A CAS number is a unique numeric code assigned to every known chemical substance, and it appears on a Material Safety Data Sheet (MSDS) or Safety Data Sheet (SDS) so you can identify exactly what chemicals are in a product. Think of it as a serial number for a chemical. While a single substance might go by dozens of different names across languages and industries, it has only one CAS number, making it the most reliable way to look up safety information, regulatory limits, and health effects.
How CAS Numbers Work
CAS stands for Chemical Abstracts Service, a division of the American Chemical Society that maintains the world’s largest database of chemical substances. Each time a new chemical is discovered or synthesized, CAS assigns it a unique registry number. The database now contains hundreds of millions of entries covering organic compounds, metals, alloys, polymers, and biological sequences.
Every CAS number follows the same format: up to 10 digits separated into three groups by hyphens. The first group has 2 to 7 digits, the second group always has 2 digits, and the third group is a single check digit. That final digit is calculated from the others using a simple formula, which lets software instantly verify whether a CAS number is valid. For example, water is 7732-18-5, ethanol is 64-17-5, and sodium chloride (table salt) is 7647-14-5.
The numbers themselves are assigned sequentially and carry no information about the chemical’s structure, properties, or hazards. They’re purely identifiers.
Where CAS Numbers Appear on an SDS
Under the Globally Harmonized System (GHS), which OSHA adopted in 2012, safety data sheets follow a standardized 16-section format. CAS numbers are required in Section 3: Composition/Information on Ingredients. This section lists each hazardous ingredient by its chemical name, common names and synonyms, and its CAS number alongside any other unique identifiers.
For a pure substance, Section 3 is straightforward: one chemical, one CAS number. For mixtures like cleaning products, paints, or adhesives, every hazardous ingredient above certain concentration thresholds gets its own line with its own CAS number. This is what makes the number so useful in practice. If you need to know the specific health risks of one ingredient in a 12-component mixture, the CAS number lets you search databases, poison control resources, and exposure guidelines without any ambiguity about which chemical you’re researching.
Why Chemical Names Alone Aren’t Enough
A single chemical can have a formal IUPAC name, several common names, brand names, and abbreviations that vary by country and industry. Acetone, for instance, is also called 2-propanone, dimethyl ketone, and propan-2-one. In some older documentation, you might see yet another trade name. All of these refer to CAS number 67-64-1. Without that number, cross-referencing safety information across different data sheets, regulatory lists, and exposure databases would be an exercise in guesswork.
This becomes especially important when you’re dealing with chemicals that have similar names but very different hazard profiles. Searching by CAS number eliminates the risk of confusing two substances.
CAS Numbers vs. Other Identifiers
CAS numbers are the global standard, but they’re not the only chemical identification system. The European Union uses EC numbers (also called EINECS numbers) for substances on its commercial chemical inventory. One key difference: an EC number sometimes groups the water-free and hydrated forms of a substance under a single entry, while CAS assigns separate numbers to the anhydrous form and its hydrated versions. So a CAS number can be more specific in certain cases, while an EC number may be broader.
You may also encounter UN numbers on shipping documents, which identify hazardous materials for transport purposes, or index numbers used in EU classification and labeling. None of these replace the CAS number on an SDS. They serve different regulatory purposes, and you’ll often see multiple identifiers listed together in Section 3 or Section 14 of the same data sheet.
When a CAS Number Is Missing
Sometimes you’ll find that Section 3 of an SDS lists an ingredient without a CAS number, or replaces the chemical name with a phrase like “proprietary ingredient” or “trade secret.” OSHA’s Hazard Communication Standard allows manufacturers to withhold the specific chemical identity or exact concentration of a hazardous ingredient if they claim it as a trade secret. However, they cannot simply leave the field blank. The SDS must explicitly state that the information is being withheld as a trade secret.
Even when a trade secret claim is made, the manufacturer still has to disclose the ingredient’s health hazards, protective measures, and first-aid information elsewhere on the SDS. And in a medical emergency, they’re required to provide the withheld identity to a treating healthcare professional. So a missing CAS number limits your ability to independently research the chemical, but it doesn’t eliminate the safety information on the document entirely.
How to Use a CAS Number
Once you have a CAS number from an SDS, you can plug it into several free resources to learn more about the substance. The National Library of Medicine’s PubChem database, NIOSH’s Pocket Guide to Chemical Hazards, and the EPA’s CompTox Chemicals Dashboard all accept CAS number searches. These tools give you toxicity data, occupational exposure limits, environmental fate information, and links to peer-reviewed studies.
If you work with chemicals regularly, building the habit of searching by CAS number rather than chemical name will save you time and reduce the chance of pulling up information on the wrong substance. It’s the single most efficient way to move from an SDS to deeper safety data.