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: METHODS : A non - randomised clinical study of patients aged > or = 65 years admitted to acute hospital wards during 1 month .
### Output Text:
METHODS : A non - randomised clinical study of patients aged > or = 65 years admitted to acute hospital wards during 1 month . 