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: Pain intensity and pain period were statistically significantly lower for the 30 - second injection than for the 10 - second injection .
### Output Text:
Pain intensity and pain period were statistically significantly lower for the 30 - second injection than for the 10 - second injection . 