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: With a median follow up of 22 months , median time to progression , progression - free survival ( PFS ) and overall survival ( OS ) were 8 . 9 , 8 . 7 , and 22 months , respectively .
### Output Text:
With a median follow up of 22 months , median time to progression , progression - free survival ( PFS ) and overall survival ( OS ) were 8 . 9 , 8 . 7 , and 22 months , respectively . 