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: Succimer treatment of the Pb - exposed rats significantly improved learning , attention , and arousal regulation , although the efficacy of the treatment varied as a function of the Pb exposure level and the specific functional deficit .
### Output Text:
<span class="chemical">Succimer</span> treatment of the <span class="chemical">Pb</span> - exposed rats significantly improved learning , attention , and arousal regulation , although the efficacy of the treatment varied as a function of the <span class="chemical">Pb</span> exposure level and the specific functional deficit . 