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: Mean ALT and AST levels were 124 . 4 + / - 73 . 4 and 103 . 1 + / - 81 IU / l , respectively .
### Output Text:
Mean ALT and AST levels were 124 . 4 + / - 73 . 4 and 103 . 1 + / - 81 IU / l , respectively . 