Everything You Need To Know About Skin pH.

The skin is our body's largest organ. It is our body's first line of defense against external aggression. This protective role is made possible by the acidic pH of around 5.5 of the skin's hydrolipidic film. The hydrolipidic film on the skin's surface is a complex emulsion composed essentially of sweat and sebum. The acidic pH of the hydrolipidic film is a key factor in barrier homeostasis, stratum corneum integrity and antimicrobial defense: it's known as the “acid barrier” or “acid mantle”. Ceramides, essential components of the stratum corneum, are synthesized at an acidic skin pH by pH-dependent enzymes.

Skin pH varies according to both endogenous and exogenous factors. Endogenous factors that can cause pH to vary include the following:

• Age: The skin pH of newborns is much higher than that of adults, with a value close to 7 (neutral pH), and decreasing over time;

• Skin zones: Skin pH is higher in certain areas of the body (armpits, groin, intra-mammary area…). In the armpits, a higher pH leads to colonization by certain resident odor-producing bacteria, such as propionic bacteria and staphylococci. Deodorants containing citrates reduce pH and inhibit bacterial activity;

• Skin type: Teenagers with oily, acne-prone skin have a rather alkaline pH.

Other endogenous factors, such as gender or skin pigmentation, can also vary the pH. Because men produce more sebum and sweat than women, their skin pH is more acidic. What's more, it has been proven that people with more matte skin have a more acidic skin pH, due to a greater presence of epidermal lipids. In addition to endogenous factors, there are exogenous factors that can vary the pH balance of skin. These include

• Certain cosmetic products: most soaps, for example. They have a basic pH of around 9 – 10, which disturbs the natural acid pH of the hydrolipidic film. This results in dry skin and the risk of infection, meaning that the pH balance is off level.

Please note: Unlike soaps, which have a basic pH that disrupts the skin's pH, cosmetics with an acidic pH of between 3 and 4, such as vitamin C or fruit acid-based products, are beneficial for the skin. This is due to the fact that the enzymes responsible for producing ceramides (lipid constituents of the skin barrier) have an optimal acidic pH. In this way, they help strengthen the skin barrier. But be careful! Some products, such as lemon juice, whose properties are touted on the Internet, have a pH that is too acidic (2.4) and are therefore irritating to the skin.

It can have unfortunate consequences when the pH balance is off. It is therefore important to use cosmetic products with a pH between 3 and 7.