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: Physicians and patients should be alert to the potential cardiac risk associated with the use of PPA , even at doses generally considered to be safe .
### Output Text:
Physicians and patients should be alert to the potential cardiac risk associated with the use of <span class="chemical">PPA</span> , even at doses generally considered to be safe . 