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: Future research should include animal studies that address mechanistic hypotheses and studies of human populations that integrate early - life exposure , molecular alterations , and latent disease outcomes .
### Output Text:
Future research should include animal studies that address mechanistic hypotheses and studies of human populations that integrate early - life exposure , molecular alterations , and latent disease outcomes . 