What Is pH?
pH (potential of hydrogen) is a logarithmic scale from 0 to 14 that measures the acidity or alkalinity of a solution. A pH of 7.0 is neutral. Values below 7.0 are acidic; values above 7.0 are alkaline (basic). Because the scale is logarithmic, each whole number represents a 10-fold change — pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6. In skincare, pH is critically important because it determines how well actives work, how enzymes function, and whether the skin's natural acid mantle is maintained.
Skin Surface pH
Healthy adult skin surface pH ranges from approximately 4.5 to 5.5 — slightly acidic. This acidity is maintained by lactic acid and amino acids from sweat, fatty acids from sebum, and the skin microbiome. The acid mantle created at this pH is crucial for barrier enzyme function (desquamation enzymes are pH-sensitive), antimicrobial defence, and microbiome balance. When skin pH rises above 6.0, barrier function deteriorates measurably.
pH and Skincare Actives
Vitamin C (L-ascorbic acid) requires a formulation pH below 3.5 for stability and effective penetration. AHAs (glycolic, lactic) are most effective at pH 3–4. BHA (salicylic acid) performs optimally at pH 3–4. Retinoids are stable across a broader pH range. Niacinamide is effective at near-neutral pH. Layering products with very different pH levels can temporarily reduce efficacy — allowing 20–30 minutes between very acidic and near-neutral products helps.
Cleanser pH: Traditional soap sits at pH 9–10. Most modern synthetic detergent cleansers (syndets) are formulated at pH 5.5–6.5. Choosing a pH-balanced cleanser is one of the highest-leverage improvements for skin health, particularly for eczema, acne, and sensitive skin.
See Acid Mantle and Acids: AHA, BHA & PHA.