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: BACKGROUND : Millions of individuals worldwide , particularly those living in rural and developing areas , are exposed to harmful levels of inorganic arsenic ( iAs ) in their drinking water .
### Output Text:
BACKGROUND : Millions of individuals worldwide , particularly those living in rural and developing areas , are exposed to harmful levels of <span class="chemical">inorganic arsenic</span> ( <span class="chemical">iAs</span> ) in their drinking water . 