CAS numbers are the global standard for identifying chemical substances, issued by the American Chemical Society. EC numbers are European identifiers assigned by the EU for regulatory purposes. Most substances have both. REACH registration uses the EC number as the primary identifier, but SDS documents and international trade typically reference CAS numbers. For product data, store both.
Two systems, one substance
Chemical substances that exist in commerce are typically identified by at least two numeric codes: a CAS Registry Number and an EC number. Both are unique identifiers for the same substance, but they come from different organizations, serve different purposes, and have different formats.
Lead is CAS 7439-92-1 and EC 231-100-4. Bisphenol A is CAS 80-05-7 and EC 201-245-8. The numbers are unrelated to each other. You cannot derive one from the other. You simply have to know both.
Format comparison
| Property | CAS Number | EC Number |
|---|---|---|
| Issued by | Chemical Abstracts Service (ACS, USA) | European Commission / ECHA |
| Format | XXXXXXX-XX-X (5-10 digits, 3 groups) | XXX-XXX-X (7 digits, 3 groups) |
| Check digit | Yes (weighted sum mod 10) | Yes (custom algorithm) |
| Total registered | 230+ million substances | ~180,000 substances |
| Scope | All known chemical substances worldwide | Substances on the European market |
| Used in | SDS Section 3, international trade, GHS | REACH registration, EU regulations |
The most obvious difference in scope: CAS covers every chemical substance ever described in scientific literature, over 230 million entries. The EC inventory covers roughly 180,000 substances that have been manufactured or imported into the European market. A newly synthesized research compound will get a CAS number immediately but may never receive an EC number unless it enters European commerce.
The European numbering systems
The "EC number" is actually an umbrella term for several European inventories:
EINECS (European Inventory of Existing Commercial Chemical Substances)
Published in 1990, EINECS listed all commercial chemical substances on the European market between January 1, 1971 and September 18, 1981. It contains about 100,106 entries. EINECS numbers range from 200-001-8 to 310-199-2.
ELINCS (European List of Notified Chemical Substances)
ELINCS covers "new" substances notified under the EU's former Dangerous Substances Directive after September 18, 1981. ELINCS numbers start from 400-010-9.
NLP (No Longer Polymers)
A smaller list of substances that were originally classified as polymers and later reclassified. NLP numbers start from 500-001-0.
List numbers
For substances registered under REACH that do not have an EINECS, ELINCS, or NLP number, ECHA assigns a "list number" starting from 601-001-0 or 700-001-0. These are sometimes informally called EC numbers as well.
When to use which
REACH registration and communication
REACH uses the EC number as the primary substance identifier. When you search the ECHA database for a registered substance, you can search by either CAS or EC number, but the registration dossier is indexed by EC number. The REACH registration number itself (e.g., 01-2119457558-25-XXXX) is linked to the EC number.
For REACH Article 33 communication (duty to inform about SVHCs), ECHA's Candidate List includes both CAS and EC numbers. Your SCIP database notifications should reference both.
Safety Data Sheets
The globally harmonized SDS format (GHS/CLP) requires both identifiers in Section 3. A properly formatted composition table looks like:
| Substance | CAS Number | EC Number | Concentration |
|---|---|---|---|
| Di-isononyl phthalate | 28553-12-0 | 249-079-5 | 15-25% |
| Lead stabilizer | 1072-35-1 | 214-005-2 | 0.5-2% |
If a supplier's SDS lists only CAS numbers and omits EC numbers, it is non-compliant with EU CLP regulation requirements.
International trade outside Europe
Outside the EU, CAS numbers are the universal standard. US regulations (TSCA), Chinese regulations (IECSC), Korean regulations (K-REACH), and Japanese regulations (CSCL/ISHL) all reference substances by CAS number. EC numbers are meaningless outside the European regulatory context.
Product data management
For electrical product data teams, the practical answer is: store both. When you receive compliance data from a European supplier, they will typically include EC numbers. When you receive data from an American or Asian supplier, they will include CAS numbers. Your system should have fields for both and cross-reference them.
Cross-referencing CAS and EC numbers
ECHA provides a free substance database at echa.europa.eu where you can look up the EC number for any CAS number and vice versa. The mapping is generally one-to-one, but there are exceptions:
- A substance may have multiple CAS numbers (if it was registered more than once in the CAS system) but only one EC number.
- A substance may have a CAS number but no EC number (if it was never marketed in Europe).
- A group entry in EINECS may correspond to multiple CAS numbers (for substances that are isomeric mixtures).
Practical example: lead in electrical products
Lead (Pb) is one of the most frequently referenced substances in electrical product compliance.
| Identifier | Value |
|---|---|
| CAS Number | 7439-92-1 |
| EC Number | 231-100-4 |
| REACH Registration | 01-2119513221-59-XXXX |
| RoHS status | Restricted (0.1% threshold) |
| SVHC | Yes (Candidate List) |
When you validate a supplier's compliance declaration, you might see the CAS number in their RoHS certificate, the EC number in their REACH documentation, and both in their SDS. All three documents are referring to the same substance. Your data system should recognize that 7439-92-1 and 231-100-4 point to the same entry.
Validation
Both CAS numbers and EC numbers include check digits. Validate CAS numbers with the free CAS number validator to catch format errors and transcription mistakes in your compliance data.
For a broader view of which substances require attention in your product catalog, use the SVHC checker to cross-reference your substance lists against the ECHA Candidate List.
Summary
CAS numbers are the global standard. EC numbers are the European standard. Both identify chemical substances, but they come from different systems and serve different regulatory contexts. For electrical product data teams managing compliance documentation, store both identifiers, validate both check digits, and cross-reference them using ECHA's substance database. The few minutes this takes per substance saves hours of confusion when a regulator, customer, or trading partner asks for documentation using the identifier you did not capture.