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: Our findings illustrate that PB may be associated with chronic liver damage , which may lead to more serious and deleterious consequences .
### Output Text:
Our findings illustrate that <span class="chemical">PB</span> may be associated with chronic liver damage , which may lead to more serious and deleterious consequences . 