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: Through 30 years of widespread use , acetaminophen has been shown to be a remarkably safe medication in therapeutic dosages .
### Output Text:
Through 30 years of widespread use , <span class="chemical">acetaminophen</span> has been shown to be a remarkably safe medication in therapeutic dosages . 