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: The difference in the percentage of patients with a 50 % reduction in their nausea was 12 . 6 % ( 95 % CI - 4 . 6 % to 29 . 8 % ) .
### Output Text:
The difference in the percentage of patients with a 50 % reduction in their nausea was 12 . 6 % ( 95 % CI - 4 . 6 % to 29 . 8 % ) . 