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: This imply an incidence of 1 . 7 / 100 , 000 treatment years .
### Output Text:
This imply an incidence of 1 . 7 / 100 , 000 treatment years . 