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One CAS Number Is Not One Cosmetic Ingredient

A CAS registry number looks like the most reliable identifier a compliance team has. Names vary by language, by supplier and by decade. The number does not. So a common shortcut in regulatory checks is to take an ingredient's CAS number and look up every restriction filed against it.

We checked how well that shortcut holds up across our database of 21,796 cosmetic ingredients. It holds up less often than the number's reputation suggests, and when it fails, it fails in a consistent direction.

How many ingredients share a number

Only 8,403 of the 21,796 ingredients carry a CAS number at all. The other 61 percent have none, so a CAS-based check cannot see them.

Among the ingredients that do carry one, there are 7,419 distinct CAS numbers. Of those, 710 are listed against more than one ingredient, and 2,730 ingredients carry at least one shared number.

Ingredients sharing one CAS number CAS numbers
2369
3109
456
549
6 to 1079
11 or more48

The largest single group is CAS 9003-11-6, a copolymer number used for poloxamers, listed against 68 separate ingredients.

Why the numbers are shared

Polymers and protein hydrolysates dominate the largest groups.

Polymer families. A CAS number is often assigned to a class of polymer rather than to one chain length. INCI names the chain lengths individually. So one number can cover dozens of copolymer grades, or twenty or so steareth grades, each of which appears as its own ingredient with its own name.

Hydrolysates and amino acid mixtures. In our data, amino acid mixtures from wheat, oat, soy, silk and corn gluten share one number, and hydrolyzed proteins from collagen, silk and pea share another. Whether that comes from a broad registry entry or from loose assignment in the source dictionary, the effect on a lookup is the same.

Sharing is not always an error. A CAS number describes a chemical substance. A cosmetic ingredient name describes something narrower: a substance from a particular source, at a particular grade, used in a particular way. The two systems were built for different jobs.

Where the shortcut goes wrong

CAS 65072-01-7 is listed against 15 ingredients in our data, including wheat, oat, soy, silk and corn gluten amino acids. It is also listed against hair keratin amino acids.

Hair keratin is of human origin. Six of the markets we track, the EU, ASEAN, China, Taiwan, Brazil and Argentina, prohibit cells, tissues or products of human origin in cosmetics. Those prohibitions are written for the hair keratin entry.

Look up restrictions by CAS number alone and wheat amino acids come back prohibited in six markets.

The same pattern shows up in CAS 73049-73-7, shared by 25 hydrolyzed proteins including collagen, silk and pea protein. One of them is hydrolyzed hair keratin, prohibited in six markets and restricted in Canada as a substance of human origin.

The direction of the failure matters. A shared number does not hide a restriction. It spreads one restriction across ingredients it was never written for, and a formulator acting on the result would drop a raw material for no regulatory reason.

Smaller traps

Partial matches. CAS numbers are strings of digits, and a lookup that tests whether one string contains another will match numbers that happen to end the same way. 112-07-2, butoxyethyl acetate, sits inside 26112-07-2, potassium methylparaben. A containment search for the first returns the paraben restrictions of the second. We found three cases of this in our data. Matching the whole number, not a substring, removes them.

Errors in the source. Ingredient dictionaries are compiled by people, and some entries carry the wrong number. In our source data, a synthetic peptide is listed with CAS 532-32-1, which is the number for sodium benzoate, a preservative. Any check keyed on that number attaches preservative limits to a peptide. We found fourteen entries of this kind.

What holds up better

The registered ingredient name, INCI in most markets, is a safer primary key for cosmetic compliance than the CAS number. Several of the restrictions in our data name a specific ingredient or a specific plant part rather than a chemical class, and a name-based lookup keeps that distinction. We saw the same thing with fragrance allergens, where the rules name individual molecules.

The CAS number still has a job. It is useful for confirming that two differently written names refer to the same substance, and for connecting cosmetic data to chemical inventories. It works best as supporting evidence, not as the lookup key.

When several ingredients share a number and carry different restrictions, the honest answer is to report that the number is ambiguous rather than pick one. When a number is shared by a whole polymer family or a class of hydrolysates, restrictions filed against one member should not be read onto the others without checking the regulation's own wording.

Methodology and sources

Ingredient data comes from the Korea Cosmetic Association ingredient dictionary. Regulatory entries are compiled from Korea's Ministry of Food and Drug Safety open data covering restricted cosmetic ingredients, and from EU CosIng Annexes, across 10 markets: the EU, Korea, ASEAN, China, Japan, Taiwan, Brazil, Argentina, Canada and the US.

Figures reflect our production database as of 2026-09-22, holding 21,796 ingredients and 31,191 regulatory entries. CAS numbers were split on commas, slashes and whitespace, and only tokens in valid CAS format were counted. A CAS number counts as shared when it appears on two or more distinct ingredient records.

Regulatory data changes. Verify against the current official source before relying on any of this for a compliance decision.

For the underlying data, see K-Beauty Cosmetic Ingredients on RapidAPI.

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