Clearly...
breathing pure oxygen for an extended period of time can have unintended side effects... so it too will eventually be declared a pollutant.
We breathe oxygen but we don't breathe 100 % oxygen it is highly flammable and
breathing pure oxygen is dangerous.
Current methods of oxygenating tumors in patients includes
breathing pure oxygen, putting patients in oxygen chambers or injecting liquids full of oxygen directly to the tumor site.
They are more likely to strike smokers, can be triggered by alcohol, and can often be soothed by
breathing pure oxygen.