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: Of the responders 72 . 2 % were still improved at the follow - up ; only one - third of them had continued application irregularly .
### Output Text:
Of the responders 72 . 2 % were still improved at the follow - up ; only one - third of them had continued application irregularly . 