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: Epidemiologic studies of the health effects of metolachlor have been limited .
### Output Text:
Epidemiologic studies of the health effects of <span class="chemical">metolachlor</span> have been limited . 