Cyclopentasiloxane (D5)

What is cyclopentasiloxane and what is it used for?

Cyclopentasiloxane is a ring-shaped silicone made of five silicon-oxygen units, known for short as D5. The same family includes four-ring D4 (cyclotetrasiloxane) and six-ring D6 (cyclohexasiloxane). On labels these mixtures are sometimes also listed as “Cyclomethicone”.

Its most important property is that it is volatile: it makes the product easy to spread on skin or hair and then evaporates without leaving a greasy feel. This is why it is chosen for silky, lightweight products and as a carrier for other ingredients.

Which products contain it?

• Roll-on and stick deodorants and antiperspirants
• Hair serums, shine sprays and heat protectants
• Lightweight moisturisers, make-up primers and foundations
• Some sunscreens and body oils

Is it harmful? The scientific assessment

The SCCS has assessed the use of D5 in cosmetics in terms of consumer health and found it generally safe; for inhalation exposure from aerosol sprays, it recommended certain concentration limits. Its penetration into the skin is very low, and most of it evaporates. Irritation and allergy are extremely rare.

The real issue is environmental. D4 and D5 have been assessed by ECHA as persistent, bioaccumulative and toxic (PBT) / very persistent and very bioaccumulative (vPvB) substances and added to the list of substances of very high concern (SVHC). D4 is also classified as suspected of harming reproductive health.

For this reason, under the REACH regulation, the EU first restricted the use of D4 and D5 above a certain concentration in rinse-off cosmetics (such as shampoo and shower gel), because these products go straight into waste water. The restriction was then extended to include D6 and to cover leave-on cosmetics and many other consumer and professional products, with transition periods phased in by product type.

Who should be careful?

• People who care about environmental impact: They can choose products without D5 or made with biodegradable alternatives.
• People who use spray products heavily in enclosed spaces: Ventilating the room during application reduces inhalation exposure.

What to do when you see it on a label

From a personal health point of view, there is no finding that should make you worry about a deodorant or serum containing D5. If you want to choose on environmental grounds, you can pick products made with dimethicone, which has large molecules and is not covered by the restriction, or with volatile emollients such as plant-derived alkanes. Products on the EU market are gradually being brought into line with these restrictions.