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A Healthy Stratum Corneum: A Key to Sunscreen Effectiveness?

The skin barrier, more specifically the stratum corneum, plays a critical role in protecting the body from external environmental factors. In an exclusive conversation with The Dermatologist, Carl R. Thornfeldt, MD, FAAD, shared his knowledge of this superficial layer of skin and the effectiveness of topical products.

Dr Thornfeldt noted that the skin barrier shields the body from pollutants and UV light, environmental factors known to damage the skin. If unchecked at the superficial layer of skin, these external factors can penetrate deeper, impacting proliferating cells and their organelles. To help reduce the impact of external factors, the health of the stratum corneum should be noted by dermatologists.

This is especially important when considering that a number of patients have a significantly diminished skin barrier function as measured by transepidermal water loss. This perturbation may affect the effectiveness of dermatologic products, including those meant to protect the skin barrier from further damage.

Sunscreen, for example, binds to the corneocyte phospholipid bilayer of the stratum corneum; it also embeds in the intracellular barrier lipids. A healthy stratum corneum provides a larger surface area and number of molecules for sunscreen molecules to bind with, adding to a prolonged substantivity and increased photoprotection.

But what if the skin barrier is not healthy? A damaged stratum corneum can render sunscreens somewhat ineffective. Instead of binding to the surface and creating a physical layer between the skin and UV light, sunscreen ingredients will bypass the superficial stratum corneum to the epidermis, thereby reducing protection by an exponential amount. Further, recent studies have noted a possible systemic absorption greater than the threshold set by the FDA (0.5 mg/dL) of certain types of sunscreen ingredients such as avobenzone, oxybenzone, octocrylene, homosalate, octisalate, and octinoxate. A damaged skin barrier may be a key factor in the heightened levels of absorption, as fewer superficial lipid and phospholipid molecules allow sunscreen to penetrate deeper, thus increasing detectable levels of these ingredients in the blood as well as risk of toxicity.

It has been hypothesized that a healthy stratum corneum may prevent deeper penetration of sunscreen into the skin. Prevention may cause the sunscreen to function more as intended—a physical blocker between external UV light damage—instead of being absorbed into the blood. While more study is needed to prove this hypothesis, dermatologists can prioritize the repair and optimization of the stratum corneum for a number of other reasons, including keeping bacteria at bay. Careful selection of topical products should include cosmeceuticals that help restore the skin barrier’s function and thickness.

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