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: Exposure to tobacco was characterized as low ( < 5 cigarettes per day , n = 88 ears ) , moderate ( 5 < or = cigarettes per day < 10 , n = 76 ) or high ( > or = 10 cigarettes per day , n = 36 ) .
### Output Text:
Exposure to tobacco was characterized as low ( < 5 cigarettes per day , n = 88 ears ) , moderate ( 5 < or = cigarettes per day < 10 , n = 76 ) or high ( > or = 10 cigarettes per day , n = 36 ) . 