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: Adverse effects were observed in 19 patients ( 4 % ) during an average follow - up of 20 months .
### Output Text:
Adverse effects were observed in 19 patients ( 4 % ) during an average follow - up of 20 months . 