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: Despite this , there was a dramatic decrease in the 50 % incidence of blinding complications of uveitis cited in earlier studies .
### Output Text:
Despite this , there was a dramatic decrease in the 50 % incidence of blinding complications of uveitis cited in earlier studies . 