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: These studies suggest that both phenacetin and acetaminophen may contribute to the burden of ESRD , with the risk of the latter being somewhat less than that of the former .
### Output Text:
These studies suggest that both <span class="chemical">phenacetin</span> and <span class="chemical">acetaminophen</span> may contribute to the burden of ESRD , with the risk of the latter being somewhat less than that of the former . 