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: Intraocular pressure - lowering surgeries were considered a success ( postoperative IOP of 6 - 21 mm Hg with or without additional IOP - lowering medication ) in 85 . 1 % of eyes at 1 year .
### Output Text:
Intraocular pressure - lowering surgeries were considered a success ( postoperative IOP of 6 - 21 mm Hg with or without additional IOP - lowering medication ) in 85 . 1 % of eyes at 1 year . 