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 Agricultural Health Study ( AHS ) is a prospective cohort study including licensed private and commercial pesticide applicators in Iowa and North Carolina enrolled 1993 - 1997 .
### Output Text:
The Agricultural Health Study ( AHS ) is a prospective cohort study including licensed private and commercial pesticide applicators in Iowa and North Carolina enrolled 1993 - 1997 . 