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: 3NT levels were significantly increased in all subcellular compartments at 6 hours and subsequently declined at 24 hours .
### Output Text:
<span class="chemical">3NT</span> levels were significantly increased in all subcellular compartments at 6 hours and subsequently declined at 24 hours . 