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: There was a significant decrease in the time to onset of arrhythmia as compared with control nonprogesterone - treated rats ( 6 . 2 + / - 1 . 3 vs . 30 . 8 + / - 2 . 5 min , mean + / - SE ) .
### Output Text:
There was a significant decrease in the time to onset of arrhythmia as compared with control nonprogesterone - treated rats ( 6 . 2 + / - 1 . 3 vs . 30 . 8 + / - 2 . 5 min , mean + / - SE ) . 