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: We have proposed that 1 , 3 - butadiene metabolites be so tested as a reality check on the epidemiological reports .
### Output Text:
We have proposed that <span class="chemical">1 , 3 - butadiene</span> metabolites be so tested as a reality check on the epidemiological reports . 