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: When both skin test and RAST for BPO were negative , single - blind , placebo - controlled challenge tests were done to ensure tolerance of PG or sensitivity to AX .
### Output Text:
When both skin test and RAST for <span class="chemical">BPO</span> were negative , single - blind , placebo - controlled challenge tests were done to ensure tolerance of <span class="chemical">PG</span> or sensitivity to <span class="chemical">AX</span> . 