INTRODUCTION
“A pivotal point of terrestrial adaptation is prevention of desiccation and maintenance of internal water homeostasisâ€.1 The critical role of the stratum corneum (SC) permeability barrier is the protection of the human body from desiccation by limiting transcutaneous movement of water and electrolytes, as well as preventing the entry into the skin of harmful substances like irritants, allergens, and microorganisms. The SC lipids, particularly ceramides, cholesterol, and free fatty acids together forming lamellar membranes in the extracellular spaces of the SC, play a key role in the integrity of SC permeability barrier commonly referred to as epidermal barrier, or skin barrier.1 The intercellular lipids of the SC together with intracellular humectants (natural moisturizing factor, NMF) endow the SC with softness and flexibility by their water holding capacity.2,3 Disruption of the epidermal barrier leads to alterations of SC proteins and lipids, increased transepidermal water loss (TEWL), decreased skin hydration status (clinically seen as dry skin), decreased skin elasticity and smoothness, increased skin reactivity to external stimuli,2,3 and even skin diseases. This review summarizes current understanding of skin barrier integrity and function, clinical consequence of impaired skin barrier integrity, impact of COVID-19 on skin health, sensitive skin in diverse populations, and management strategies.
Ethnicity and Skin Barrier Function
Several methods have been used to understand differences in skin barrier among Caucasian, Asian, and African American ethnic groups including the measurement of TEWL, tape stripping to examine stratum corneum layers, lipid content analysis, and irritation with sodium lauryl sulfate.4,5,6 For TEWL, the evidence indicates that African American skin has greater TEWL than Caucasian skin.5 However, for Asians, the data is inconsistent, with some studies showing TEWL similar to African American skin7 and some showing TEWL lower than Caucasian or Hispanic skin. Other studies have compared differences of skin barrier in different skin pigmentation types (Fitzpatrick phototypes) instead of ethnicity. The study by Reed et al comparing Fitzpatrick skin type II and III of Asians and Caucasians to types IV and VI of Asian, Hispanic, and African American backgrounds with TEWL measured after tape stripping, showed that phototypes IV and V required more tape stripping than phototypes II and III to achieve the same TEWL.8 This led to the conclusion that darker skin may have better barrier integrity and is thus able to withstand insults more than lighter skin. Other studies have supported this theory by demonstrating that African American skin has more corneocyte layers, with a more compactly packed stratum corneum due to increased intercellular cohesiveness.9,10 This connection of
Ethnicity and Skin Barrier Function
Several methods have been used to understand differences in skin barrier among Caucasian, Asian, and African American ethnic groups including the measurement of TEWL, tape stripping to examine stratum corneum layers, lipid content analysis, and irritation with sodium lauryl sulfate.4,5,6 For TEWL, the evidence indicates that African American skin has greater TEWL than Caucasian skin.5 However, for Asians, the data is inconsistent, with some studies showing TEWL similar to African American skin7 and some showing TEWL lower than Caucasian or Hispanic skin. Other studies have compared differences of skin barrier in different skin pigmentation types (Fitzpatrick phototypes) instead of ethnicity. The study by Reed et al comparing Fitzpatrick skin type II and III of Asians and Caucasians to types IV and VI of Asian, Hispanic, and African American backgrounds with TEWL measured after tape stripping, showed that phototypes IV and V required more tape stripping than phototypes II and III to achieve the same TEWL.8 This led to the conclusion that darker skin may have better barrier integrity and is thus able to withstand insults more than lighter skin. Other studies have supported this theory by demonstrating that African American skin has more corneocyte layers, with a more compactly packed stratum corneum due to increased intercellular cohesiveness.9,10 This connection of
