British Standard for the storage, transportation and maintenance of media for use in data processing and information storage. The standard originally came out in 1973 (ancient!) but was replaced in 1988 (which is still pretty old). A summary of its recommendations appears on the DPC site here.

The Standard only deals with tape and CD-ROMs.  For long term storage of CD-ROMs, an environment of 18-22 deg C and 35-45% RH is recommended. Operating conditions can fall outside these but you need to have acclimatisation procedures.

You should note that the BSI’s own site says “this standard has been declared obsolescent as it is no longer felt to be relevant,” though they will still charge you £72 to buy a copy. (BSI site accessed 9 Feb 2008).