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: Epidemic of liver disease caused by hydrochlorofluorocarbons used as ozone - sparing substitutes of chlorofluorocarbons .
### Output Text:
Epidemic of liver disease caused by <span class="chemical">hydrochlorofluorocarbons</span> used as <span class="chemical">ozone</span> - sparing substitutes of <span class="chemical">chlorofluorocarbons</span> . 