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: Of 6 patients with irreversible visual impairment , 4 patients had diabetes mellitus , glaucoma and a history of heavy smoking .
### Output Text:
Of 6 patients with irreversible visual impairment , 4 patients had diabetes mellitus , glaucoma and a history of heavy smoking . 