The highest carbon dioxide levels observed in the warmest interglacial peaks was about 300ppm. The pre-industrial levels of 285 were pretty typical of the warmest part of the natural glacial cycle. 

Ozone is considered a pollutant as well, even though stratospheric ozone is quite important to our well-being. Yes, plants use it to grow, but that doesn't mean more is better. (for that logic, someone whose house is flooded should be thrilled, since water is similarly essential for plant growth!)