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 the present study , oxygen radical production in patients with ALF due to paracetamol overdose was compared with that of healthy volunteers .
### Output Text:
In the present study , <span class="chemical">oxygen</span> radical production in patients with ALF due to <span class="chemical">paracetamol</span> overdose was compared with that of healthy volunteers . 