Dimethicone.

What it does

On skin it smooths roughness, adds silky slip, temporarily blurs fine lines and acts as a light occlusive that slows water loss; it is also the key ingredient in many medical silicone scar gels. On hair it coats the strand for shine and detangling. Because it is non-volatile (it does not evaporate) it stays put, which is what makes it effective but also what drives the build-up debate.

Benefits

• Makes skin and hair feel instantly silky and smooth and improves spreadability
• Forms a breathable, water-repellent film that reduces moisture loss without feeling heavy
• Temporarily fills and blurs fine lines and uneven texture
• Very low irritancy and low comedogenicity; used in scar-care gels to soften and flatten scars

Side effects

• Considered safe and non-irritating by the Cosmetic Ingredient Review panel; rated essentially non-irritating
• Not water-soluble, so on hair it can be harder to wash out and may build up over time, leaving hair limp or dull until clarified
• The build-up concern is real but often overstated — it applies to non-volatile silicones like dimethicone and is readily removed with a clarifying/sulfate shampoo; it does not 'suffocate' skin or hair