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: Nausea and vomiting were infrequent , with 14 % of patients experiencing grade 1 or 2 and only 2 % experiencing grade 3 or 4 .
### Output Text:
Nausea and vomiting were infrequent , with 14 % of patients experiencing grade 1 or 2 and only 2 % experiencing grade 3 or 4 . 