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 chelation improves learning , attention , and arousal regulation in lead - exposed rats but produces lasting cognitive impairment in the absence of lead exposure .
### Output Text:
<span class="chemical">Succimer</span> chelation improves learning , attention , and arousal regulation in <span class="chemical">lead</span> - exposed rats but produces lasting cognitive impairment in the absence of <span class="chemical">lead</span> exposure . 