How to maintain the pH of your 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 horny layer and antimicrobial defence : we refer to this as "acid barrier" or "acid mantle".

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 pH of newborns' skin 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 cause variations in pH. In addition to endogenous factors, exogenous factors can also alter the skin's pH. Among these, we primarily find:

• Some cosmetic products : This is the case with soaps for example. Indeed, these have a basic pH around 9 - 10 which will disrupt the natural acidic pH of the hydrolipidic film, thus causing a 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 based on 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 for the skin.

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

As a reminder, the acidic pH of the skin preserves the integrity and cohesion of the stratum corneum and protects the skin from microbial infections. Indeed, when the skin's pH increases, the enzymes responsible for the production of ceramides, which have an optimal acidic pH, are inactivated, compromising the structure and function of the stratum corneum. Moreover, while the enzymes synthesising lipids decrease, other enzymes increase at acidic pH: the serine proteases. These lead to a reduction in corneodesmosomes, by degradation of desmoglein-1, which have the role of ensuring the cohesion of the elements of the stratum corneum.

When the pH of the skin is disrupted, the skin no longer resists external aggressions and no longer retains its hydration.

This is favourable to the development of dermatoses, such as atopic dermatitis. Indeed, it has been demonstrated that the higher the pH, the more intense the itching and the greater the dryness. Besides the fact that in atopic dermatitis the structure of the horny layer is altered, colonisation by the Staphylococcus aureus is a common characteristic of affected patients and is considered a major pathogenic factor of atopic dermatitis. However, the adhesion of Staphylococcus aureus to human keratinocytes increases with the rise in pH.

The growth of protective bacteria in the skin microbiota is optimal at acidic pH levels, whereas pathogenic bacteria thrive at neutral and basic pH levels (Staphylococcus aureus, Cutibacterium acnes). Thus, a disruption in skin pH disturbs the skin's microbiota and hinders anti-microbial protection. The risk of infection is therefore higher. Moreover, serine proteases, which are activated by a high pH, trigger the synthesis of cytokines that cause inflammation.

Maintaining the skin's pH balance is therefore essential in order to preserve the integrity of the stratum corneum and the protective role of the skin.