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: 23 582 patient - years of observation have been completed so far , 10 684 on active drugs and 12 898 on placebos .
### Output Text:
23 582 patient - years of observation have been completed so far , 10 684 on active drugs and 12 898 on placebos . 