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: SETTING : Twenty - nine university , university - affiliated , or community hospitals during a 6 - year period ( total of 117 cumulative hospital - years ) .
### Output Text:
SETTING : Twenty - nine university , university - affiliated , or community hospitals during a 6 - year period ( total of 117 cumulative hospital - years ) . 