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: The median TDF exposure was 316 days ( interquartile range 120 - 740 ) .
### Output Text:
The median <span class="chemical">TDF</span> exposure was 316 days ( interquartile range 120 - 740 ) . 