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: Mean follow - up time is 20 . 6 months ( range 6 - 48 ) and all except 4 patients have been followed more than one year .
### Output Text:
Mean follow - up time is 20 . 6 months ( range 6 - 48 ) and all except 4 patients have been followed more than one year . 