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: The incidence rates of events were determined in all patients ( n = 7934 ) and in users of low - dose ( < or = 325 mg daily ) aspirin ( n = 1051 ) and nonusers of aspirin ( n = 6883 ) .
### Output Text:
The incidence rates of events were determined in all patients ( n = 7934 ) and in users of low - dose ( < or = 325 mg daily ) <span class="chemical">aspirin</span> ( n = 1051 ) and nonusers of <span class="chemical">aspirin</span> ( n = 6883 ) . 