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: Animal studies and mechanistic studies indicate that nicotine could play a role in accelerating atherosclerosis , but evidence among humans is too inadequate to be definitive about such an effect .
### Output Text:
Animal studies and mechanistic studies indicate that <span class="chemical">nicotine</span> could play a role in accelerating atherosclerosis , but evidence among humans is too inadequate to be definitive about such an effect . 