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 ' characteristics were : male / female ratio 20 / 13 ; median age 57 ( 27 - 75 ) years ; median WHO status 1 ( 0 - 2 ) .
### Output Text:
Patients ' characteristics were : male / female ratio 20 / 13 ; median age 57 ( 27 - 75 ) years ; median WHO status 1 ( 0 - 2 ) . 