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: Additional studies are needed to address uncertainties related to confounding and to examine temporal aspects of the associations we observed .
### Output Text:
Additional studies are needed to address uncertainties related to confounding and to examine temporal aspects of the associations we observed . 