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: Temporary hearing loss is a known toxicity of treatment with DFMO , thus a comprehensive approach was developed to analyze serial air conduction audiograms .
### Output Text:
Temporary hearing loss is a known toxicity of treatment with <span class="chemical">DFMO</span> , thus a comprehensive approach was developed to analyze serial air conduction audiograms . 