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: Previous exposures to enflurane have to be disclosed to avoid mistakes in clinical interpretation of the EEG .
### Output Text:
Previous exposures to <span class="chemical">enflurane</span> have to be disclosed to avoid mistakes in clinical interpretation of the EEG . 