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: Intracranial pressure increases during alfentanil - induced rigidity .
### Output Text:
Intracranial pressure increases during <span class="chemical">alfentanil</span> - induced rigidity . 