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 study was based on the medical records of about 540 , 000 women born between 1941 and 1981 .
### Output Text:
The study was based on the medical records of about 540 , 000 women born between 1941 and 1981 . 