Why Some Cosmetic Ingredients Have No CAS Number — Or Several — And How To Look Them Up

Search 'bentonite cas no' and you get a single, tidy answer: 1302-78-9. Search 'sodium lauroyl methyl isethionate cas no' and the tidy answer disappears — the EU's own cosmetic ingredient inventory records no CAS number for it at all. Neither result is a data error. Both are normal outcomes of how CAS numbers and EC numbers actually get assigned to cosmetic raw materials, and formulators and regulatory staff who don't know why end up either paralysed by a blank field or trusting the wrong number pulled from a supplier catalogue.

This article explains what a CAS Registry Number and an EC number actually identify, why certain classes of cosmetic ingredient — polymers, ethoxylates, botanical extracts, natural clays, reaction mixtures — routinely end up with a generic, shared, multiple or absent identifier, and how to look one up safely when a supplier Safety Data Sheet and the INCI list don't agree. All examples below are pulled live from cosingchecker.com's ingredient and identifier records.

What a CAS number and an EC number actually are

A CAS Registry Number is assigned by CAS, a division of the American Chemical Society, to a specific chemical substance recorded in the CAS Registry. The registry covers far more than simple organic compounds: CAS describes its Registry as covering, besides organic and inorganic compounds, polymers and non-structurable materials of unknown or variable composition and biologics, the category usually abbreviated UVCB. CAS numbers are structural or compositional identifiers, not legal ones: CAS assigns them regardless of whether a substance is permitted, restricted or banned anywhere.

An EC number is a different kind of identifier, issued for EU regulatory purposes rather than by a chemistry abstracting service. It is a seven-digit code in the form XXX-XXX-X, and it comes from one of three source lists that together make up the EC Inventory published by the European Chemicals Agency (ECHA): EINECS, for substances that were already on the EU market between 1 January 1971 and 18 September 1981; ELINCS, for substances notified as new under the old Dangerous Substances Directive after that date; and the NLP list ('No-Longer Polymers'), for substances that were classified as polymers under the old rules but stopped being exempt when the definition of 'polymer' changed. A substance can have an EC number without a CAS number, a CAS number without an EC number, or — for genuinely novel or non-standard materials — neither.

CosIng, the European Commission's cosmetic ingredient database, and cosingchecker.com's mirror of it record both fields per entry where they exist, alongside the INCI name, the annex placement and the restriction text. Across the whole EU Annex II-VI dataset the site currently holds, 2,874 distinct CAS numbers and 2,646 distinct EC numbers are recorded (the site's stats endpoint calls these cas_group_count and ec_group_count) against 2,353 total annex entries — already a sign that identifiers are not issued one-for-one with entries: several INCI names can share one CAS number (see bentonite below), and one entry can list several (see Quaternium-18 Bentonite below).

Why some ingredient classes rarely get a clean, single CAS number

A handful of ingredient categories systematically produce messy identifiers, for structural reasons rather than data gaps.

• Polymers: CAS names polymers by their monomer(s) or structural repeating unit, not by exact molecular weight or chain length, so materials that differ only in degree of polymerisation are often not distinguished by a separate number.
• Ethoxylates (INCI names ending '-eth-n', such as deceth-3): these are polyethylene glycol ethers with a stated average number of ethylene oxide units, not a single molecule — each named degree of ethoxylation can get its own CAS entry, but the underlying material is itself a distribution of chain lengths, which is why adjacent members of a series sometimes carry a CAS number and sometimes don't: in the site's data deceth-3 and deceth-9 carry one, and deceth-7 carries none.
• Botanical extracts: an extract's composition depends on the plant part, extraction solvent and process, so CAS treats many extracts as UVCB substances rather than single compounds, and a wide range of suppliers' extracts of the same plant can legitimately carry different or no CAS numbers.
• Natural clays such as bentonite: bentonite is not one compound but a natural, variable mineral (montmorillonite-dominant clay with impurities), which is why CosIng assigns the same CAS/EC pair to bentonite and to several other minerals with overlapping composition (see the worked example below).
• Reaction mixtures and salts of reaction products: CosIng entries describing 'the reaction product of X and Y' or 'salts with' another substance often have no independent CAS number at all, because the mixture itself was never separately registered.

Worked example: one CAS number, four INCI names (bentonite)

Bentonite is the cleanest illustration of why 'one substance, one CAS number' breaks down for minerals. In the site's Annex IV colorant/mineral data, entry IV/119 is recorded under CAS 1302-78-9 and EC 215-108-5, described as 'natural hydrated aluminium silicate... containing calcium, magnesium or iron carbonates, ferric hydroxide, quartz-sand, mica, etc. as impurities.' That single identifier pair is associated on the site with four different identified INCI names: BENTONITE, CI 77004, KAOLIN and ALUMINUM SILICATE. cosingchecker.com/cas/1302-78-9/ lists every ingredient record sharing that CAS number, and cosingchecker.com/ec/215-108-5/ lists every record sharing that EC number — for this pair, both pages currently show the same single underlying annex entry shared across the four INCI names.

A related but different pattern shows up one step down the alphabetised bentonite family: HYDROGENATED TALLOWALKONIUM BENTONITE (CAS 71011-24-0, EC 275-124-3) and STEARALKONIUM BENTONITE are each modified organoclays with their own distinct CAS numbers, because reacting bentonite with a quaternary ammonium compound produces a different registered substance. QUATERNIUM-18 BENTONITE, by contrast, is recorded on the site with two CAS numbers separated by a slash — '68953-58-2 / 1340-69-8' — reflecting that the ingredient description covers a class of quaternary ammonium salts with bentonite rather than one fixed reaction product. And QUATERNIUM-18/BENZALKONIUM BENTONITE has no CAS or EC number recorded at all. Four ingredients, four different identifier patterns, one shared root material.

Worked example: deceth-3 and the ethoxylate series

DECETH-3 (decanol, ethoxylated to an average of 3 moles of ethylene oxide) is recorded on the site with CAS 26183-52-8 and no EC number. Searching the site's ingredient data for 'deceth-3' returns 14 related entries — DECYLTETRADECETH-30, ISODECETH-30, POTASSIUM TRIDECETH-3 CARBOXYLATE, PPG-2-DECETH-3 and others — and the pattern is inconsistent by design: OCTYLDODECETH-30 has both a CAS number (32128-65-7) and an EC number (500-080-1), while ISODECETH-30, POTASSIUM TRIDECETH-3 CARBOXYLATE and PPG-2-DECETH-3 have neither recorded. This is the ethoxylate rule in practice: coverage of CAS and EC numbers across a homologous '-eth-n' series is uneven, because each named degree of ethoxylation is a separate registration decision, not an automatic consequence of the parent alcohol having a number.

Worked examples: a clean single CAS, and a genuine none

Not every preservative or specialty ingredient is messy. METHYLCHLOROISOTHIAZOLINONE — the 'MCI' half of the MCI/MI preservative pair, restricted under Annex V entry V/39 — has one CAS number (26172-55-4) and one EC number (247-500-7) recorded on the site, consistent with it being a single, well-defined molecule (5-chloro-2-methyl-2H-isothiazol-3-one). METHYLENE DI-T-BUTYLCRESOL, also known as DBMC, likewise carries a single CAS number (119-47-1) and EC number (204-327-1); the site's Annex II data shows it listed as entry II/1682, added under Regulation (EU) 2022/1531 and flagged as Reprotoxic Category 1B in that record — a site record, not a safety verdict, and readers should check the regulation text linked from the entry for the current legal status.

SODIUM LAUROYL METHYL ISETHIONATE — a common mild sulfate-alternative surfactant in cleansers — is the genuine 'none' case: the site's ingredient record for it (description: 'Dodecanoic acid, methyl-2-sulfoethyl ester, sodium salt (1:1)') carries no CAS number and no EC number, and it also has zero recorded EU annex evidence and zero jurisdiction rules in the site's data. That absence describes what is missing from the data the site has indexed — it is not evidence that the ingredient is unregulated or that no CAS number exists anywhere; a supplier or a chemistry database may hold one that CosIng has simply never recorded against this INCI name.

How to look an identifier up, and what to do when sources disagree

Start from the INCI name, not the number, because the INCI name is the label declaration you're trying to match. cosingchecker.com/search/ lets you search by INCI name, CAS number or EC number and returns every matching ingredient record. Once you have a candidate identifier, cosingchecker.com/cas/ and cosingchecker.com/ec/ pages (for example cosingchecker.com/cas/1302-78-9/) each list every ingredient record on the site that shares that exact identifier — which is the fastest way to see, as with bentonite above, whether a number is shared across several INCI names before you assume it points to one substance.

When a supplier's Safety Data Sheet quotes a CAS number that doesn't match the INCI list, or quotes none at all, treat the mismatch as a prompt to check identity, not as something to resolve by picking whichever number looks more official. A shared CAS number across several INCI names (like bentonite's) means the number alone cannot tell you which specific labelled ingredient you have — you need the full record, including the chemical description, to confirm the match. Conversely, a missing CAS number on an EU regulatory record does not mean the supplier's number is wrong; UVCB materials, extracts and reaction mixtures are exactly where different databases and different suppliers most often diverge, because each may be describing a slightly different manufacturing process under the same INCI name. In all of these cases the identifier is evidence of identity, not a substitute for reading the full entry, and it says nothing on its own about whether an ingredient is permitted in a given market — that is a separate question, answered by the annex or hotlist entry, not the identifier.

Sources

• CAS Registry (CAS, a division of the American Chemical Society) — https://www.cas.org/cas-data/cas-registry
• CAS Registration Criteria Overview — https://web.cas.org/marketing/pdf/CAS-Registration-Criteria-Overview.pdf
• EC Inventory (EINECS/ELINCS/NLP) — European Chemicals Agency — https://echa.europa.eu/information-on-chemicals/ec-inventory
• cosingchecker.com ingredient record: bentonite — https://cosingchecker.com/ingredients/32125-bentonite/
• cosingchecker.com ingredient record: deceth-3 — https://cosingchecker.com/ingredients/75433-deceth-3/
• cosingchecker.com ingredient record: methylchloroisothiazolinone — https://cosingchecker.com/ingredients/35333-methylchloroisothiazolinone/
• cosingchecker.com ingredient record: sodium lauroyl methyl isethionate — https://cosingchecker.com/ingredients/86105-sodium-lauroyl-methyl-isethionate/
• cosingchecker.com ingredient record: methylene di-t-butylcresol — https://cosingchecker.com/ingredients/35336-methylene-di-t-butylcresol/
• cosingchecker.com CAS-number index page — https://cosingchecker.com/cas/1302-78-9/
• cosingchecker.com EC-number index page — https://cosingchecker.com/ec/215-108-5/
• cosingchecker.com identifier and annex search — https://cosingchecker.com/search/
• Regulation (EU) 2022/1531 (adds DBMC / methylene di-t-butylcresol to Annex II) — https://eur-lex.europa.eu/eli/reg/2022/1531