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 patients affected were younger ( p < 0 . 05 ) and the site of the dural puncture was higher ( p < 0 . 01 ) than those individuals without pain .
### Output Text:
The patients affected were younger ( p < 0 . 05 ) and the site of the dural puncture was higher ( p < 0 . 01 ) than those individuals without pain . 