The consequences of a disrupted skin pH.

The skin is the largest organ of our body. It's our body's first line of defense against external influences. This protective function is made possible by the acidic pH of about 5.5 of the skin's hydrophilic lipid membrane . The hydrolipidic film on the surface of the skin is a complex emulsion composed mainly of sweat and sebum. The acidic pH of the hydrolipidic film is an important factor in the homeostasis of the barrier, the integrity of the horny layer, and the antibacterial defense.

The skin's pH varies depending on both endogenous and exogenous factors. Among the endogenous factors, 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 (armpits, groin, intra-mammary area...). In the armpits, a higher pH leads to colonization by certain resident bacteria that produce odors, such as propionic bacteria and staphylococci. Therefore, 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.

• Gender : Due to a higher production of sebum and sweat, men have a more acidic skin pH than women.

• Skin Pigmentation : Studies have proven that matte skins have a more acidic skin pH than fair skins. This is explained by the higher presence of epidermal lipids, which result in a lower pH and better integrity of the barrier function.

As mentioned earlier, there are endogenous factors, such as age or skin type, that influence the value of skin pH. However, certain external factors can disrupt the pH balance of the skin.

This is the case with soaps, for instance. Indeed, soaps have a basic pH around 9 - 10 which disrupts the natural acidic pH of the hydrolipidic film, thus leading to skin dryness and a risk of infection. Therefore, it is necessary to favor cleansers with a pH between 4 and 7.

Note : The same applies to solid shampoos that do not respect the hydrolipidic film on the scalp.

Unlike soaps that, with a basic pH, disrupt the skin's pH balance, cosmetics with an acidic pH between 3 and 4, such as products containing vitamin C or fruit acids, are beneficial for the skin because theenzymes responsible for the production of ceramides(lipids that form the skin barrier) have an optimal acidic pH. Therefore, 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.

Furthermore,the use of cosmetics unsuitable for your skin type can disrupt the pH balance of the skin. For instance, the pH of acne-prone skin is alkaline. In this case, using a soap, which also has an alkaline pH, exacerbates acne because the bacteria responsible for acne thrive in a basic pH environment.

No matter your skin type, it is recommended to favor 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 deactivated, compromising the structure and function of the stratum corneum.

Furthermore, while the enzymes synthesizing lipids decrease, other enzymes increase at acidic pH: the serine proteases. These lead to a decrease in corneodesmosomes, through the degradation of desmoglein-1, which are responsible for ensuring the cohesion of the elements of the stratum corneum.

When the pH of the skin is deregulated, the skin no longer resists external aggressions and does not retain its hydration. This is conducive to the development of dermatoses such as atopic dermatitis. Indeed, it has been shown that the higher the pH, the more intense the itching and the greater the dryness.

Beyond the fact that in atopic dermatitis the structure of the stratum corneum is altered, colonization by Staphylococcus aureus is a common characteristic of affected patients and is considered a major pathogenic factor in 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 microbiome is optimal at acidic pH levels, while pathogenic bacteria thrive at neutral and basic pH levels (Staphylococcus aureus, Cutibacterium acnes). Therefore, a disruption in skin pH disturbs the skin's microbiome and hinders anti-microbial protection. The risk of infections is thus increased. Moreover, serine proteases, which are activated by a high pH, trigger the synthesis of cytokines that cause inflammation.

• YOSIPOVITCH G. & al. Skin pH: From Basic Science to Basic Skin Care. ActaDermato-Venereologica (2013).

• CRITON S. & others. Evaluation of pH in bathing soaps and shampoos for skin and hair care. American Journal of Dermatology (2014).