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: Regular audiologic evaluation is imperative in all thalassemic patients so that early changes may be recognized and treatment may be judiciously adjusted in order to prevent or reverse hearing impairment .
### Output Text:
Regular audiologic evaluation is imperative in all thalassemic patients so that early changes may be recognized and treatment may be judiciously adjusted in order to prevent or reverse hearing impairment . 