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: Both risperidone and haloperidol produced D2 occupancy levels between approximately 60 and 90 % at standard clinical doses .
### Output Text:
Both <span class="chemical">risperidone</span> and <span class="chemical">haloperidol</span> produced D2 occupancy levels between approximately 60 and 90 % at standard clinical doses . 