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 increased nicotine exposure from the lozenge has raised questions about the relative safety of the lozenge and gum .
### Output Text:
The increased <span class="chemical">nicotine</span> exposure from the lozenge has raised questions about the relative safety of the lozenge and gum . 