The skin is a complex organ that maintains the body’s internal environment, and a primary function is the prevention of dehydration. This defense against water loss is measured as Transepidermal Water Loss (TEWL), which is the passive evaporation of water from the deeper layers through the skin’s surface and into the air. While some water loss is natural, excessive TEWL indicates a compromised skin barrier, leading to dryness, flakiness, and increased sensitivity. The skin barrier, primarily the outermost layer of the epidermis, is the body’s sophisticated defense mechanism, regulating moisture flow to maintain the skin’s health and homeostasis. Keeping this barrier intact is fundamental to retaining hydration.
The Primary Structural Barrier
The outermost layer of the skin, the stratum corneum (SC), forms the physical boundary that minimizes water evaporation. This layer is often described using the “brick and mortar” model to illustrate its unique architecture. The “bricks” are the corneocytes, which are flattened, dead skin cells filled with keratin protein. These corneocytes are surrounded by a continuous intercellular lipid matrix, which acts as the “mortar” holding the structure together. This organized arrangement creates a highly tortuous pathway that any escaping water molecule must navigate, significantly slowing the rate of evaporation.
Essential Lipid Components
The sealing power of the skin barrier comes from the specialized fats that make up the intercellular “mortar.” These lipids are a blend of three main components: ceramides, cholesterol, and free fatty acids. Ceramides constitute the largest proportion of this matrix, typically making up between 30% and 50% of the total lipid content. They are instrumental in locking in moisture and forming the backbone of the barrier structure.
Cholesterol provides structural stability to the lipid matrix, helping to maintain the barrier’s strength and flexibility. Free fatty acids, such as linoleic and palmitic acids, are the third component, which helps prevent water loss and maintain the skin’s slightly acidic pH balance. The skin barrier functions optimally when these three lipids are present in a balanced molar ratio, often cited around 3:1:1 for ceramides, cholesterol, and free fatty acids, respectively. This precise ratio allows the lipids to arrange themselves into highly structured, layered sheets called lamellae, creating an impermeable seal.
Internal Hydration Molecules
While the lipid matrix provides the external seal, internal water retention is managed by the Natural Moisturizing Factor (NMF). NMF is a complex mixture of water-soluble compounds found within the corneocytes, the “bricks” of the stratum corneum. These compounds are byproducts of the breakdown of filaggrin protein, which occurs as skin cells move up through the epidermis.
The composition of NMF includes a variety of free amino acids, along with derivatives such as pyrrolidone carboxylic acid (PCA), urea, lactic acid, and various inorganic ions. These molecules are powerful humectants, meaning they attract and bind water. By holding moisture inside the corneocytes, NMF maintains the cells’ plumpness and flexibility, which is necessary for the overall integrity of the skin barrier. NMF makes up approximately 20% to 30% of the dry weight of the stratum corneum.
Factors That Compromise Barrier Function
Numerous internal and external factors can degrade the skin’s protective structure, leading to a breakdown of the lipid seal and increased TEWL.
Environmental Stressors
Environmental stressors, such as low humidity, wind, and air pollution, can physically strip moisture and damage the lipid structures of the stratum corneum. Air pollutants and particulate matter can induce oxidative stress, which further disrupts barrier integrity.
Skincare Practices
Harsh skincare practices are a common source of compromise, particularly the overuse of strong soaps or high-pH cleansers. These products dissolve protective lipids and wash away the water-soluble NMF components. Over-exfoliation, whether mechanical or chemical, also physically damages the stratum corneum, creating gaps that allow water to escape more readily.
Biological Factors
Biological factors also influence barrier function, including the natural process of aging, which reduces the skin’s ability to synthesize lipids and NMF. Certain genetic conditions and inflammatory skin disorders, such as atopic dermatitis and eczema, are characterized by an inherent impairment in the ability to produce adequate amounts of ceramides and NMF. When these factors disrupt the SC balance, the resulting spike in TEWL makes the skin vulnerable to chronic dryness and sensitivity.