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 only significantly increased risk reported was for venous thrombo - embolism in women taking combined continuous HT : their absolute risk remained low , at less than 1 / 500 .
### Output Text:
The only significantly increased risk reported was for venous thrombo - embolism in women taking combined continuous HT : their absolute risk remained low , at less than 1 / 500 . 