Background: pH is known as an important parameter in epidermal barrier function and homeostasis. Aim: The impact of age and body site on skin surface pH (pH_SS) of women was evaluated in vivo. Methods: Time domain dual lifetime referencing with luminescent sensor foils was used for pH_SS measurements. pH_SS was measured on the forehead, the temple, and the volar forearm of adult females (n = 97, 52.87 ± 18.58 years, 20–97 years). Every single measurement contained 2,500 pH values due to the luminescence imaging technique used. Results: pH_SS slightly increases with age on all three investigated body sites. There are no significant differences in pH_SS between the three investigated body sites. Conclusion: Adult pH_SS on the forehead, the temple and the volar forearm increases slightly with age. This knowledge is crucial for adapting medical skin care products.

Background/Aims: The effects of a 10% α-hydroxy acid (AHA) oil/water (O/W) emulsion on the pH of human skin surface (pH_ss) and stratum corneum (SC; pH_sc) were evaluated in vivo. Methods: The AHA O/W emulsion was applied to an area on the volar forearm of male volunteers (n = 12), and then wiped off after 10 min. Prior to application and over the following 3 h, the pH_ss was measured. We used glass electrode measurements and time domain dual lifetime referencing (tdDLR) with luminescent sensor foils. In another experiment (n = 5), the impact of the AHA O/W emulsion on the pH_sc gradient was measured by tape stripping of the SC of the volar forearm after application of the AHA O/W emulsion. Results: Compared with pH_ss values prior to treatment [5.2 ± 1.7 (tdDLR) or 4.8 ± 0.5 (electrode)], the pH_ss was significantly reduced 10 min after application [4.0 ± 0.3 (tdDLR) or 4.1 ± 0.1 (electrode)] and the pH_ss remained significantly reduced over the measurement period of 3 h [after 3 h: 4.4 ± 0.2 (tdDLR) or 4.5 ± 0.3 (electrode)]. The AHA O/W emulsion significantly reduced the pH_sc even down to deep layers of the SC. Conclusion: After a 10-min application time, the 10% AHA O/W emulsion reduces the pH_ss (for at least 3 h) and pH_sc in deep layers of the SC.