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: Patients ' mean age was 53 . 9 years , their mean weight was 193 . 9 pounds , and they smoked a mean of 25 . 2 cigarettes per day at baseline .
### Output Text:
Patients ' mean age was 53 . 9 years , their mean weight was 193 . 9 pounds , and they smoked a mean of 25 . 2 cigarettes per day at baseline . 