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: Effects of calcium channel blockers on bupivacaine - induced toxicity .
### Output Text:
Effects of <span class="chemical">calcium</span> channel blockers on <span class="chemical">bupivacaine</span> - induced toxicity . 