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 objective of this study was to determine the effects of maternal smoking on transient evoked otoacoustic emissions ( TEOAEs ) of healthy neonates .
### Output Text:
The objective of this study was to determine the effects of maternal <span class="chemical">smoking</span> on transient evoked otoacoustic emissions ( TEOAEs ) of healthy neonates . 