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: In addition , both change and peak , showed significant positive correlations with migraine headache intensity ( P < 0 . 001 ) .
### Output Text:
In addition , both change and peak , showed significant positive correlations with migraine headache intensity ( P < 0 . 001 ) . 