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: Long - term oestrogen - only HT significantly increased the risk of venous thrombo - embolism , stroke and gallbladder disease ( after one to two years , three years and seven years ' use respectively ) , but did not significantly increase the risk of breast cancer .
### Output Text:
Long - term <span class="chemical">oestrogen</span> - only HT significantly increased the risk of venous thrombo - embolism , stroke and gallbladder disease ( after one to two years , three years and seven years ' use respectively ) , but did not significantly increase the risk of breast cancer . 