Skin pH / Surface Acid Film
A thin, slightly acidic film on the surface of the skin with a natural pH of approximately 4.5–5.5, formed by a mix of sebum, sweat, amino acids, lactic acid and other secretions. The acid mantle is a critical component of barrier function: it inhibits the growth of pathogenic bacteria (C. acnes, S. aureus) while supporting beneficial microbiome species, activates enzymes responsible for lamellar body secretion (ceramide production), and regulates desquamation. Alkaline products (soaps with pH 9–10) disrupt the acid mantle and require 1–2 hours for natural restoration.
Many skincare actives (AHAs, BHAs, Vitamin C) require a low pH (below 4) to function. Applying a high-pH product before an acid will neutralise the acid and render it ineffective. Always apply pH-dependent actives to clean, un-buffered skin.
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