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: Limiting depth of sedation during spinal anesthesia is a simple , safe , and cost - effective intervention for preventing postoperative delirium in elderly patients that could be widely and readily adopted .
### Output Text:
Limiting depth of sedation during spinal anesthesia is a simple , safe , and cost - effective intervention for preventing postoperative delirium in elderly patients that could be widely and readily adopted . 