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: Two subsequent CO2 - rebreathing tests were performed in healthy young volunteers .
### Output Text:
Two subsequent <span class="chemical">CO2</span> - rebreathing tests were performed in healthy young volunteers . 