Skin

Largest organ

How does the skin work?

A layered barrier that keeps water in, keeps microbes out, senses touch and makes vitamin D with UVB light.

Layers

Epidermis (outer barrier, constantly renewing), dermis (collagen, vessels, nerves, hair follicles, sweat glands), subcutaneous fat.

UV

UV radiation can damage DNA in skin cells — the main reason sun protection matters.

Products

Most cosmetic ingredients act on the outermost layer; the barrier limits how much passes through.

Evidence & sources

• Skin makes vitamin D when exposed to UVB light. Evidence: established. Known: Basic physiology. Unknown: Production varies with season, latitude, age, skin color and sunscreen use. Source: NIH Office of Dietary Supplements, Vitamin D Fact Sheet for Health Professionals (Current revision).

Connected topics

• UV Radiation
• Sunscreen
• Vitamin D
• Vitamin C
• Shampoo
• Moisturizer
• Soap & Cleansers
• Skin Barrier

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General education only, not medical advice. Drafted Oct 2026 · pending qualified expert review.