What a CAS Number Is and Where Filings Actually Use It

You'll see CAS numbers on safety data sheets, supplier certificates of analysis, and sometimes stamped right on a drum of raw material. It's tempting to treat that string of digits as the thing your filing needs. It isn't, not directly. The filing needs the INCI name. The CAS number is what confirms you've got the INCI name right.

What CAS actually is

CAS stands for Chemical Abstracts Service, and a CAS number is a unique registry identifier assigned to a specific chemical substance. It's formatted as a string of digits broken into groups, something like 112-34-5, and it exists so that chemists, regulators, and suppliers worldwide can point at one substance without ambiguity, regardless of what any given manufacturer decides to call it that week.

INCI names do something similar but for a different audience. INCI (International Nomenclature of Cosmetic Ingredients) is the standardized system for naming cosmetic ingredients on labels and in filings. Water is Aqua. Glycerin is Glycerin. The two systems overlap heavily, but they're not the same list, and they don't map one to one in every case.

Why the mismatch happens

A few situations come up constantly when you're building an ingredient list:

• One substance, multiple CAS numbers. Some ingredients have been registered more than once over the decades, often because of how a substance was manufactured or characterized at the time. You might see two different CAS numbers on two different supplier documents for what is functionally the same INCI entry.
• Botanical extracts with no CAS number at all. A plant extract is a complex mixture, not a single molecule, so it frequently has no CAS registry entry. It still has a valid INCI name, and that's what goes on the filing.
• Trade names hiding the real identity. A supplier blend sold under a brand name isn't a filing entry by itself. You need to expand it into its component INCI names, and the CAS numbers on the underlying components are usually your best confirmation that you've identified them correctly.

Where CAS earns its keep

Despite not being the primary filing field, the CAS number is genuinely useful in a few spots:

1. Cross-checking supplier paperwork. When a certificate of analysis and a safety data sheet both list the same CAS number, you can be confident they're describing the same substance, even if the trade name or spelling of the INCI varies slightly between documents.
2. Reading section 3 of an SDS. That's the composition section, and it typically lists each component's CAS number alongside its concentration or concentration range. It's often the fastest way to reverse-engineer what's actually inside a blend.
3. Resolving ambiguous or generic ingredient names. If a supplier calls something "preservative blend A12" with no further detail, the CAS numbers in the SDS composition section are usually your only real path to identifying what you're working with.

A quick example

Say you're formulating a lotion and your preservative supplier's product sheet lists a blend under a trade name, with an SDS that shows two components in section 3, each with its own CAS number and a percentage range. You'd take each CAS number, confirm the corresponding INCI name, then multiply each component's percentage within the blend by your product's use level of the blend itself. That gives you the real concentration of each individual ingredient in the finished formula, which is the number that actually matters against any prohibited or restricted list.

What goes on the actual filing

Health Canada's Cosmetic Notification Form asks for ingredients by INCI name with a concentration or concentration range, not by CAS number. The same pattern holds broadly across other cosmetic regimes: the ingredient list on a label and in most notification systems is INCI-based. The CAS number does its work behind the scenes, in your documentation, not on the form itself.

Keep a record either way

Even though CAS numbers rarely show up on the finished filing, it's worth keeping them in your internal formula records. If a supplier reformulates a blend or an ingredient gets flagged under a different name somewhere down the line, having the CAS numbers documented alongside your INCI list makes it much faster to confirm whether anything actually changed in your product or just in the paperwork.

This is exactly the kind of mapping problem Cosmetic Comply was built to handle. Feed in your ingredient list and it matches each entry to its INCI name and CAS number, expands supplier blends into their real components, and carries the percentages through so the concentration that hits the prohibited and restricted list check is the actual one, not a guess based on a trade name.