For Chemical Abstracts Society Registry Number, a unique identifier for chemical substances. The CASRN, while not required, is a staple of an MSDS, or Materials Safety Data Sheet. This is particularly useful in the field of Industrial Hygiene given that there is not a one to one mapping between chemicals and their names as listed on an ingredient sheet (a particular chemical having dozens of alternate names is not unusual). For example, the names Tetracholoroethylene and Perchloroethelyene refer the same chemical. Using the CASRN number, 127-18-4, aids in the ready identification of this chemical, no matter what synonym it is described by.

The CAS is a division of the American Chemical Society.

The last digit of the CAS Registry Number is a checksum. For example, TCE, listed above, has CASRN 127-18-4. The checksum is (5*1+4*2+3*7+2*1+1*8 mod 10) = 4.

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