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: Up until now , the only factor studied has been the effect of the diameter of the spinal needle on post - operative sensorineural hearing loss .
### Output Text:
Up until now , the only factor studied has been the effect of the diameter of the spinal needle on post - operative sensorineural hearing loss . 