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: Four out of six cases with blood loss > or = 1000 ml occurred in the 200 microg group .
### Output Text:
Four out of six cases with blood loss > or = 1000 ml occurred in the 200 microg group . 