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: CONCLUSIONS : These are the first data , to our knowledge , to show that treatment with any chelating agent can alleviate cognitive deficits due to Pb exposure .
### Output Text:
CONCLUSIONS : These are the first data , to our knowledge , to show that treatment with any chelating agent can alleviate cognitive deficits due to <span class="chemical">Pb</span> exposure . 