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: Further studies are needed in order to establish a potential negative effect of maternal smoking on the neonate ' s hearing acuity .
### Output Text:
Further studies are needed in order to establish a potential negative effect of maternal <span class="chemical">smoking</span> on the neonate ' s hearing acuity . 