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: BACKGROUND : There is growing pressure for clinicians to prescribe chelation therapy at only slightly elevated blood lead levels .
### Output Text:
BACKGROUND : There is growing pressure for clinicians to prescribe chelation therapy at only slightly elevated blood <span class="chemical">lead</span> levels . 