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 no observed effect level ( NOEL ) for this contaminant appears to be approximately 1mg / kg , corresponding to a contamination level of approximately 3 % .
### Output Text:
The no observed effect level ( NOEL ) for this contaminant appears to be approximately 1mg / kg , corresponding to a contamination level of approximately 3 % . 