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 rate of hypotony ( IOP < / = 5 mm Hg ) following IOP - lowering surgery ( 42 . 5 % ) was not different from that of implanted eyes not subjected to surgery ( 35 . 4 % ) ( P = . 09 ) .
### Output Text:
The rate of hypotony ( IOP < / = 5 mm Hg ) following IOP - lowering surgery ( 42 . 5 % ) was not different from that of implanted eyes not subjected to surgery ( 35 . 4 % ) ( P = . 09 ) . 