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: With mild toxicity , a reduction to 30 or 40 mg / kg per dose should result in a reversal of the abnormal results to normal within four weeks .
### Output Text:
With mild toxicity , a reduction to 30 or 40 mg / kg per dose should result in a reversal of the abnormal results to normal within four weeks . 