A CAS number on a Safety Data Sheet (SDS) is a unique numerical code assigned to a specific chemical substance so it can be identified without confusion. You’ll find it in Section 3 of the SDS, listed alongside each ingredient’s chemical name, common name, and concentration. It exists because chemicals often go by multiple names, and a single standardized number eliminates any ambiguity about exactly which substance you’re dealing with.
How CAS Numbers Work
CAS stands for Chemical Abstracts Service, a division of the American Chemical Society that has been assigning these identifiers since 1965. Each CAS number is a sequence of digits separated by two hyphens, like 7732-18-5 (water) or 67-56-1 (methanol). Every number maps to one and only one substance. No two chemicals share a CAS number, and no single chemical has more than one.
This matters because chemical naming is messy. A single substance can have a systematic scientific name, a common name, several trade names, and abbreviations that vary by country and industry. Isopropyl alcohol, for instance, is also called 2-propanol, isopropanol, IPA, and rubbing alcohol. Its CAS number, 67-63-0, cuts through all of that. Whether you’re reading an SDS written in Germany or Texas, the number points to the same substance.
Where CAS Numbers Appear on an SDS
Under the Globally Harmonized System (GHS) format that OSHA requires, every SDS follows a standardized 16-section layout. CAS numbers appear in Section 3: Composition/Information on Ingredients. For a pure substance, you’ll see one CAS number listed with the chemical name, common names, and any relevant impurities or stabilizing additives. For a mixture, like a cleaning product or industrial solvent blend, Section 3 lists each hazardous ingredient separately with its own CAS number and its concentration or concentration range in the product.
There is one exception. If a manufacturer claims an ingredient as a trade secret, OSHA’s Hazard Communication Standard allows them to withhold the specific chemical identity, including the CAS number, from Section 3. However, they must still make that information available to health professionals and employees who need it for medical or safety purposes.
Why CAS Numbers Matter for Safety
The practical value of a CAS number is speed and precision. If someone is exposed to a chemical at work, the CAS number lets you pull up exact toxicity data, first aid protocols, and exposure limits without guessing which “sodium hydroxide” or “ethylene glycol” variant you’re actually dealing with. Poison control centers, emergency responders, and occupational health professionals all rely on these numbers to look up the right safety information instantly.
Government agencies use CAS numbers for the same reason. The EPA uses them to identify hazardous substances under environmental reporting laws. OSHA’s Hazard Communication Standard specifically defines “specific chemical identity” as including the CAS Registry Number. Regulatory databases worldwide are indexed by CAS number, making it the fastest way to cross-reference a chemical across different agencies, countries, and languages.
How to Use a CAS Number
If you find a CAS number on an SDS and want more information about that chemical, you can search it in several free databases. CAS Common Chemistry is the registry’s own open lookup tool. The National Library of Medicine’s PubChem database and the EPA’s CompTox Chemicals Dashboard both accept CAS number searches and return detailed safety, toxicity, and regulatory data.
When comparing products, CAS numbers let you quickly check whether two SDSs are describing the same ingredient under different names. This is especially useful when you’re evaluating substitute products or verifying that a supplier’s formulation matches what you expected. Two products might list completely different ingredient names on the label but contain the same chemical, and matching CAS numbers will confirm it.
If an SDS you’re reviewing is missing CAS numbers for its ingredients, that’s either a trade secret claim (which should be noted on the document) or a sign the SDS may not be compliant with current OSHA requirements. A properly formatted, GHS-compliant SDS will include CAS numbers for every identified hazardous component.