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: Of 16 men treated for at least 2 months , six ( 37 . 5 % ) showed a fall in absolute PSA by a median of 48 % .
### Output Text:
Of 16 men treated for at least 2 months , six ( 37 . 5 % ) showed a fall in absolute PSA by a median of 48 % . 