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: Prenatal exposure to non - selective COX inhibitors increases the risk of VSD and MD when compared to historic control but not with selective COX - 2 inhibitors .
### Output Text:
Prenatal exposure to non - selective COX inhibitors increases the risk of VSD and MD when compared to historic control but not with selective COX - 2 inhibitors . 