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: However , trends for both lifetime and intensity - weighted lifetime days of metolachor use were positive and statistically significant with an unexposed reference group .
### Output Text:
However , trends for both lifetime and intensity - weighted lifetime days of <span class="chemical">metolachor</span> use were positive and statistically significant with an unexposed reference group . 