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: Percutaneous absorption of warfarin causing coagulopathy , reported three times in the past , is a significant risk if protective measures , such as gloves , are not used .
### Output Text:
Percutaneous absorption of <span class="chemical">warfarin</span> causing coagulopathy , reported three times in the past , is a significant risk if protective measures , such as gloves , are not used . 