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: Clinical studies of pipe smokers and people using transdermal nicotine support the idea that toxins other than nicotine are the most important causes of acute cardiovascular events .
### Output Text:
Clinical studies of pipe smokers and people using transdermal <span class="chemical">nicotine</span> support the idea that toxins other than <span class="chemical">nicotine</span> are the most important causes of acute cardiovascular events . 