When we say we are breathing oxygen we are breathing O 2 . The air we breathe contains O 2 along with many other gases. Ozone is not stable. As stated above ozone is toxic. Ozone is also toxic to plants at levels of 0.01 to 0.05 parts per million with an exposure time of several hours.
Ozone can also cause breathing disorders such as bronchitis.
Parts per million is a way of measuring the concentration of a gas. An ozone concentration of 0.01 parts per million means that for every 100 million particles there will be 1 ozone molecule. This is not very much ozone and it is still harmful to plants. Ozone does exist in the atmosphere. Everyone has heard discussions of the problems with the ozone layer.