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: Skeletal movements occurred in 50 % of patients ; 30 % experienced respiratory upset , one sufficiently severe to necessitate abandoning the technique .
### Output Text:
Skeletal movements occurred in 50 % of patients ; 30 % experienced respiratory upset , one sufficiently severe to necessitate abandoning the technique . 