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: Finally , we strongly recommend to prospectively and regularly perform audiologic assessment in sick preterm children as hearing loss is of delayed onset and in most cases bilateral and severe .
### Output Text:
Finally , we strongly recommend to prospectively and regularly perform audiologic assessment in sick preterm children as hearing loss is of delayed onset and in most cases bilateral and severe . 