Skin pH

The viable epidermis is near-neutral, but the skin surface, the acid mantle, is acidic at about pH 4–6. Several things keep it acidic:

• lactic acid and amino acids in sweat;
• free fatty acids in sebum;
• urocanic acid and pyrrolidone carboxylic acid (PCA) made when Filaggrin breaks down into Natural Moisturising Factor;
• deeper down, the NHE1 proton pump and secretory phospholipase A2 (Kumar & Das, IJDVL 2023; Panther & Jacob 2015).

Newborn skin starts near-neutral and acidifies over the first year, fastest in the first two months. In infants, pH is highest on the cheeks and other convex surfaces, which matches where infant eczema first appears (Panther & Jacob 2015). Filaggrin loss alone may be partly offset by the other acidifying pathways. Skin buffering capacity depends heavily on filaggrin-derived amino acids, so children with filaggrin-deficient skin are less able to absorb an alkaline insult.

Filaggrin [supports] Skin pH Natural Moisturising Factor [part-of] Skin pH Skin pH [part-of] skin barrier

What a raised pH does. In Atopic Dermatitis, lesional skin has the highest pH and perilesional skin is intermediate. Even unaffected skin in atopic children is less acidic than in healthy controls (Panther & Jacob 2015). A rising pH causes damage in three ways:

• Proteases: the serine proteases kallikrein 5 and 7 have neutral pH optima. At higher pH they break down corneodesmosomes (desmoglein 1) too fast, loosening the stratum corneum. Kallikrein 7 also activates IL-1β, which drives inflammation. Persistent activity blocks lamellar body secretion (Kumar & Das 2023; Hachem 2005, cited in the 2024 J Invest Dermatol acid-mantle review).
• Lipids: the enzymes that make Ceramides work best at pH 4.5–5.6 and slow down as pH rises, which weakens the barrier further (Skin Barrier Dysfunction).
• Microbes: Staphylococcus aureus grows better at neutral pH. The skin’s own antimicrobial peptide dermcidin killed >90% of S. aureus at pH 5.5 but only 60% at pH 6.5 (Kumar & Das 2023). Higher skin pH has been linked to more S. aureus and to worsening eczema severity (Hülpüsch et al. 2020, cited in the JID review).

Skin pH [causes] Skin Barrier Dysfunction Skin pH [relates] Staphylococcus aureus Skin pH [regulates] Ceramides Skin pH [relates] Skin Microbiome

Soaps raise pH. Alkaline soap can lift skin pH by about 3 units, and the effect lasts around 90 minutes. Neutral and alkaline soaps are known to provoke eczema lesions in susceptible people. Atopic skin buffers poorly, so the effect is magnified (Panther & Jacob 2015). Even plain water raises pH briefly, and occlusion raises it too (Kumar & Das 2023). Soap also strips lipids and NMF, and surfactants such as Sodium Lauryl Sulfate (found in Aqueous Cream) do further barrier damage. Hard Water leaves more of this residue on the skin. This is the reason UK guidance tells parents to wash toddlers with Soap Substitutes instead of soap.

Sodium Lauryl Sulfate [worsens] Skin pH Soap Substitutes [prevents] Skin Barrier Dysfunction Hard Water [relates] Skin pH

Syndets and acidic products. Syndet Cleansers (synthetic detergents) clean with mild surfactants at a neutral or acidic pH. Unlike soap, they disturb the acid mantle less and for a shorter time (Kumar & Das 2023; Cosmetics 2025 pH review). Acidic leave-on products are promising mostly in lab work. Acidified moisturisers prevented eczema-like disease in mouse models. In human skin, 4% lactic acid (pH 3.7–4.0) raised stratum corneum ceramides and improved barrier function within 4 weeks (Kumar & Das 2023). Colloidal Oatmeal may help partly by normalising pH (Panther & Jacob 2015). Bleach Baths complicate the picture: a 1:1000 dilution of 6% hypochlorite has a pH of about 9, yet such baths can help through their antimicrobial effect. Their net effect on skin pH has not been measured.

Syndet Cleansers [supports] Skin pH Colloidal Oatmeal [supports] Skin pH Bleach Baths [relates] Skin pH

Conflict: [Panther & Jacob, J Clin Med 2015] state that “maintaining a normal pH through appropriate therapy is well known to benefit” atopic dermatitis. The same review [2015] and [Kumar & Das, IJDVL 2023] report a “paucity of current evidence on how to clinically approach pH management”, with acidic-emollient benefits shown mainly in mice. No RCT of pH-targeted cleansers or emollients in toddlers was found. Unresolved. Added to open_questions.

Connections

• Natural Moisturising Factor — NMF acids keep stratum corneum acidic, source: Agrawal & Woodfolk 2014
• Filaggrin — FLG loss raises pH, source: Elias & Wakefield 2011
• Ceramides — raised pH impairs ceramide-generating lipid processing, source: Elias & Wakefield 2011
• Staphylococcus aureus — favoured by neutral pH; dermcidin killing falls from >90% to 60% between pH 5.5 and 6.5, source: Kumar & Das 2023
• Soap Substitutes — avoid alkaline soap, which lifts pH ~3 units for 90 min, source: Panther & Jacob 2015
• Syndet Cleansers — neutral/acidic cleansers preserve the acid mantle, source: Kumar & Das 2023
• Bleach Baths — alkaline (≈pH 9) yet antimicrobial, net pH effect unmeasured, source: Panther & Jacob 2015
• Skin Barrier Dysfunction — raised pH drives protease-mediated barrier breakdown, source: Kumar & Das 2023
• Colloidal Oatmeal — may act partly by normalising pH, source: Panther & Jacob 2015