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: OBJECTIVE : This study describes associations of ozone and fine particulate matter with Parkinson ' s disease observed among farmers in North Carolina and Iowa .
### Output Text:
OBJECTIVE : This study describes associations of <span class="chemical">ozone</span> and fine <span class="chemical">particulate matter</span> with Parkinson ' s disease observed among farmers in North Carolina and Iowa . 