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: After lithium treatment , calcium levels were higher than either baseline levels or control levels .
### Output Text:
After <span class="chemical">lithium</span> treatment , <span class="chemical">calcium</span> levels were higher than either baseline levels or control levels . 