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: RESULTS : In exposed neonates , TEOAEs mean response ( across frequency ) and mean amplitude at 4000Hz was significantly lower than in non - exposed neonates .
### Output Text:
RESULTS : In exposed neonates , TEOAEs mean response ( across frequency ) and mean amplitude at 4000Hz was significantly lower than in non - exposed neonates . 