Cyclopentasiloxane (D5)
In plain terms
A volatile silicone that gives products a silky, fast-evaporating slip. The risk to your skin is low, but it is very persistent in the environment and builds up in living things, which is why the EU is phasing it out of cosmetics. Easy to swap out.
What the evidence shows
Cyclopentasiloxane, almost always labeled D5, is a lightweight silicone that spreads easily and then evaporates, leaving a smooth, dry finish; it turns up in hair serums, antiperspirants, makeup primers, and creams. On skin safety the EU's expert committee judged it acceptable for use in cosmetics, and it rarely irritates. The concern sits elsewhere. D5 is classed as very persistent and very bioaccumulative, meaning it does not break down readily and accumulates in living things, and the European Chemicals Agency identified cosmetics as the main route by which it reaches the environment, where it collects in water, sediment, and wildlife. It is also less inert in the body than once assumed: a study on human skin found cyclic silicones can diffuse into and through skin rather than fully evaporating, and D5 has been measured in human blood, fat, and breast milk. Harm to people from those traces has not been shown, so this is a precautionary and environmental call, not a claim that it damages your skin. Because of the persistence, the EU has limited D5 to a trace 0.1% in rinse-off products since 2020 and extends that cap to all cosmetics, including leave-on, from June 2027, a step expected to cut emissions by up to 90%. A close relative, D4, is a separate story: it is banned in EU cosmetics and is the siloxane flagged for hormone and fertility effects, so the two should not be treated as one. Since many other emollients do the same job, D5 is simple to skip.
On the label
Listed as cyclopentasiloxane or D5, and sometimes folded into cyclomethicone. To avoid it, check hair serums, antiperspirants, primers, and light creams, and choose products that use non-volatile silicones like dimethicone or plant oils instead. This is about reducing a persistent substance going forward, not any sharp risk from products you have already used.