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: CONCLUSIONS : Ethambutol usage is associated with permanent visual loss and should be avoided if possible or used with caution and proper ophthalmological follow - up .
### Output Text:
CONCLUSIONS : <span class="chemical">Ethambutol</span> usage is associated with permanent visual loss and should be avoided if possible or used with caution and proper ophthalmological follow - up . 