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: Seventy - three percent ( 73 / 99 ) of the study participants were treated for headache and 70 % ( 70 / 99 ) for nausea .
### Output Text:
Seventy - three percent ( 73 / 99 ) of the study participants were treated for headache and 70 % ( 70 / 99 ) for nausea . 