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: Venous pain occurred in 68 % of patients and 50 % had redness , pain or swelling related to the injection site , in some cases lasting up to three weeks after anaesthesia .
### Output Text:
Venous pain occurred in 68 % of patients and 50 % had redness , pain or swelling related to the injection site , in some cases lasting up to three weeks after anaesthesia . 