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: Five hundred fifty - three patients , 264 taking the lozenge and 289 taking the gum , used the study product for > or = 4 days per week during the first 2 weeks ( evaluable population ) .
### Output Text:
Five hundred fifty - three patients , 264 taking the lozenge and 289 taking the gum , used the study product for > or = 4 days per week during the first 2 weeks ( evaluable population ) . 