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: Further long - term follow - up studies will be necessary to fully evaluate the impact of dexamethasone on neurological and cognitive development .
### Output Text:
Further long - term follow - up studies will be necessary to fully evaluate the impact of <span class="chemical">dexamethasone</span> on neurological and cognitive development . 