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: CONCLUSIONS : The plausibility of an effect of ambient concentrations of these pollutants on Parkinson ' s disease risk is supported by experimental data demonstrating damage to dopaminergic neurons at relevant concentrations .
### Output Text:
CONCLUSIONS : The plausibility of an effect of ambient concentrations of these pollutants on Parkinson ' s disease risk is supported by experimental data demonstrating damage to dopaminergic neurons at relevant concentrations . 