Soaps, surfactants, and detergents, especially those products with a high pH, may excessively remove NMF and skin lipids, enhancing skin surface pH and triggering AD flares.2,24 Filaggrin and its degradation products play a key role in the control of TEWL and skin pH.6 Filaggrin undergoes proteolysis to release hygroscopic amino acids at the surface of the SC, when the outer skin starts to become dehydrated.6 The acidic pH of skin acts as an antimicrobial defensive mechanism to limit bacterial colonization.20-24 In healthy skin, even if the pH is increased, filaggrin proteolysis can contribute acidic amino acids to return the skin to the optimal slightly acidic pH.6 However, in individuals at risk for developing AD, such as newborns and elderly individuals, skin surface pH may remain high.15,16 Frequent washing with alkaline soap reduces buffer capacity by washing away inherent buffering components, enhancing the risk for irritation, thus triggering AD flares.6,11,18,24-26
Statement 8: The use of cleansers and moisturizers with a physiological skin surface pH (4.0–6.0) may allow for skin barrier repair, decreased inflammation, accelerated pH recovery, and increased antimicrobial defense.
Cleansers enable the removal of dirt and oil, and clears pores of debris to prevent dirt buildup, allowing sebum to reach the skin surface unimpeded.27 Surfactants within skin cleansers solubilize and remove debris and oil; however, interaction of surfactants with the SC may cause erythema, dryness, skin barrier impairment, enhanced skin pH, and sensorial irritation (Table 1).27-29 The use of gentle cleanser that employs advanced vehicles with a near-physiologic pH (4.0–6.0) and milder surfactants, and that rinses clean, leaving no residue, may help in maintaining skin barrier function.27-29 Cleansing and moisturizing may help manage pH levels of the skin surface, enabling sufficient water retention and improving dry, flaky, and atopic skin.28,29 A cleanser composed of a lower concentration of free surfactant micelles, as well as polymer-surfactant complexes, has been shown to be less aggressive than alkaline soaps.25
Although there are data24 supporting enhancing skin pH can cause irritation when using cleansers with a high pH (9–10), conclusive evidence on lowering skin pH when using near physiological skin pH (5.0–7.0) products is lacking.24 Cork et al18 plotted the skin barrier function with arbitrary units against the first 3 years after birth. At birth, in children with no genetic pre-disposition to AD, skin barrier function is insufficient while skin pH is high.18 The skin barrier function gradually improves over a period of about 3 years, becoming more acidic.18 However, depending on the degree of pre-disposition to AD and environmental factors such as the use of soap and detergents, which enhance skin pH, skin barrier defect may become exacerbated. On the other hand, effective treatment, including low pH moisturizer, can improve skin barrier function, thereby acidifying the SC pH.18 Improved knowledge about the central roles a defective skin barrier and dry skin may play in AD is increasingly recognizing the benefits of daily and ongoing use of moisturizers. Current treatments aim to reduce inflammation and to restore skin barrier function. Those moisturizers that contain humectants such as ceramides have shown benefits over standard emollients.33
In healthy skin, filaggrin proteolysis supports restoring the slightly acidic pH, whereas in skin affected by inflammatory skin conditions such as AD, the pH remains elevated.6 Skin pH values are higher in patients with active AD lesions than in asymptomatic individuals.18,19 Furthermore, the elevated level of skin pH can be expected to delay skin barrier recovery and facilitate barrier breakdown.18 These mechanisms confirm that skin pH values are an important indicator for skin health, the severity of AD, as well as a predictor of AD flares.
The pH and hydrophilic index of a product may give important information to choose a suitable moisturizer for AD.24,34 According to the panel, topical products with near-physiologic pH (4.0–6.0) are considered the best option for AD. Currently, pH values of moisturizers are frequently unknown to physicians and can range widely from 3.7–8.2, some of the frequently used moisturizers having a physiologic pH of 4.0–6.0.34
The choice of cleanser/moisturizer mostly depends on individual preference; to enable adherence to treatment, the cleanser/ moisturizer should be found pleasant to use by the patient. However, according to the panel, the ideal agent should be safe, effective, inexpensive, and free from additives, fragrances, perfumes, and sensitizing agents.35 Additionally, they stated, the cleanser and moisturizer should have a physiologic pH (4.0–6.0) or lower to support skin barrier repair.