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 , they also suggest that succimer treatment should be strongly discouraged for children who do not have elevated tissue levels of Pb or other heavy metals .
### Output Text:
However , they also suggest that <span class="chemical">succimer</span> treatment should be strongly discouraged for children who do not have elevated tissue levels of <span class="chemical">Pb</span> or other heavy metals . 