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: Lower doses of azelastine were statistically more effective than placebo only during portions of the first 3 weeks of the study .
### Output Text:
Lower doses of <span class="chemical">azelastine</span> were statistically more effective than placebo only during portions of the first 3 weeks of the study . 