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: The association was stronger with longer duration of use when compared to shorter duration of use and was more pronounced in recent users than in remote users .
### Output Text:
The association was stronger with longer duration of use when compared to shorter duration of use and was more pronounced in recent users than in remote users . 