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 used Poisson regression to evaluate relations between two metrics of metolachlor use ( lifetime days , intensity - weighted lifetime days ) and cancer incidence .
### Output Text:
We used Poisson regression to evaluate relations between two metrics of <span class="chemical">metolachlor</span> use ( lifetime days , intensity - weighted lifetime days ) and cancer incidence . 