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: Operations lasting up to 2h could be performed in the absence of sophisticated equipment such as pulse oximeters or ventilators in patients on spontaneous ventilation breathing air / oxygen only .
### Output Text:
Operations lasting up to 2h could be performed in the absence of sophisticated equipment such as pulse oximeters or ventilators in patients on spontaneous ventilation breathing air / <span class="chemical">oxygen</span> only . 