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: Our case illustrates the serious consequences of medical errors that can be avoided through improved medication labeling and staff supervision .
### Output Text:
Our case illustrates the serious consequences of medical errors that can be avoided through improved medication labeling and staff supervision . 