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: RESULTS : Pb exposure produced lasting impairments in learning , attention , inhibitory control , and arousal regulation , paralleling the areas of dysfunction seen in Pb - exposed children .
### Output Text:
RESULTS : <span class="chemical">Pb</span> exposure produced lasting impairments in learning , attention , inhibitory control , and arousal regulation , paralleling the areas of dysfunction seen in <span class="chemical">Pb</span> - exposed children . 