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: Phasic pain was applied by means of short pulses of CO2 to the nasal mucosa ( stimulus duration 500 ms , interval approximately 60 s ) , and tonic pain was induced in the nasal cavity by means of dry air of controlled temperature , humidity and flow rate ( 22 degrees C , 0 % relative humidity , 145 ml . s - 1 ) .
### Output Text:
Phasic pain was applied by means of short pulses of <span class="chemical">CO2</span> to the nasal mucosa ( stimulus duration 500 ms , interval approximately 60 s ) , and tonic pain was induced in the nasal cavity by means of dry air of controlled temperature , humidity and flow rate ( 22 degrees C , 0 % relative humidity , 145 ml . s - 1 ) . 