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: Patients who experience a fall in hemoglobin concentrations of 2 g / dL or more at week 2 after the start of treatment should be monitored with particular care .
### Output Text:
Patients who experience a fall in hemoglobin concentrations of 2 g / dL or more at week 2 after the start of treatment should be monitored with particular care . 