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: This patient had hearing threshold changes ( 15 - 20 dB ) at 6 - 10 kHz on the opposite side also .
### Output Text:
This patient had hearing threshold changes ( 15 - 20 dB ) at 6 - 10 kHz on the opposite side also . 