Overview
The cyclosiloxanes are a small family of volatile silicone ring molecules. The three of regulatory interest are D4 (octamethylcyclotetrasiloxane), D5 and D6. They evaporate readily, which makes them useful in cosmetics and cleaning products and as feedstock for silicone polymers. The same volatility, combined with their persistence and tendency to accumulate in living things, is what drives concern.
Where they're restricted
All three are on the REACH Candidate List as substances of very high concern. Under REACH Annex XVII they are already restricted in wash-off cosmetic products, such as rinse-off shower gels and shampoos, where the material is washed down the drain after use. Broader restrictions covering more product types are under discussion.
D4, D5 and D6 are flagged for being persistent and bioaccumulative rather than for acute toxicity. The worry is long-term build-up in the environment, not immediate harm.
Typical uses
These siloxanes feature in personal care and cosmetic formulations, in some cleaning products, and as building blocks for the silicone polymers used across many industries. Reformulation away from the cyclic forms is ongoing in consumer products.
Note: general educational information, not legal advice. Check the official source before relying on it.