Encyclopedia · silicone
Cyclopentasiloxane
A volatile silicone that spreads easily and evaporates, used to carry heavier silicones in serums; now restricted in the EU for environmental reasons.
What it is
A cyclic silicone with five silicon units, known as D5. The Cosmetic Ingredient Review panel reviewed cyclomethicone and its cyclic relatives and found minimal skin absorption and no irritation or sensitisation signal. It concluded they are safe in the present practices of use and concentration. Johnson 2011
How it works
It acts as a light carrier that evaporates and leaves the heavier silicone behind. This is general formulation knowledge. It is not itself a lasting conditioner.
What the studies show
- No hair-efficacy studies were found for D5 alone.
- Safety: the Cosmetic Ingredient Review found consumer use acceptable. Johnson 2011
Side effects and safety
The EU restricts D4, D5 and D6 under its chemicals regulation (Regulation 2024/1328). Cosmetics may not contain 0.1% or more of D5 or D6, with leave-on products covered from 6 June 2027, and D4 is separately banned. Regulation 2024/1328 The driver is persistence and bioaccumulation in the environment. The UK status was not checked.
Bottom line
Cosmetically fine, environmentally contentious, and increasingly absent from EU products. There is no reason to seek it out. We are moderately sure about the regulation and unsure about any hair benefit.
Studies 2
Johnson W Jr et al. Safety assessment of cyclomethicone, cyclotetrasiloxane, cyclopentasiloxane, cyclohexasiloxane, and cycloheptasiloxane. Int J Toxicol. 2011
Result: Minimal skin absorption and no irritation or sensitisation signal; safe in the present practices of use and concentration.
Read the study ↗European Commission. Regulation (EU) 2024/1328 amending Annex XVII to Regulation (EC) No 1907/2006 as regards octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6). 2024
Result: Restricts D4, D5 and D6 so that cosmetics may not contain 0.1% or more by weight, with leave-on products covered from 6 June 2027.
Read the study ↗Related
- DimethiconeThe standard hair silicone, a water-insoluble oil that coats the fibre for slip, shine and friction control without repairing anything.
- DimethiconolA dimethicone with hydroxyl end groups, usually supplied as a thick gum emulsion for longer-lasting slip in rinse-off conditioners.
- Silicones in hair careSilicones coat hair to cut friction and add shine without repairing it; buildup is real but manageable, and the scare claims are unsupported.