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 also significantly increased the risk of stroke and gallbladder disease .
### Output Text:
Long - term <span class="chemical">oestrogen</span> - only HT also significantly increased the risk of stroke and gallbladder disease . 