In the presence of a strong base such as KOH, the polymer/ring mixture is equilibrated, allowing complete conversion to the more volatile cyclic siloxanes:[5] D4 and D5 are also precursors to the polymer.

[8] As the smallest cyclic dimethylsiloxane that does not experience considerable ring strain,[9] D4 is one of the most abundant siloxanes in the environment, e.g. in landfill gases.

"[13] In the European Union, D4 was characterized as a substance of very high concern (SVHC) due to its PBT and vPvB properties and was thus included in the candidate list for authorisation.

[14][clarification needed] D4 shall not be placed on the market in wash-off cosmetic products in a concentration equal to or greater than 0.1% by weight of either substance, after 31 January 2020.

[15] Conversely, a detailed review and analysis of the science by the State of Washington in 2017 led to the removal of D4 from their CHCC listing.