Cyclotetrasiloxane (Cyclomethicone) is a chemical compound silicone D4 invented in 1980 by Dow Corning, a mixture of cyclic compounds dimethyl polysiloxane.

Silicones have high chemical stability but low biodegradability and high bioaccumulation potential.

Safety. It is an ingredient that has a high bioaccumulation and persistence and toxicity properties of the nonreproductive organs. Banned in cosmetics if it contains D4 silicone. However, when used in cosmetic products, Cyclomethicone is considered a safe ingredient.
https://ec.europa.eu/health/scientific_committees/consumer_safety/docs/sccs_o_029.pdf

What it is used for and where
Cosmetics
Hair conditioning agent. A large number of ingredients with specific purposes can co-exist in a hair shampoo: cleansers, conditioners, thickeners, mattifying agents, sequestering agents, fragrances, preservatives, special additives. However, the indispensable ingredients are the cleansers and conditioners as they are necessary and sufficient for hair cleansing and manageability. The others act as commercial and non-essential auxiliaries such as: appearance, fragrance, colouring, etc. Hair conditioning agents have the task of increasing shine, manageability and volume, and reducing static electricity, especially after treatments such as colouring, ironing, waving, drying and brushing. They are, in practice, dispersing agents that may contain cationic surfactants, thickeners, emollients, polymers. The typology of hair conditioners includes: intensive conditioners, instant conditioners, thickening conditioners, drying conditioners.

Skin conditioning agent - Emollient. Emollients have the characteristic of enhancing the skin barrier through a source of exogenous lipids that adhere to the skin, improving barrier properties by filling gaps in intercorneocyte clusters to improve hydration while protecting against inflammation. In practice, they have the ability to create a barrier that prevents transepidermal water loss. Emollients are described as degreasing or refreshing additives that improve the lipid content of the upper layers of the skin by preventing degreasing and drying of the skin. The problem with emollients is that many have a strong lipophilic character and are identified as occlusive ingredients; they are oily and fatty materials that remain on the skin surface and reduce transepidermal water loss. In cosmetics, emollients and moisturisers are often considered synonymous with humectants and occlusives.

Solvent. It is the substance for dissolving or dispersing surfactants, oils, dyes, flavourings, bactericidal preservatives in solution.
Synonyms: Octamethylcyclotetrasiloxane
Cyclomethicone
EC number 209-136-7
Trade name:

CAS: 556-67-2 293-51-6