweat and the environment. It is also affected by things like the application of cosmetics, detergents, water exposure, and occlusive wrappings (like bandages). Certain skin diseases such as contact dermatitis, acne, and ichthyosis can also have an effect on skin pH. The fact that some skin diseases tend to raise the pH of the skin acid mantle suggests that a suitable treatment for these conditions might be the use of products that have lower pH’s (like pH 3.5 - 4.5).
Skin mantle and cosmetic formulating
While you won’t be able to completely protect the skin acid mantle when formulating skin cleansing products, it is a good idea to create products that are slightly acidic so you minimize the damage your products are doing. This is why cleansers should be formulated in a range of 4.0 - 6.0 and even moisturizing lotions should be formulated on the acidic side.
