Effect of Post-Washing Time Interval on Skin Hydration Following Moisturizer Application on the Ventral Forearm in Healthy Volunteers

Background: Early post-washing moisturization is widely recommended, yet the optimal application interval remains unclear in healthy adults. Objective: To evaluate whether different post-washing moisturizer application times influence stratum corneum hydration (SCH) and transepidermal water loss (TEWL). Materials and Methods: This controlled experimental study employed a within-subject repeated-measures design in 36 healthy adults aged 21-65 years (18 females, 18 males), stratified into three age groups (21-35, 36-50, 51-65 years; 12 each). Five standardized circular sites on the ventral forearms were assigned to no moisturization (control) or moisturizer application at 2, 5, 10, or 15 minutes after water-only washing. SCH (Corneometer®) and TEWL (Tewameter®) were measured at baseline and at 30, 60, and 90 minutes after application under controlled temperature and relative humidity. Results: Baseline SCH and TEWL were comparable across conditions. In overall analyses, moisturization significantly increased SCH and reduced TEWL versus the untreated control at 30, 60, and 90 minutes (all p<0.001). SCH did not differ among the four application-timing conditions after Bonferroni adjustment (all pairwise p>0.05). Earlier application conferred modest TEWL advantages: the 2-minute condition was lower than the 10- and 15-minute conditions at 30, 60, and 90 minutes (all p<0.05), and the 5-minute condition was lower than the 15-minute condition at the same time points (all p<0.05). Exploratory sex- and age group-stratified analyses showed similar SCH patterns, while TEWL timing effects were more apparent in females and in older age groups, particularly at 90 minutes. Conclusion: Moisturization within 2-15 minutes after washing substantially improves SCH and reduces TEWL versus no moisturization, while earlier application (approximately 2-5 minutes) may confer a small additional TEWL-related barrier benefit.