Ozone is formed by the reaction of an oxygen atom with an oxygen molecule.
The difference between good ozone and bad ozone depends on which layer of the earth's atmosphere the ozone is found. Good ozone, or stratospheric ozone, is found in the stratospheric layer of the atmosphere. The stratospheric layer reaches from ten to thirty-one miles above the earth's surface. Within this layer, good ozone is made and destroyed naturally all of the time. The layer of stratospheric ozone varies in thickness for this very reason.