1. What are ozone and the ozone layer?
2. What have humans done to the ozone layer?
3. How is increased surface UV-B radiation harmful to global ecosystems?
4. What have we done to protect the ozone layer?
5. Is ozone depletion related to global warming?
6. Are CFC-replacements really ozone friendly?

1. What are ozone and the ozone layer?

Ozone (O3) is a molecule made up of three oxygen atoms. The oxygen we breathe (O2) is similar but has only two oxygen atoms. High up in a region of the upper atmosphere known as the stratosphere, light rays can break down breathable oxygen into two individual oxygen atoms. Single oxygen atoms are quite reactive, and ozone is formed when one of these lone oxygens bump into and combine with O2.

But ozone isn't very stable either. If a high-energy light ray, in particular ultraviolet-B (UV-B), strikes an ozone molecule, it too will break down, back to the lone oxygen and O2. Other molecules naturally found in the stratosphere, such as nitrous oxide, can also react with O3 and break it down.

Over time, as the atmosphere formed, the reactions that create and break down ozone came into a dynamic equilibrium. The result was a small residual amount of ozone concentrated in a band between nine and twenty-two miles high in the stratosphere. This is the band popularly referred to as the ozone layer. But the equilibrium is delicate, and ozone is rare even in the ozone layer. For every ten million molecules of air, two million are breathable O2, and only three are ozone. Yet this small amount of ozone is enough to prevent most UV-B radiation from reaching the surface of the earth.

2. What have humans done to the ozone layer?

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.

3. How is increased surface UV-B radiation harmful?

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.

4. What has been done to protect the ozone layer?

Through extensive research, scientists identified the human-produced chemicals that are responsible for the destruction of stratospheric ozone. As evidence emerged on the extent of the threat to the ozone layer, the international community agreed to control ozone-depleting substances and schedule a timetable for completely phasing them out. This agreement is known as the Montreal Protocol and is a monumental achievement in international cooperation and environmental protection. Furthermore, the protocol provides for an on-going process so that, as the scientific understanding of ozone depletion improved, the phasing out process could be accelerated. The agreement also provides a powerful precedent for similar international efforts to deal with global warming and loss of biodiversity.

In the United States, the Environmental Protection Agency is charged with enforcing the terms of the Montreal Protocol. The treaty provisions are given the force of law through the Clean Air Act of 1990. Accordingly, chlorofluorocarbon, carbon tetrachloride, and methyl chloroform production ended at the end of 1995; methyl bromide is currently scheduled to be phased out by 2001; and all hydrochlorofluorocarbons will be phased out by 2030.

5. Is ozone depletion related to global warming?

No. Ozone depletion and global warming are separate problems, though some agents contribute to both. Chlorofluorocarbons (CFCs) are the principle cause of ozone deletion, but they also happen to be potent heat-trapping gases. Still, CFCs are responsible for less than 10 percent of total atmospheric warming, far less than the 63 percent contribution of carbon dioxide. Thus, attention paid to CFCs has been on their ozone depletion role. This will change as CFCs are phased out and replaced by hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs such as R-134a). These chemicals have little or no effect on the ozone layer but are strong heat-trapping gases. As their concentration in the atmosphere is already rising, the likely net effect in the future is that reductions in the CFC-related contribution to global warming will be offset by the presence of HCFCs and HFCs.

6. Are CFC-replacements really ozone friendly?

The chemicals that are currently replacing CFCs are either HCFCs (hydrochlorofluorocarbons) or HFCs (hydrofluorocarbons such as R-134a). HCFCs have less chlorine in them and are less susceptible to the reactions that release chlorine in the stratosphere. But they are still ozone-depleting chemicals -- they just destroy far less ozone than CFCs.

For example, while CFC-12 has an ozone depleting potential rating of 1.0, HCFCs have ratings from 0.02 - 0.1. HCFCs will eventually be phased out by 2030, as stated in the Montreal Protocol. HFCs do not contain chlorine, so they don't contribute to ozone destruction at all. However, since both of these groups are potent heat-trapping gases, they are a stop-gap measure, the lesser of two evils. Eventually, we are going to need a permanent replacement for all these kinds of chemicals.