Cyclopentasiloxane comedogenic rating matters less than D5’s 2027 EU leave-on restriction

A brand owner sends two questions about the leave-on serum you make for them: what is the comedogenic rating of its cyclopentasiloxane, and is the ingredient safe? The first has no answer in PubChem or in the EU regulation that restricts the substance. Nor does the regulation rule on skin safety: its recitals give environmental grounds for the restriction. Its dated limit reaches your formula whether or not anyone settles either question. For a leave-on product sold in the EU, the date that decides the reformulation is 6 June 2027.

Cyclopentasiloxane comedogenic rating: is there one, and is D5 safe?

No cyclopentasiloxane (D5) comedogenic rating appears in PubChem’s record (CID 10913) or in the EU regulation that restricts it. What is settled is regulatory: Commission Regulation (EU) 2024/1328 bars placing leave-on cosmetics containing 0.1% or more of D5 by weight on the EU market after 6 June 2027, on environmental grounds.

The questions behind the rating, whether cyclopentasiloxane clogs pores or is linked to acne, are about finished products on skin, and neither PubChem nor the regulation settles them. Treat any rating you are quoted as a claim that needs its own evidence: ask for the test method, the concentration tested and the vehicle. A number for the neat raw material says nothing on its own about your formula.

Handling questions belong to the current Safety Data Sheet (SDS) for your grade and the safety section of the product page. What the REACH restriction decides is whether D5 can stay in a formula sold there (the separate EU cosmetics safety assessment is outside this post), and that turns on what happens after the product leaves the skin.

Why did the EU restrict cyclopentasiloxane?

Commission Regulation (EU) 2024/1328 gives its reasons in its recitals. Recital 2 records that on 13 June 2018 ECHA’s Member State Committee identified D4, D5 and D6 as SVHC with vPvB properties (very persistent and very bioaccumulative). The same recital records D5 and D6 as having PBT properties when they contain 0.1% or more by weight of D4.

The Annex XV dossier, submitted on 20 March 2019, found Union-wide action necessary to address risks to the environment. The Risk Assessment Committee (RAC) opinion of 28 November 2019 confirmed concerns when D4, D5 and D6 in consumer and professional products end up in the aquatic and atmospheric compartments. RAC also concluded that wide-dispersive uses in cosmetic products are the main source of releases (recitals 4 to 6).

In recital 15, the Commission concluded that emissions of D4, D5 and D6 from consumer and professional products pose an unacceptable risk. Its recitals make no finding about skin, either way. The post on the environmental impacts of silicones in manufacturing covers the wider emissions picture.

PubChem data show why the destination matters. Cyclopentasiloxane dissolves in water only to 1.7 × 10⁻² mg/L at 25 °C and has a log Kow of 8.06, so it moves out of water into fats and organic matter. The post on why cyclopentasiloxane does not dissolve in water covers that side. A Henry’s law constant of 33.0 atm-cu m/mole at 25 °C means it escapes readily from water to air.

The property formulators buy D5 for, its volatility, is also one route by which a leave-on product releases it. The regulation targets the emissions, to water and to air, not volatility as such. PubChem’s experimental-properties record reports D5 evaporating from aluminum at mean rates of 0.029 mg per cm² per minute at 23 °C and 0.060 at 32 °C. Cosmetic formulations changed the rate, the largest effect a 2-fold decrease for a face cream at 32 °C.

Wash-off products went first: Regulation (EU) 2018/35 of 10 January 2018 restricted placing D4 and D5 on the market in wash-off cosmetic products, as the original entry 70 (recital 1). The 2024 text replaces that entry in its entirety and reaches well beyond cosmetics.

What does Regulation (EU) 2024/1328 restrict, and from when?

Regulation (EU) 2024/1328 was adopted on 16 May 2024, published in the Official Journal on 17 May 2024 and, under its Article 2, entered into force on 6 June 2024. Each later date in entry 70 falls on 6 June.

Entry 70 of REACH Annex XVII, as replaced by the regulation, lists octamethylcyclotetrasiloxane (D4, CAS 556-67-2), decamethylcyclopentasiloxane (D5, CAS 541-02-6) and dodecamethylcyclohexasiloxane (D6, CAS 540-97-6).

Paragraph 1 bars placing each of them on the market on its own, as a constituent of other substances or in mixtures, at 0.1% or more by weight of the respective substance, after 6 June 2026. The one use ban is paragraph 2, on dry-cleaning solvent. Paragraphs 3 to 7 move those dates or lift the rules for named uses.

Two readings of the text matter to a formulator. Its limit is per substance: “by weight of the respective substance” means D4, D5 and D6 are each measured against 0.1% on their own. And under the same regulation D6 is no way out: it carries the same threshold, and the text can be read two ways on its date in wash-off products.

Paragraph 3(b) applies paragraph 1 after 6 June 2027 “for all cosmetic products other than the ones mentioned in paragraph 3(a)”, and 3(a) names only D4 and D5; on that reading, D6 in a wash-off product falls under 6 June 2027. But 3(a) mentions, and defines, “wash-off cosmetic products”, and recital 18 gives the deferral to “cosmetic products other than wash-off cosmetic products”. On that cautious reading, paragraph 1’s general date of 6 June 2026 applies, so a wash-off product with 0.1% or more of D6 is restricted now.

The leave-on date was a policy choice. The Agency’s Socio-Economic Assessment Committee (SEAC) agreed with a five-year deferral for leave-on cosmetic products (recital 10). The Commission set three years, citing high emissions from that product group and RAC’s view that a longer transition means more emissions (recital 18).

Can a cyclopentasiloxane formula still be placed on the EU market?

Under entry 70, a leave-on cosmetic product containing 0.1% or more by weight of D4, D5 or D6 may not be placed on the EU market after 6 June 2027. Entry 70 is one rule among several: other EU rules, including those on cosmetic products, still apply before and after that date. Five actions follow:

1. Plan the cut-over by the date, not the production lot. Entry 70 has no clause for stock made before 6 June 2027.

2. Specify residual cyclics, and replace D5 blends. Does residual D5 count toward your limit? Paragraph 1(b) covers D5 “as a constituent of other substances” and paragraph 5 lifts 1(b) for D5 “as a constituent of a silicone polymer on its own”, so one reading treats the residue as part of the polymer. Paragraph 6 lifts 1(c), the mixtures point, for polymer residues only in its points (a) to (j), each with its own cap, none cosmetic, so the other counts residues in your cosmetic. Plan to the second.

   Put D4, D5 and D6 content on the Certificate of Analysis (CoA) specification of each silicone and multiply by its use level to estimate its share. Blends supplied in cyclopentasiloxane, such as dimethiconol or crosspolymer blends, carry D5 as their main fluid, not a residue: replace them.

   A blend sold to you as a raw material is a mixture, not a cosmetic product, so on a cautious reading the 2027 cosmetic deferral does not cover it and its supply has been under paragraph 1 since 6 June 2026 unless a paragraph 4 derogation applies.

3. Split hair care by product type. Under paragraph 3(a), a rinse-off conditioner is a wash-off product (washed off with water after application), held below 0.1% D5 after 31 January 2020; a leave-in serum falls under 6 June 2027. D6 in a rinse-off conditioner is the case the text can be read two ways on: treat it as restricted now, on the 6 June 2026 reading, unless counsel confirms 2027. The post on whether cyclopentasiloxane is good or bad for hair covers what D5 does on hair.

4. Separate the drum from the finished product. Paragraph 4(a) exempts placing D4, D5 and D6 on the market “for the following industrial uses”, including “in the formulation or (re)packing of mixtures”. Read broadly, D5 shipped to an EU industrial formulation site falls under 4(a), and the 0.1% rule lands on the cosmetic made from it. Read narrowly, it covers “activities on industrial sites” (recital 11) where the listed use can be shown, and not D5 bound for a product paragraph 1 already restricts.

   Plan to the narrow reading. On either reading, D5 placed on the market on its own for a use no derogation covers has been under paragraph 1 after 6 June 2026.

5. Ask for D4 content on the CoA of any D5 you still buy. Recital 2 of the regulation records D5 as having PBT properties when it contains 0.1% or more by weight of D4.

Reformulating is a project, not a swap. Cyclopentasiloxane works as a volatile emollient, solvent and viscosity-control fluid in skin, hair and make-up products, with a viscosity of 3.9 cSt at 25 °C and a boiling point of 210 °C in PubChem. A replacement has to match that evaporation profile and feel. Short linear siloxanes (discrete compounds, each with its own CAS number) and volatile hydrocarbons are candidates, but entry 70 not naming them is no regulatory clearance.

ECHA’s Candidate List of substances of very high concern, checked on 2 October 2026, lists two short linear siloxanes for vPvB properties. Octamethyltrisiloxane (L3, CAS 107-51-7) was added on 21 January 2025 and decamethyltetrasiloxane (L4, CAS 141-62-8) on 25 June 2025. It also lists a short branched siloxane, 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (EC 241-867-7), added on 25 June 2025 for vPvB properties.

Hexamethyldisiloxane (L2, CAS 107-46-0) and dodecamethylpentasiloxane (L5, CAS 141-63-9) were not listed on 2 October 2026; that is a snapshot of the list, not a clearance. Check any replacement’s Candidate List status before you switch.

Dimethicone is a separate case: a polymer sold in viscosity grades. Any grade can carry residual D4, D5 and D6, the only three that count under entry 70. As in action 2, one reading treats them as part of the polymer (paragraphs 1(b) and 5); the cautious one counts them toward your formula’s limit (paragraph 6).

The lowest-viscosity grades can consist largely of short linear siloxanes such as L3 and L4, not just traces of them; those are a Candidate List matter, with obligations separate from entry 70. Ask for L3, L4, D4, D5 and D6 content on the CoA of any low-viscosity grade.

The selection guide to silicones in cosmetics compares the silicone options, and the post on cyclopentasiloxane in skin-care formulation covers the job D5 does in a base. Its other uses, as a dry-cleaning solvent and a siloxane intermediate, are in the overview of cyclopentasiloxane benefits, uses and properties.

What do PubChem and the EU text not settle?

• Comedogenicity, acne and pores. No rating and no finding, in either direction.
• The safety of a finished product on skin. That belongs to the product’s own testing; handling belongs to the current SDS.
• Markets outside the EU. Everything above reads the EU text only; check each destination market’s own rules.
• Enforcement detail. How authorities apply paragraph 4(a), the 2027 date to goods already in distribution and the D6 wash-off date: confirm these with your regulatory lead.

Methodology: regulatory statements quote or paraphrase Commission Regulation (EU) 2024/1328 and its EUR-Lex record; Candidate List entries come from ECHA’s ECHA CHEM database; physical data come from PubChem CID 10913. This is a reading of the published text, not legal advice.