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: We investigated an epidemic of liver disease in nine industrial workers who had had repeated accidental exposure to a mixture of 1 , 1 - dichloro - 2 , 2 , 2 - trifluoroethane ( HCFC 123 ) and 1 - chloro - 1 , 2 , 2 , 2 - tetrafluoroethane ( HCFC 124 ) .
### Output Text:
We investigated an epidemic of liver disease in nine industrial workers who had had repeated accidental exposure to a mixture of <span class="chemical">1 , 1 - dichloro - 2 , 2 , 2 - trifluoroethane</span> ( <span class="chemical">HCFC 123</span> ) and <span class="chemical">1 - chloro - 1 , 2 , 2 , 2 - tetrafluoroethane</span> ( <span class="chemical">HCFC 124</span> ) . 