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: The chronic iron overload may be more destructive to dopaminergic neurons than the acute iron overload .
### Output Text:
The chronic <span class="chemical">iron</span> overload may be more destructive to dopaminergic neurons than the acute <span class="chemical">iron</span> overload . 