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: These results suggest that peripheral iron dextran can increase the iron level in the SN , where excessive iron causes the degeneration of dopaminergic neurons .
### Output Text:
These results suggest that peripheral <span class="chemical">iron dextran</span> can increase the <span class="chemical">iron</span> level in the SN , where excessive <span class="chemical">iron</span> causes the degeneration of dopaminergic neurons . 