Octamethylcyclotetrasiloxane (D4)

Octamethylcyclotetrasiloxane (D4) is a cyclic volatile methylsiloxane (cVMS) used in silicone polymer manufacture and as a carrier solvent in some glazes, polishes, and spray-applied detail products. It evaporates rapidly from the surface, leaving the active polymer or wax behind. **Environmental concern:** D4 is identified by ECHA as a Substance of Very High Concern (SVHC) on the basis of PBT (persistent, bioaccumulative, toxic to aquatic life) and vPvB (very persistent, very bioaccumulative) properties. EU REACH restricts D4 in wash-off cosmetic products above 0.1%. D4 reaches surface waters primarily through wastewater treatment plant effluent because it does not biodegrade. Even at trace formulation concentrations, products containing D4 contribute to long-term sediment and aquatic-organism accumulation. **Human health:** GHS classification Repr. 2 (H361f — suspected of damaging fertility) is based on reproductive toxicity studies in rats. The substance is also a flammable liquid (Flam. Liq. 3, H226). At the trace concentrations typical of consumer detail products (< 1%), single-application inhalation exposure is brief; the regulatory concern is chronic occupational exposure or repeated dermal/inhalation contact at higher concentrations. Listed on the ECHA Candidate List of Substances of Very High Concern.