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: It is important to recognize this reaction when it occurs and to avoid further quinine exposure , since the reaction seems to be recurrent .
### Output Text:
It is important to recognize this reaction when it occurs and to avoid further <span class="chemical">quinine</span> exposure , since the reaction seems to be recurrent . 