You make a good point. I went back over McCabe's section about breathing ozone (page 232) and he basically says the same thing: that ozone will do it's job and oxidize crud that has built up in your lungs. Therefore, an ozone newby will probably gag on ozone while their body tries to cope with breathing it. But a clean person can tolerate much more.
Personal experience bears this out. I do know that when I've had a whiff of straight medical grade, no matter how clean I am, I will gag and start coughing something aweful. Regardless if it is oxidizing more or hurting my lung tissue, I'm not sure. All I know is that it doesn't feel good and I won't do it unless by accident.