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: All patients were observed for 6 hours after each challenge , and controlled again after 24 hours to exclude delayed reactions .
### Output Text:
All patients were observed for 6 hours after each challenge , and controlled again after 24 hours to exclude delayed reactions . 