Everything you need to know about the pH of the skin.

The skin is the largest organ of our body. It serves as our body's first line of defence against external aggressions.This protective role is made possible thanks to the acidic pH around 5.5 of the skin's hydrolipidic film. The hydrolipidic film, located on the skin's surface, is a complex emulsion primarily composed of sweat and sebum. The acidic pH of the hydrolipidic film is a key factor in the homeostasis of the barrier, the integrity of the stratum corneum, and antimicrobial defence : we refer to this as "acid mantle" or "acidic barrier". Indeed, ceramides, which are essential constituents of the stratum corneum, are synthesised at an acidic pH by pH-dependent enzymes.

The pH of the skin varies depending on endogenous factors, but also exogenous ones. Among the endogenous factors that can alter the pH, we can mention:

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

• Skin Zones : The pH of the skin is higher in certain areas of the body (armpits, groin, intra-mammary area...). In the armpits, a higher pH leads to colonisation by certain resident bacteria that produce odours, such as propionic bacteria and staphylococci. Thus, deodorants containing citrates reduce the pH and inhibit bacterial activity;

• Skin Type : Indeed, teenagers with oily and acne-prone skin tend to have a more alkaline skin pH.

Other endogenous factors, such as gender or skin pigmentation, can also affect pH levels. Indeed, due to a higher production of sebum and sweat than women, men have a more acidic skin pH. Furthermore, it has been proven that individuals with darker skin have a more acidic skin pH due to a higher presence of epidermal lipids. Besides endogenous factors, there are exogenous factors that can alter pH levels. Among them, we primarily find:

• Some cosmetic products : This is the case with soaps for instance. Indeed, these have a basic pH around 9 - 10 which disrupts the natural acidic pH of the hydrolipidic film, thus causing skin dryness and a risk of infections.

Please note : Unlike soaps which, with a basic pH, disrupt the skin's pH, cosmetics with an acidic pH between 3 and 4, such as products containing vitamin C or fruit acids, are beneficial for the skin because the enzymes responsible for the production of ceramides (lipid constituents of the skin barrier) have an optimal acidic pH. Thus, they help to strengthen the skin barrier. Beware! Some products like lemon juice, whose properties are praised on the internet, have a too acidic pH (2.4) and are therefore irritating to the skin.

Deregulating the skin's pH leads to unfortunate consequences for the skin. Therefore, it is important touse cosmetic products with a pH between 3 and 7.