Is Skin Watertight?

No, skin is not completely watertight, despite often being described as waterproof. While skin acts as an excellent barrier, preventing significant water loss and entry of pathogens, it's not impermeable.

The statement that skin is "waterproof" is a simplification. The epidermis, the outermost layer of skin, contains a lipid-rich layer (stratum corneum) that significantly reduces water permeability. This layer, along with the natural oils produced by sebaceous glands, creates a barrier that prevents excessive water loss and keeps most external water out. [Source: Skin is a waterproof, flexible, but tough protective covering for your body.][Source: https://www.aocd.org/?page=SkinFacts]

However, this barrier isn't perfect. Skin possesses pores, which are openings for sweat glands and hair follicles. While these pores don't penetrate the entire depth of the skin, they do allow for some water passage. [Source: https://www.reddit.com/r/explainlikeimfive/comments/188bmof/eli5_how_is_human_skin_water_proof_yet_full_of/] Additionally, prolonged submersion in water can lead to some water absorption through the skin. [Source: https://www.reddit.com/r/explainlikeimfive/comments/rjd63m/eli5_if_skin_is_waterproof_why_does_moisturiser/]

Furthermore, the effectiveness of the skin's barrier function varies depending on factors like individual skin condition, environmental conditions, and the nature of the substance interacting with the skin. For example, moisturizers and sunscreens can penetrate the skin due to their formulation and the skin's semi-permeable nature. [Source: https://www.reddit.com/r/explainlikeimfive/comments/rjd63m/eli5_if_skin_is_waterproof_why_does_moisturiser/]

Examples:

• Swimming: While skin protects against significant water loss during swimming, prolonged immersion can lead to some water absorption, especially in areas with more sensitive skin.
• Moisturizers: These penetrate the skin to hydrate it, demonstrating that the skin barrier isn't completely impermeable.
• Medical Procedures: In medical procedures requiring watertight seals, like those involving spinal surgery, advanced multi-layer closures are needed to prevent leakage of cerebrospinal fluid. [Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC11268763/]

In contrast, a frog's skin is not watertight, readily absorbing water through its skin. [Source: https://www.quora.com/Is-a-frogs-butt-watertight]

Conclusion

Skin provides a significant barrier against water, but it is not entirely waterproof. It's more accurate to describe it as water resistant.