Good ozone is generally considered the ozone found in the ozone layer of the earth's atmosphere. The ozone layer is a
layer in Earth's atmosphere which contains a few parts per million of ozone (O3). This layer absorbs 97-99% of the sun's
high frequency ultraviolet light, which is potentially damaging to life on Earth. It is mainly located in the lower portion of the
stratosphere from approximately 15 km to 35 km above Earth's surface. Ozone in the earth's stratosphere is created by
ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms
(atomic oxygen); the atomic oxygen then combines with unbroken O2 to create O3. The ozone molecule is also unstable
and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process
called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere.
Humans have damaged the ozone layer by adding molecules containing chlorine or bromine that lead to ozone destruction. The largest group among these are chlorofluorocarbons (CFCs). At ground level, these molecules are stable and have many uses in industrial and domestic applications. However, when they are released into the atmosphere, they drift up to the stratosphere, pushed by winds and atmospheric mixing.
At that high altitude, energetic light rays (UV-C radiation) can break down such molecules in a reaction that liberates an atom of chlorine (Cl). This chlorine atom can react with ozone and break it down to chlorine oxide and O2. Chlorine oxide will break down as well, releasing the Cl to go on destroying ozone. In fact, one Cl can destroy up to 10,000 ozone molecules!
As a result, ozone in the stratosphere has been reduced to such an extent that a hole opens up above Antarctica each spring that has, in each of the past four years, measured 8.2 million square miles -- larger than the United States and Canada combined! The problem is not limited to Antarctica, however. Stratospheric ozone is being reduced over much of the globe and research shows that this allows more dangerous UV-B to reach the surface of the earth.
The harmfulness of UV-B stems from the high energy of these light rays, which allows them to penetrate deeply into water, leaves, and skin. Because of this, increased UV-B radiation can harm the metabolism of cells and even damage genetic material. Although living organisms have always been exposed to some UV-B, cellular repair mechanisms evolved to protect against its damaging effects. The problem with increased UV-B is that it causes more damage than the natural protection can cope with. Increased UV-B radiation leads to increased incidence of such problems as skin cancer, eye damage and cataracts, and possible inhibition of immune system function in humans as well as other animals. Plants also suffer under increased UV-B, and their vulnerability could result in reduced crop yields, damage to forest ecosystems, and decreased populations of phytoplankton in the world's oceans.
So, unlike ground level ozone which is harmful to environment, stratospheric ozone is protective to the environment. Thus, this is why this ozone is called "good".