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: Its effect was determined in terms of attenuation of hearing loss , measured by shifts in the auditory brainstem response threshold .
### Output Text:
Its effect was determined in terms of attenuation of hearing loss , measured by shifts in the auditory brainstem response threshold . 