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: Mild hypoxia ( SO2 < 90 % ) was the most common event ( 11 patients ) ; 3 patients ( 2 % ) presented transient hypoxia due to upper airway obstruction by probe introduction and 8 ( 5 . 8 % ) due to hypoxia caused by MZ use .
### Output Text:
Mild hypoxia ( SO2 < 90 % ) was the most common event ( 11 patients ) ; 3 patients ( 2 % ) presented transient hypoxia due to upper airway obstruction by probe introduction and 8 ( 5 . 8 % ) due to hypoxia caused by <span class="chemical">MZ</span> use . 