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: A total of 4 , 499 patients with dyslipidemia requiring drug intervention was enrolled at 1 , 081 sites .
### Output Text:
A total of 4 , 499 patients with dyslipidemia requiring drug intervention was enrolled at 1 , 081 sites . 