Healthy skin is slightly acidic, with a pH typically between 4.5 and 5.5 — the "acid mantle", part of the skin's natural defence. Correct pH supports barrier enzymes and surface balance.
Harsh, high-pH cleansers (like some classic soaps) can temporarily disrupt this. pH-balanced, gentle cleansers — and avoiding very hot water — help maintain surface balance. Consult a physician on cleanser choice for your skin type.