The CAS Number on an SDS is a unique numeric identifier assigned by the Chemical Abstracts Service to one specific chemical substance, and it appears in Section 3 (Composition/Information on Ingredients) of a Safety Data Sheet alongside the chemical name and concentration. It looks like a string of digits broken into three groups by hyphens, such as 67-64-1 for acetone. The CAS Registry Number identifies the substance itself, not the mixture, not the product, and not the hazard. It is the one field on an SDS that stays constant across manufacturers, languages, and countries, which is why regulators, downstream users, and poison control centers rely on it.
What a CAS Number Actually Is and How It Is Built
A CAS Registry Number is a numeric tag the American Chemical Society's Chemical Abstracts Service assigns to a chemical as it enters the scientific literature. The service maintains the registry, and as of 2026 it lists more than 200 million unique substances, including organics, inorganics, polymers, and even some biological sequences. Each number is permanent. Once assigned, it is never reassigned to a different substance, even decades later.
The format follows a check-digit scheme. A CAS Number has two to ten digits split into three groups by hyphens, and the final group after the last hyphen is a single check digit. To validate it, read the digits from right to left skipping the hyphens, multiply each by its position (1, 2, 3...), sum the products, and divide by 10. If the remainder is zero, the number is valid. For acetone, 67-64-1, the check digit is 1, and the arithmetic confirms it. This matters because a single transposed digit on an SDS can point to the wrong chemical entirely.
The number tells you nothing about hazard, toxicity, or physical state by itself. It is a lookup key, not a rating. Two substances with adjacent CAS numbers can have wildly different hazards. In plain terms: the CAS Number is the chemical's legal name in numeric form, not its warning label.
Why the CAS Number Sits in Section 3 of an SDS
Section 3 of a Safety Data Sheet is titled Composition/Information on Ingredients, and that is where the CAS Number lives. Under the Globally Harmonized System, which the US adopted through OSHA's Hazard Communication Standard in 2012, Section 3 must identify each hazardous ingredient by chemical name and concentration. The CAS Number is the standard way to make that identification unambiguous.
The reason is linguistic and legal. A chemical can carry dozens of trade names, synonyms, and translations. "Acetone" is also sold as propanone, dimethyl ketone, and 2-propanone. A formulator in Ohio and a buyer in Germany may use different names for the same substance. The CAS Number collapses all of that into one string that every database, regulator, and toxicologist recognizes.
A 2021 update to OSHA's Hazard Communication Standard tightened Section 3 requirements further, and a subsequent 2024 rule aligned US SDS formatting more closely with GHS revision 7. In practice, if a hazardous ingredient makes up 1 percent or more of a mixture, or 0.1 percent or more if it is a carcinogen, it should appear in Section 3 with its CAS Number. Below those thresholds, disclosure is often optional, which is a limitation covered later.
Where to Find It and How to Read It on a Real SDS
On a printed or PDF Safety Data Sheet, the CAS Number appears in the table in Section 3, usually in a column labeled CAS No., CAS Registry Number, or CAS #. The same number may also appear in Section 1 under product identifier if the product is a single substance rather than a mixture. On a mixture SDS, Section 3 lists several rows, one per hazardous ingredient, each with its own CAS Number and concentration range.
Watch for these patterns when reading Section 3:
- A single CAS Number with no concentration range usually means a pure substance, not a blended product.
- Concentration ranges like 30 to 60 percent are common because exact formulation is often proprietary.
- The word "trade secret" may replace a CAS Number when the manufacturer claims confidentiality, which is legal under specific conditions.
- A CAS Number followed by "and isomers" or "and salts" signals a broader substance category, not one exact molecule.
- Mixtures may show a CAS Number that belongs to the mixture as a whole, distinct from the ingredients listed below it.
- Duplicate numbers across rows with different names usually indicate synonyms for the same ingredient.
If Section 3 lists no CAS Number at all, the SDS is incomplete under GHS expectations and should be treated as suspect.
How CAS Numbers Compare to Other Identifiers
The CAS Number is one of several chemical identifiers, and each serves a different purpose. Knowing which to use prevents costly mismatches in purchasing, labeling, and regulatory reporting.
| Identifier | Example (acetone) | Who assigns it | Best used for | Key limitation |
|---|---|---|---|---|
| CAS Registry Number | 67-64-1 | Chemical Abstracts Service | Universal substance lookup, SDS Section 3, regulatory lists | Says nothing about hazard or concentration |
| EC Number | 200-662-2 | European Chemicals Agency | EU REACH and CLP compliance | EU-centric, not always assigned to new substances |
| UN Number | 1090 | United Nations (transport) | Shipping and dangerous goods classification | Covers transport hazard class, not identity |
| RTECS Number | AL3150000 | NIOSH | Occupational toxicity reference | Not a regulatory identifier, rarely on SDS |
| IUPAC Name | propan-2-one | IUPAC nomenclature | Precise structural naming | Long, language-dependent, error-prone in typing |
| SMILES String | CC(=O)C | Cheminformatics convention | Software and database search | Not human-readable, not on most SDS |
The practical rule: use the CAS Number for identity, the UN Number for shipping, and the EC Number for European regulatory work. Mixing them up is a common compliance error, especially when a shipping clerk uses a CAS Number where a UN Number is required on a transport document.
How CAS Numbers Behave Across Mixtures, Polymers, and Trade Secrets
Mixtures are where CAS Numbers get slippery. A blended solvent may show five or six CAS Numbers in Section 3, one per hazardous ingredient, and none for the mixture itself. Some manufacturers do obtain a CAS Number for a proprietary mixture, but that number identifies the blend, not the components, and it will not appear on pure-substance databases.
Polymers present another edge case. A polymer like polyethylene has a CAS Number for the repeat unit and often a separate number for the finished resin. Regulatory lists sometimes target the monomer, such as ethylene oxide at CAS 75-21-8, because the monomer carries the hazard while the polymer does not. A formulator reading Section 3 may therefore see a monomer CAS Number in a product that contains no free monomer at all, listed because residual monomer can remain above trace levels.
Trade secret claims are the most consequential edge case. Under GHS and OSHA rules, a manufacturer can withhold a specific chemical identity and CAS Number if the ingredient qualifies as a trade secret and the SDS substitutes a generic description. The catch is that the actual CAS Number must still be disclosed to a treating physician or nurse in a medical emergency, and to workers or their representatives on request under defined conditions. In plain terms: the number can be hidden from the public sheet but not from the people who need it to treat an exposure.
Correcting Four Common Misconceptions
Misconception one: the CAS Number on an SDS tells you how dangerous the product is. It does not. Acetone (67-64-1) and hydrogen cyanide (74-90-8) both have valid CAS Numbers, and one is a common solvent while the other is lethal at low exposure. Hazard lives in Sections 2, 8, and 11, not in the number.
Misconception two: a higher CAS Number means a newer or more hazardous chemical. The registry assigns numbers roughly in order of first literature appearance, but the sequence is not chronological in any reliable way, and it carries no hazard meaning. Some of the oldest known substances have high numbers because they entered the registry late.
Misconception three: the same CAS Number guarantees the same SDS. The number identifies the substance, but SDS content varies by manufacturer, jurisdiction, and revision date. Two SDSs for the same CAS Number can differ in Section 3 concentration ranges, Section 8 exposure limits, and even Section 2 classification if one follows an older GHS revision. Always check the revision date on page one.
Misconception four: a missing CAS Number means a safe product. It usually means an incomplete SDS, a trade secret claim, or a mixture below disclosure thresholds. None of those is a safety signal.
What to Actually Do With the CAS Number
The CAS Number is most useful as a cross-checking tool. Use it to confirm you have the right SDS for the right substance before you file it, ship it, or store it. If the CAS Number in Section 3 does not match the one on your purchase order, stop and resolve the discrepancy before the material moves.
Practical actions that prevent the most common errors:
- Verify the CAS Number's check digit before entering it into any inventory or EHS system.
- Search regulatory lists, such as EPA's TSCA Inventory or the EU's REACH candidate list, by CAS Number rather than by trade name.
- When comparing SDSs from two suppliers for the same product, align them by CAS Number in Section 3 to spot ingredient differences.
- Store the CAS Number in your chemical inventory database as the primary key, with trade names as secondary fields.
- Flag any SDS where Section 3 lists a hazardous ingredient above 1 percent without a CAS Number for follow-up with the supplier.
These steps take minutes and prevent mislabeled shipments, wrong-substance purchases, and audit findings.
Limits of the CAS Number and What It Cannot Tell You
The CAS Number is a precise identity tool with real boundaries. It does not describe concentration, purity, physical form, or hazard. It does not tell you whether the substance is a solid, liquid, or gas at room temperature. It does not indicate whether the SDS you are reading is current, since revisions happen without changing the number.
A subtler limit is that the same CAS Number can cover a family of related substances. "Xylene" as a category includes three isomers with separate numbers (o-xylene 95-47-6, m-xylene 108-38-3, p-xylene 106-42-3) plus a mixed-isomer number (1330-20-7). An SDS may use any of these depending on what the supplier actually sells. Reading only the CAS Number without the chemical name and concentration can therefore give a false sense of precision.
Finally, the registry is not a regulatory authority. Inclusion in the CAS Registry does not mean a substance is approved, restricted, or safe. That determination belongs to bodies like EPA, OSHA, and ECHA, and those bodies maintain their own lists keyed to CAS Numbers. The number is the common thread that lets you connect the SDS to those lists, and that connection is where its real value lies.