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: However , very few studies have evaluated whether chelation improves cognitive outcomes in Pb - exposed children , or whether these agents have adverse effects that may affect brain development in the absence of Pb exposure .
### Output Text:
However , very few studies have evaluated whether chelation improves cognitive outcomes in <span class="chemical">Pb</span> - exposed children , or whether these agents have adverse effects that may affect brain development in the absence of <span class="chemical">Pb</span> exposure . 