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: OBJECTIVES : This work summarizes research on the molecular mechanisms that underlie the increased risk of cancer development in adulthood that is associated with early - life iAs exposure .
### Output Text:
OBJECTIVES : This work summarizes research on the molecular mechanisms that underlie the increased risk of cancer development in adulthood that is associated with early - life <span class="chemical">iAs</span> exposure . 