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: A significant increase in the hepatic and brain P450 monooxygenases was also observed when the duration of exposure of low dose ( 2 . 5 mg / kg ) of lindane was increased from 5 days to 15 or 21 days .
### Output Text:
A significant increase in the hepatic and brain P450 monooxygenases was also observed when the duration of exposure of low dose ( 2 . 5 mg / kg ) of <span class="chemical">lindane</span> was increased from 5 days to 15 or 21 days . 