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: Safety assessments were made at baseline and at 2 , 4 , 6 , and 12 weeks after the start of product use .
### Output Text:
Safety assessments were made at baseline and at 2 , 4 , 6 , and 12 weeks after the start of product use . 