I agree with rog. When you're not used to ozone, it is reducing the impurities in the lung by oxidation/reduction so quickly that the body is challenged to deal with the by-products produced, resulting in tighness of chest, involuntary coughing, headache etc. As the toxins are reduced slowly over time with mild exposure to ozone, the lungs are able to accept greater amounts without discomfort. However, no matter how clean your lungs get, it is never a pleasant experience or a good idea to breathe strong ozone directly from a medical generator.
Best practice when starting is be aware of what you're body's telling you and act accordingly.