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: 164 patients ( mean age + / - standard deviation [ SD ] 81 . 6 + / - 6 . 8 years ) were admitted .
### Output Text:
164 patients ( mean age + / - standard deviation [ SD ] 81 . 6 + / - 6 . 8 years ) were admitted . 