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: Among these 47 patients the mean ( + / - SD ) age was 32 . 5 + / - 12 . 1 years ; 76 % ( 34 / 45 ) were men .
### Output Text:
Among these 47 patients the mean ( + / - SD ) age was 32 . 5 + / - 12 . 1 years ; 76 % ( 34 / 45 ) were men . 