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: Much of the benefit is sustained at two years , although some improvements , such as those on the ipsilateral side and in axial symptoms , wane within the first year .
### Output Text:
Much of the benefit is sustained at two years , although some improvements , such as those on the ipsilateral side and in axial symptoms , wane within the first year . 