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: The mean H concentration during hypotension in the inspiratory gas was 0 . 7 + / - 0 . 1 vol % , the mean E concentration 1 . 6 + / - 0 . 2 vol % , and the mean I concentration 1 . 0 + / - 0 . 1 vol % .
### Output Text:
The mean <span class="chemical">H</span> concentration during hypotension in the inspiratory gas was 0 . 7 + / - 0 . 1 vol % , the mean <span class="chemical">E</span> concentration 1 . 6 + / - 0 . 2 vol % , and the mean <span class="chemical">I</span> concentration 1 . 0 + / - 0 . 1 vol % . 