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: The fact that 5 of the 6 patients who received concurrent calcium channel blockers died , should alert clinicians to be more cautious when using these two drugs simultaneously .
### Output Text:
The fact that 5 of the 6 patients who received concurrent <span class="chemical">calcium</span> channel blockers died , should alert clinicians to be more cautious when using these two drugs simultaneously . 