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: Grade 1 or 2 polyneuropathy affected 56 % of patients while only one ( 2 % ) experienced severe polyneuropathy .
### Output Text:
Grade 1 or 2 polyneuropathy affected 56 % of patients while only one ( 2 % ) experienced severe polyneuropathy . 