In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The mean lowest BIS score occurring during these procedural sedations was 66 . 9 ( + / - 14 . 4 ; range 33 to 91 ) .
### Output Text:
The mean lowest BIS score occurring during these procedural sedations was 66 . 9 ( + / - 14 . 4 ; range 33 to 91 ) . 