Q.01

How do I look up a chemical by its CAS number?

To look up a chemical by CAS number, type the number into your question on the RegAffairs AI landing page, together with what you want to know and where. For example: "What is the CLP Annex VI harmonised classification for CAS 50-00-0?" or "Is CAS 117-81-7 on the EU Candidate List, and is it restricted under REACH Annex XVII?" You get a cited answer, and the page shows the research trail: the searches run and the documents read.

Tips from the user manual:

  • Be specific. Specific questions work best. Along with the CAS number, name the regulation, annex or list you care about (Candidate List, Annex XIV, Annex XVII, exposure limits, MRLs, cosmetics ingredient annexes, BPR active-substance status) and the jurisdiction (EU, GB, US, Japan and so on).
  • Check the citations. Each claim points to the exact regulation, annex entry or paragraph it came from, so you can open the source.
  • Negative findings are explicit. If a CAS number isn't on a list, the answer says so and names the list and version that was searched. It doesn't guess.
  • You can also search from Claude or ChatGPT. If you connect the RegAffairs AI MCP server (https://mcp.regaffairsai.com/mcp), you can look up substances, classifications, exposure limits, ingredients, MRLs, guidance and approvals with sources from inside those assistants. Setup guides are at regaffairsai.com/integrations, and the MCP documentation describes the tools and their inputs.
  • Your free allowance. The Free plan gives 10 questions per calendar month, and follow-ups in the same thread count as questions. Opening cited sources or expanding the research trail doesn't count.

If you give me a CAS number and the regulations or jurisdictions you're interested in, I can run the lookup now.

Sources: RegAffairs AI user manual

Q.02

What's the difference between a CAS number and an EC number?

An EC number is the EU's official identifier for a substance. A CAS number is the Chemical Abstracts Service number, which the EU texts treat as an extra aid to identification. The two don't always map one-to-one, so on the same EU entry they can describe the substance with different precision.

EC number

  • CLP Annex VI (point 1.1.1.2) says "the EC number, i.e. EINECS, ELINCS or NLP, is the official number of the substance within the European Union." [1]
  • It is a seven-digit number in the format XXX-XXX-X. The starting number tells you which EU list it comes from [1]:
    • EINECS (European Inventory of Existing Commercial Chemical Substances) numbers start at 200-001-8.
    • ELINCS (European List of Notified Substances) numbers start at 400-010-9.
    • NLP ("No-longer-polymers" list) numbers start at 500-001-0.
  • The EINECS listing also matters legally. Under REACH Article 3(20), a substance listed in EINECS meets one of the criteria for a "phase-in substance". [2]

CAS number

  • CLP Annex VI (point 1.1.1.3) says the Chemical Abstracts Service number "is also included to assist identification of the entry." [1]

Why they don't always match

  • One EINECS number covers both the anhydrous and hydrated forms of a substance. Those forms frequently have different CAS numbers. [1]
  • Annex VI lists the CAS number for the anhydrous form only. As the regulation says, the CAS number shown "does not always describe the entry as accurately as the EINECS number." [1]
  • In practice: if you search Annex VI by the CAS number of a hydrate, you may not find the entry that covers it. Searching by EC number is more reliable.

How the law uses each one: SDS Section 3 (REACH Annex II, point 3.2.4)

  • The EC number, if available, "shall be given" in line with CLP. [3]
  • The CAS number, if available, "may also be given", as may the IUPAC name. [3]
  • Neither is needed where the substance is disclosed under an alternative chemical name under CLP Article 24. [3]
  • REACH Annex VI on substance identification also accepts either one ("CAS or EC number") for identifying the agents used to treat the surface of a nanoform. [4]

Approach: I read the current consolidated texts of the CLP Regulation (EC) No 1272/2008 and the REACH Regulation (EC) No 1907/2006. The datasets don't include the Chemical Abstracts Service's own description of how it assigns CAS numbers.

Q.03

What's a CUS number?

A CUS number is the identifier the European Commission's tax and customs department (DG TAXUD) gives each product in the European Customs Inventory of Chemical Substances (ECICS). It's a customs identifier, not a regulatory one under REACH or CLP. The Commission says the CUS code is "an easy identifier to search the database and to communicate with customs" (CLEN Action 6 page).

What it's for

  • ECICS links chemicals to their CN code, the 8-digit EU customs tariff code. A CUS number points you to the tariff classification a substance has been given. CAS, EC and UN numbers, where they exist, sit alongside it to identify the substance (ECICS page).
  • The inventory "mainly covers pure chemicals such as pharmaceuticals, pesticides, chemical weapons precursors, narcotic drugs and precursors, ozone-depleting substances or hazardous chemicals" (ECICS page).
  • ECICS was first published in 1974 and became "CLEN Action 6" in 2009 (CLEN Action 6 page).

Format and example The format is seven digits, a dash, then a check digit. For example, the ECICS entry for benzene, pure, reads: CUS 0011535-5, CN code 29022000, CAS 71-43-2, EC 200-753-7, UN 1114 [5].

How it compares with CAS and EC numbers

  • A CAS number identifies the chemical itself.
  • An EC number is the substance's official identifier in the EU (for CLP and REACH).
  • A CUS number identifies a product entry in the EU customs inventory. You use it mainly for tariff classification and when dealing with customs authorities.

Legal status ECICS isn't binding. The Commission says its information "is not binding and does not give rise to protection of any kind" (ECICS page). So it helps you classify goods, but it doesn't replace a formal tariff classification decision.

Neither Commission page I opened, nor the ECICS2 user's guide, spells out what the letters "CUS" stand for.

Approach: I looked up the EU ECICS customs inventory and read the European Commission's ECICS and CLEN Action 6 pages and the ECICS2 user's guide.

Q.04