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: Finally , the dose response for cardiovascular events of nicotine appears to be flat , suggesting that if nicotine is involved , adverse effects might be seen with relatively low - level cigarette exposures .
### Output Text:
Finally , the dose response for cardiovascular events of <span class="chemical">nicotine</span> appears to be flat , suggesting that if <span class="chemical">nicotine</span> is involved , adverse effects might be seen with relatively low - level cigarette exposures . 