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: Subjects ranged in age from 18 to 60 years of age and had at least a 2 - year history of spring allergic rhinitis , confirmed by positive skin test to spring aeroallergens .
### Output Text:
Subjects ranged in age from 18 to 60 years of age and had at least a 2 - year history of spring allergic rhinitis , confirmed by positive skin test to spring aeroallergens . 