The cyclosiloxanes commonly known as cyclotetrasiloxane (D4) and cyclopentasiloxane (D5) are used for their anti-static, emollient, humectant, solvent, viscosity-controlling and hair conditioning properties in cosmetic products.

However, concerns have been raised about their toxicity and effects on the environment, and now it is recognised that cyclotetrasiloxane (D4) is toxic to human reproduction systems.

It also accumulates in the environment with unpredictable long term effects.

Cyclopentasiloxane (D5) can also be harmful to the environment and in January 2018 the EU commission published an update amending Annex XVII to Regulation (EC) No 1907/2006 of REACH.

After 31 January 2020, the concentration of D4/D5 in cosmetic products placed on the market that are rinsed off after application should be less than 0.1% by weight of either substance. This has now been amended to include cyclohexasiloxane (D6) and it is expected that this restriction will be extended to include leave-on products.