A relationship between pH and enlarged pores?

The skin is the largestorganof our body.It'sour body's first line of defence againstexternal influences. This protective functionismade possible by the acidic pHof around 5.5ofthe skin's hydrophilic lipid membrane. This observation, first made by researcher Marchionini in 1929, has been confirmed by subsequent studies, including those by Korting in 1992.

The hydrolipidic film on the surface of the skin is a complex emulsion primarily 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 antibacterial defence. This is referred to as the acid mantle of the skin.

The pH of the skin varies depending on endogenous as well as exogenous factors. Among the endogenous factors, 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 (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. 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 sweat production, men have a more acidic skin pH than women.

• Skin Pigmentation : Studies have proven that darker skin types have a more acidic skin pH than lighter skin types. This is due to the higher presence of epidermal lipids, specifically fatty acids, which result in a lower pH and better integrity of the barrier function.

Several studies have highlighted the connection between skin pH and acne. Indeed, enlarged pores are one of the physical manifestations of acne. Therefore, it can be said that a change in the skin's pH may be linked to the dilation of pores.

A 2017 study compared the skin pH of subjects with acne and those without. The majority of acne cases had a high skin pH. The average pH of these cases was higher than the normal reference value (pH 4.5-5.5 for women, 4-5.5 for men) and that of the control subjects.

Thus, the increase in facial skin pH in patients with acne under basal conditions reflects a chronic state of instability in the stratum corneum, which could predispose individuals to the onset and/or recurrence of acne. This could be a common area through which traditional pathomechanisms might operate in acne. The integration of measures aimed at maintaining the pH of the stratum corneum during treatment could prove beneficial, hence the use of certain acidic molecules such as retinoids.

Furthermore, the bactericidal activity, due to the dermcidin and nitrites present in sweat, is optimal at a pH of 5.5. When the skin's pH increases, the resident bacterial flora undergoes a change: the colonisation and activity of P. acnes and Staphylococcus aureus increase while the action of antimicrobial peptides decreases, predisposing the epithelium to vulgar acne.

Finally, regarding the sebum produced by the sebaceous glands, it is observed that it has a reduced content of free fatty acids which play a part in the acid mantle that covers the skin. The resulting effect could be detrimental to the skin's pH, making it more alkaline.

• Chaitra Prakash & al., Skin Surface pH in Acne Vulgaris: Insights from an Observational Study and Review of the Literature, J Clin Aesthet Dermatol. (2017)

• YOSIPOVITCH G. & al. Skin pH : From basic science to basic skin care. Acta Dermato-Venereologica (2013).