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: Twenty - two patients in the affected group had abnormal audiograms with deficits mostly in the high frequency range of 4 , 000 to 8 , 000 Hz and in the hearing threshold levels of 30 to 100 decibels .
### Output Text:
Twenty - two patients in the affected group had abnormal audiograms with deficits mostly in the high frequency range of 4 , 000 to 8 , 000 Hz and in the hearing threshold levels of 30 to 100 decibels . 