Causes of Zone Depletions:

Manufactured compounds are also capable of altering atmospheric ozone levels. Chlorine,

released from CFCs, (15k JPG) and bromine (Br), released from halons, are two of the most

important chemicals associated with ozone depletion. Halons are primarily used in fire

extinguishers. CFCs are used extensively in aerosols, air conditioners, refrigerators, and cleaning

solvents. Two major types of CFCs are trichlorofluorocarbon (CFC13), or CFC-11,and

dichlorodifluoromethane (CF2Cl2), or CFC-12. Trichlorofluorocarbon is used in aerosols, while

dichlorodifluoromethane is typically used as a coolant.

CFCs were originally created to provide a substitute for toxic refrigerant gases and to reduce the

occupational hazard of compressor explosions. Near Earth's surface, chloroflourocarbons are

relatively harmless and do not react with any material, including human skin. For 50 years they

appeared to be the perfect example of a benign technical solution to environmental and

engineering problems, with no negative side effects. While CFCs remain in the troposphere, they

are virtually indestructible. They are not water soluble and cannot even be washed out of the

atmosphere by rain. We now understand that the very quality that made them seem so safe –their

stability – is what makes them so dangerous to the Earth system. CFCs remain in the troposphere

for more than 40 years before their slow migration to the stratosphere is complete. Even if we

were to end their production and use at this very moment, they will continue to contribute to

ozone destruction far into the future.

In the stratosphere, high energy UV radiation causes the CFC molecules to break down through

photodissociation. Atomic chlorine, a true catalyst for ozone destruction, is released in the

process. Chlorine initiates and takes part in a series of ozone-destroying chemical reactions and

emerges from the process unchanged. The free chlorine atom initially reacts with an unstable

oxygen containing compound (such as ozone) to form chlorine monoxide (ClO):

Cl + O3 → ClO + O2

The ClO molecule then reacts with atomic oxygen to produce molecular oxygen (O2) and more

atomic chlorine. The regenerated Cl atom is then free to initiate a new cycle.

ClO + O → Cl + O2

As shown in figures ,

This destructive chain of reactions will continue over and over again, limited only by the amount of chlorine available to fuel the process.

Chlorine occurs naturally in the oceans. However, the majority of chlorine in the atmosphere has originated with man-made chemicals. Without the dissociation of manufactured chlorofluorocarbons, there would be almost no chlorine in the stratosphere. CFC-12 concentrations were less than 100 parts per trillion by volume when they were first measured in the 1960s. Between 1975 and 1987, concentrations more than doubled from less than 200 parts per trillion by volume to more than 400 parts per trillion by volume. The amount of chlorine in the stratosphere also increased by a factor of 2 to 3. Scientists believe that continued buildup of CFCs could lead to severe ozone loss (61k JPG) worldwide. Thus, ongoing studies are essential to provide a necessary understanding of the causes of ozone depletion.The history of CFCs demonstrates that human activities can have unexpected long-term effects on the environment.

Effects of Ozone Depletion:

Increased UV:

Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the

absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone

layer decreases exponentially with the slant-path thickness and density of the layer. When

stratospheric ozone levels decrease, higher levels of UVB reach the Earth’s surface. UV-driven

phenolic formation in tree rings has dated the start of ozone depletion in northern latitudes to the

late 1700s.

In October 2008, the Ecuadorian Space Agency published a report called HIPERION, a study of

the last 28 years data from 10 satellites and dozens of ground instruments around the world

among them their own, and found that the UV radiation reaching equatorial latitudes was far

greater than expected, with the UV Index climbing as high as 24 in some very populated cities;

the WHO considers 11 as an extreme index and a great risk to health. The report concluded that

depleted ozone levels around the mid-latitudes of the planet are already endangering large

populations in these areas. Later, the CONIDA, the Peruvian Space Agency, published its own

study, which yielded almost the same findings as the Ecuadorian study.

Increased production of vitamin D:Vitamin D is produced in the skin by ultraviolet light. Thus, higher UVB exposure raises human vitamin D in those deficient in it. Recent research (primarily since the Montreal Protocol) shows that many humans have less than optimal vitamin D levels. In particular, in the U.S. population, the lowest quarter of vitamin D (<17.8 ng/ml) were found using information from the National Health and Nutrition Examination Survey to be associated with an increase in all-cause mortality in the general population. While blood level of Vitamin D in excess of 100 ng/ml appear to raise blood calcium excessively and to be associated with higher mortality, the body has mechanisms that prevent sunlight from producing Vitamin D in excess of the body's requirements.

Biological effects:

The main public concern regarding the ozone hole has been the effects of increased surface UV

radiation on human health. So far, ozone depletion in most locations has been typically a few

percent and, as noted above, no direct evidence of health damage is available in most latitudes.

Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the

effects could be substantially more dramatic. As the ozone hole over Antarctica has in some

instances grown so large as to affect parts of Australia, New Zealand, Chile, Argentina,

and South Africa, environmentalists have been concerned that the increase in surface UV could

be significant.

Ozone depletion would magnify all of the effects of UV on human health, both positive

(including production of Vitamin D) and negative (including sunburn, skin cancer, and

cataracts). In addition, increased surface UV leads to increased tropospheric ozone, which is a

health risk to humans.

Malignant melanoma:Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15–20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet fully understood, but it appears that both UVB and UVA are involved. Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women. A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.

Effects on crops:An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV radiation and would be affected by its increase.Despite mechanisms to reduce or repair the effects of increased ultraviolet radiation, plants have a limited ability to adapt to increased levels of UVB, therefore plant growth can be directly affected by UVB radiation.

Effects on non-human animals:A November 2010 report by scientists at the Institute of Zoology in London found that whales off the coast of California have shown a sharp rise in sun damage, and these scientists "fear that the thinning ozone layer is to blame". The study photographed and took skin biopsies from over 150 whales in the Gulf of California and found "widespread evidence of epidermal damage commonly associated with acute and severe sunburn", having cells that form when the DNA is damaged by UV radiation. The findings suggest "rising UV levels as a result of ozone depletion are to blame for the

observed skin damage, in the same way that human skin cancer rates have been on the increase in

recent decades.

Overview of Ozone Depletion on Environment: