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: This case - control study was conducted in the defined population comprising members of Kaiser Permanente of Northern and Southern California .
### Output Text:
This case - control study was conducted in the defined population comprising members of Kaiser Permanente of Northern and Southern California . 