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: Almost certainly , nicotine via its hemodynamic effects contributes to acute cardiovascular events , although current evidence suggests that the effects of nicotine are much less important than are the prothrombotic effects of cigarette smoking or the effects of carbon monoxide .
### Output Text:
Almost certainly , <span class="chemical">nicotine</span> via its hemodynamic effects contributes to acute cardiovascular events , although current evidence suggests that the effects of <span class="chemical">nicotine</span> are much less important than are the prothrombotic effects of cigarette smoking or the effects of <span class="chemical">carbon monoxide</span> . 