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: Because pre - existing paralysis is of a relevant frequency ( up to 3 % ) , a preoperative evaluation of vocal cord function before carotid endarterectomy under local anesthesia is recommended to avoid intraoperative bilateral paralysis .
### Output Text:
Because pre - existing paralysis is of a relevant frequency ( up to 3 % ) , a preoperative evaluation of vocal cord function before carotid endarterectomy under local anesthesia is recommended to avoid intraoperative bilateral paralysis . 