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: Two halogenated anesthetics , enflurane and isoflurane , have been associated with an allergic - type hepatic injury both alone and following previous exposure to halothane .
### Output Text:
Two halogenated anesthetics , <span class="chemical">enflurane</span> and <span class="chemical">isoflurane</span> , have been associated with an allergic - type hepatic injury both alone and following previous exposure to <span class="chemical">halothane</span> . 