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 excess event rate was 1 . 8 per 1 , 000 woman - years ( 95 % CI - 0 . 5 - 4 . 1 ) , and the number needed to treat to cause 1 event was 170 ( 95 % CI 100 - 582 ) over 3 . 3 years .
### Output Text:
The excess event rate was 1 . 8 per 1 , 000 woman - years ( 95 % CI - 0 . 5 - 4 . 1 ) , and the number needed to treat to cause 1 event was 170 ( 95 % CI 100 - 582 ) over 3 . 3 years . 