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: Data collected on patients seen from September 1 , 1996 , to December 31 , 1997 , included the cause of the pulmonary hypertension and its severity .
### Output Text:
Data collected on patients seen from September 1 , 1996 , to December 31 , 1997 , included the cause of the pulmonary hypertension and its severity . 