Silicones in hair products: what they do and how to spot them

What silicones do in a hair product

Silicones are polymers built on repeating silicon-oxygen (siloxane) bonds.¹ In hair products they form a thin film over the hair shaft that reduces friction during combing, helps limit moisture loss and adds shine.¹ The same film acts as a barrier against humidity, which helps keep frizz down, and it reduces friction on bleached or dyed hair.¹ A dermatology review calls silicones the most active and most used ingredient in hair conditioners.²

They are not one ingredient. Reviews sort them by whether they dissolve in water, whether they evaporate, and which chemical groups they carry¹, and silicones that differ in how they deposit and dissolve act differently on hair.³

Four common names, four behaviours

Dimethicone is the most used, and it adsorbs better on untreated hair and near the root than on the tips.³ Amodimethicone is aimed at damaged hair: its amine groups become positively charged in water and deposit on the negatively charged cuticle², and one review says it binds selectively to the most damaged areas.¹ Another adds a caution: amino silicones are not necessarily more attached to hair than dimethicone, depending on the size of the molecule and the charge of the whole formula.³

Cyclopentasiloxane is volatile: it evaporates after application and leaves a light finish¹, and the EU database lists it as a solvent as well as a conditioning agent.⁶ Dimethiconol and its esters are used as skin or hair conditioning agents, according to the US Cosmetic Ingredient Review.⁹

Build-up: what the evidence supports

The concern is specific rather than general. Water-insoluble silicones such as dimethicone and amodimethicone can accumulate if they are not properly cleansed, and a 2025 review notes that this shows most on fine or oily hair.¹ Shampoo surfactants do remove silicone conditioning agents², but removing water-insoluble ones takes strong surfactants, such as sulfate clarifying shampoos, and the same review warns that using those too often may dry the hair.¹

Lighter silicones leave less behind: water-soluble ones rinse out more easily, and volatile ones evaporate.¹ To avoid excessive build-up, the review advises alternating silicone and silicone-free products, or using a clarifying shampoo now and then, especially on fine or oily hair.¹ Neither review cited here puts a number on how often build-up happens; it is described, not measured.

Cyclic silicones and EU rules: D4, D5 and D6

Three ring-shaped silicones are restricted under the EU's chemicals law, REACH, and the reason given is the environment rather than your hair. In 2018 a committee of the European Chemicals Agency identified D4, D5 and D6 as very persistent and very bioaccumulative, and the Commission later found an unacceptable risk from their release from consumer and professional products.⁷

• D4 (octamethylcyclotetrasiloxane) is on the EU list of substances cosmetics must not contain, Annex II, entry 1388.¹⁰
• D4 and D5 in wash-off products such as shampoo: not allowed at 0.1% or more by weight after 31 January 2020.⁷
• D6 in any cosmetic, and D4 and D5 in leave-on products such as serums and leave-in conditioners: the same 0.1% limit applies after 6 June 2027.⁷

On labels, D5 is Cyclopentasiloxane and D6 is Cyclohexasiloxane: the CAS numbers in the EU ingredient database match those in the restriction.⁶¹¹⁷ Cyclomethicone names a mixture of ring-shaped silicones of this kind.¹² Looking at people rather than the environment, the US Cosmetic Ingredient Review found minimal absorption through skin and no data suggesting that these cyclic silicones irritate or sensitise skin.¹²

How to spot silicones on a label

• Names ending in -methicone: Dimethicone, Amodimethicone, Cyclomethicone, and longer names built on them, such as Cetyl Dimethicone or Aminopropyl Dimethicone.¹³
• Names ending in -siloxane: Cyclopentasiloxane, Cyclohexasiloxane.⁶¹¹
• Dimethiconol, and esters named after it.⁹
• Two traps: Polysilicone-15, a UV filter, does not follow the pattern; the EU lists it under the chemical name dimethicodiethylbenzalmalonate, allowed up to 10% (Annex VI, entry 26).¹⁰ Silica, the mineral silicones are made from, is a different ingredient with a similar name.³
• "Silicone-free": EU claims guidance says the claim should not be made if the product contains silicone.¹⁴ It does not tell you what conditions the hair instead.

A silicone on the list is not a verdict on the product. The label gives no concentration, and how much stays on your hair depends on the formula, your hair and how you wash it. To see what every name on a list does, paste it into the ingredient checker.