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: Both , production of reactive oxygen species as well as activation of NF - kappaB have been implicated in severe neuronal damage in different sub - regions of the hippocampus as well as in the surrounding cortices .
### Output Text:
Both , production of reactive <span class="chemical">oxygen</span> species as well as activation of NF - kappaB have been implicated in severe neuronal damage in different sub - regions of the hippocampus as well as in the surrounding cortices . 