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: Follow - up air conduction done at least 6 months after end of treatment showed an adjusted mean difference in hearing thresholds of 1 . 08 dB ( - 0 . 81 to 2 . 96 dB ; P = 0 . 26 ) between treatment arms .
### Output Text:
Follow - up air conduction done at least 6 months after end of treatment showed an adjusted mean difference in hearing thresholds of 1 . 08 dB ( - 0 . 81 to 2 . 96 dB ; P = 0 . 26 ) between treatment arms . 