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The “normal” pH of the skin surface of most body parts is moderately acidic in the range of pH 4.1–5.8 (95% interval; arithmetic mean 4.9) [8]. Exceptions are axilla, genitals, and anus, which show pH values between 6.1 and 7.4 and are thus named the acid gaps of the body [9]. These areas are prone to infection and eczematous skin reaction. Also, there is a higher pH in skin areas exhibiting a higher density of sebaceous glands. Measurements of the buffer capacity in the special areas of the body are not described.

Skin pH is higher in newborns (approximately 6.0) than in older children, owing to incomplete acidification, reaching an acidic (“normal”) skin pH after about 4 weeks [10]. This acidic pH remains rather constant until the 6th decade of life. Thereafter, there is an increase of pH [11–14] and reduced buffer capacity [15, 16]. pH is increased and buffer capacity is probably reduced in pre- vs. postmenopausal women [17].