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: RESULTS : Person - years of exposure among non - users of aspirin were : 75 , 761 to acetaminophen , 42 , 671 to rofecoxib 65 , 860 to celecoxib , and 37 , 495 to NS - NSAIDs .
### Output Text:
RESULTS : Person - years of exposure among non - users of <span class="chemical">aspirin</span> were : 75 , 761 to <span class="chemical">acetaminophen</span> , 42 , 671 to <span class="chemical">rofecoxib</span> 65 , 860 to <span class="chemical">celecoxib</span> , and 37 , 495 to NS - NSAIDs . 