Index numbers are used to identify specific chemical substances which hold a Harmonised Classification in the EU. This means that not every chemical on the market will have an Index number. They are valid for identifying chemical substances on CLP labels and on the Safety Data Sheet.
Index numbers take the form of XXX-XXX-XX-X.
They are published in all the places where the EC list of Harmonised Classifications appears, that is:
- in the CLP Regulation (Table 3.1, Annex VI)
- on the Classification and Labelling Inventory
- in the spreadsheet of Harmonised Classifications
Index Numbers can cover multiple related substances
Unlike CAS and EC numbers, which are unique substance identifiers, the Index Number can be used to cover different (but chemically similar) substances which have the same Harmonised Classification hazards. This can mean that there are multiple product names, EC numbers and CAS numbers in the same table cell of the published Harmonised Classification List (Table 3 to Annex VI of the CLP Regulation).
An example of this is Linalool, which is a natural substance distilled from plants like coriander. It occurs in two isomers, d-linalool and l-linalool, and can also occur as a mixture, dl-linalool. All three forms are covered under a single Index Number in the Harmonised Classifications, having been added under the 10th ATP to CLP.
This infographic can be downloaded as a pdf: single HC mult subs 1.2 .
Using Index Numbers for Substance identification on the label and SDS
Reading the ECHA guidance, it appears that the EU would prefer Index numbers to be used for Harmonised Classifications instead of EC numbers and CAS numbers.
However, they are not well recognised in industry, and EC and CAS numbers tend to be used instead for Harmonised Classifications, in the same way that they are used for substances which do not hold a Harmonised Classification.
This is probably down to several reasons:
- the fact that an Index Number may identify more than one substance means that it cannot be used as a unique substance identifier in some cases, and people prefer to use numbers consistently, hence the preference for EC and CAS numbers which are (usually) unique substance identifiers
- the format of the Index Numbers is longer and less memorable than (particularly) the CAS number, so people will be more reluctant to use it than CAS and EC numbers
- if the SDS (and label) is to be used under GHS in non-EU jurisdictions, the Index Number will not be valid, and it makes sense to keep information on the label and SDS as consistent as possible globally. This also helps to explain why CAS numbers are popular as unique substance identifier numbers, as they are used world-wide in many jurisdictions.
Overall, it is easy to see why the chemical industry does not use Index Numbers as substance identifiers, instead preferring the EC and CAS numbers.