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: Mechanisms Underlying Latent Disease Risk Associated with Early - Life Arsenic Exposure : Current Research Trends and Scientific Gaps .
### Output Text:
Mechanisms Underlying Latent Disease Risk Associated with Early - Life <span class="chemical">Arsenic</span> Exposure : Current Research Trends and Scientific Gaps . 