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: The influence of smoking during pregnancy on the developing cochlea has not been estimated , although smoking has been positively associated with hearing loss in adults .
### Output Text:
The influence of <span class="chemical">smoking</span> during pregnancy on the developing cochlea has not been estimated , although <span class="chemical">smoking</span> has been positively associated with hearing loss in adults . 