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: Following intervention , 7 out of 21 affected patients recovered , 10 remained stable and 4 demonstrated aggravation .
### Output Text:
Following intervention , 7 out of 21 affected patients recovered , 10 remained stable and 4 demonstrated aggravation . 