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 then used hyperbaric oxygen at an absolute pressure of 2 atm , 5 days a week for 8 consecutive weeks .
### Output Text:
We then used hyperbaric <span class="chemical">oxygen</span> at an absolute pressure of 2 atm , 5 days a week for 8 consecutive weeks . 