The magnitude of the interference depends on how much of these catalytic molecules are present in the stratosphere and their lifetimes. A single atom of chlorine can destroy thousands of ozone molecules during its lifetime in the stratosphere. The Table B.1 below summarizes the lifetimes and the ozone depletion potential (ODP)54 of common ozone -depleting chemicals. Many of the ODSs also happen to be greenhouse gases and their global warming potentials (GWP) are indicated in the table. Chlorine-containing compounds that play this
54 The Ozone Depletion Potential (ODP) of a compound is defined as the ratio of the total amount of ozone destroyed by a given amount of the compound to the amount of ozone destroyed by the same mass of CFC-11.
catalytic role particularly well include the chlorofluorocarbons (CFCs), carbon tetrachloride, halons, methyl chloroform, methyl bromide, and hydrochlorofluo-rocarbons (HCFCs). These, particularly the CFCs, were once extensively used in refrigeration (~32%), cleaning (~20%), propellant (~18%), and foam blowing applications (~28%) (their use has since been severely restricted by the Montreal Protocol). Bromine, though more efficient at destroying ozone relative to chlorine, is fortunately present at a much lower concentration in the stratosphere. However, the effect of bromine compounds (mostly methyl bromide emissions from natural sources) can be quite significant in determining the ozone concentrations at mid latitudes.
The primary consequence of the loss of stratospheric ozone is that more of the extraterrestrial UV-B radiation (290 nm to 315 nm in wavelength) reaches the Earth. A wide range of measurements has established this relationship between ozone levels and the UV radiation reaching the Earth's surface. In 1994 the UV-B levels reaching earth's surface were found to be 8-10% higher than those found 15 years earlier at 45°N and °S, with even higher in the polar regions. The increase in UV-B across the Earth is not uniform; there is negligible ozone depletion in the tropics (20°N to 20°S of the equator) and the maximum reported depletions in the "ozone hole" at the pole55. Antarctica has unique weather conditions56 such as polar winds and polar stratospheric clouds that create particularly effective conditions for halogen-catalyzed ozone depletion. This lead to the appearance of a seasonal "ozone hole" first observed in 1985, where over half the overhead ozone within it was depleted. Recently reported increases in erythe-mal UV radiation from 1970 values was about 7% in Northern mid latitudes in spring, 6% in southern mid-latitudes, 130% in Antarctic spring, and 22% in Arctic Spring . These values are probably close to the highest possible increase in UV radiation to be experienced on earth's surface, as the best models suggest the concentration of ozone-depleting substances in the stratosphere to have peaked around 2000.
The increase in UV-B anticipated as a result of ozone depletion is relatively small compared to the natural variations in UV-B in most places on earth57. However, unlike with natural variations, ozone-depletion consistently increases the UV-B levels. Changes that rely on cumulative exposure (such as certain types of skin cancer) will therefore be affected by any increase in UV-B despite the
55 The most severe depletion of ozone in the ozone hole was observed in the 2000-20001 season in the Antarctica. The area of the hole was over 28 million sq.m. and the ozone levels were about 40 percent of the normally expected level!
56 The lower stratosphere at the South Pole is the coldest spot on Earth! The polar clouds with minute ice crystals that form in the region effectively facilitate the breakdown halogen-containing molecules into reactive forms and into halogen radicals.
57 The increased in UV-B radiation due to ozone depletion at mid latitudes is well within that obtained by moving several hundred kilometers towards the equator. At the population level (as opposed to individual level) an equivalent move would still be expected to result in increased health and other risks.
natural variability. Aerosol and pollutant build up in the atmosphere in urban areas, can generally decrease the UV-B levels, but at a high environmental cost.
In 1974 Molina and Rowland proposed the catalytic pathway for ozone depletion by stable halogen-containing chemicals such as the CFCs. The Montreal Protocol on Substances that Deplete the Ozone Layer established in 1987 sought to restrict the production and emission of chlorofluorocarbons and related compounds by signatory countries. With mounting evidence of damage to the ozone layer the protocol was soon amended (by the 1990 London Amendments) to call for a ban on the production of the most damaging of the ozone-depleting compounds by 2000 (2010 for developing countries). Subsequent amendments revised the year to 1996 for developed countries (Copenhagen Amendments in 1992), and added further restrictions to strengthen the protocol. These corrective measures have drastically reduced the input of ODSs into the atmosphere allowing the photochemical processes to reestablish the integrity off the ozone layer. Recent measurements made show a decrease in the abundance of ozone-depleting gases in the atmosphere. This decrease is almost entirely due to the outstanding success of the Montreal Protocol and associated Amendments. It is likely that the levels of ODSs will remain at near-peak levels for a few decades and then decrease significantly. Based on the present rate of recovery, the ozone layer is expected to return to normal levels (pre-1980 levels) by the middle of the next century.
The increase in UV-B radiation content in terrestrial solar radiation is undesirable and can lead to severe disruptions in different areas of human activity. These impacts and their significance along with the areas of uncertainty associated with each, are discussed in detail in a special recent issue of the Journal of Photochemistry and Photobiology (volume 46 B: Biology, October 1998). The anticipated effects of ozone depletion such as the increase in the incidence of a given disease, is based on two quantities; the radiation amplification factor, RAF, and the biological amplification factor BAF, for the particular effect. RAF is the ratio of the (percentage increase in the UV effective radiation) to the (percentage decrease in stratospheric ozone concentration). BAF is the ratio of the (percentage increase in the incidence of the disease or effect) to the (percentage increase in the UV effective radiation). The product of these two factors allows the estimation of the overall percentage increase in incidence of the disease per percentage ozone depletion. Two particularly important areas of impact58, however, need to be pointed out.
a) Human and Animal Health Impacts: Several major organ systems in human beings, particularly components of the eye, the outer layers of the skin, and cells from the immune system (Langerhan's cells) that are affected via exposure through skin, are routinely exposed to solar radiation.
58 A particularly informative web site that discusses the impacts of increased UV-B in sunlight is the Columbia University's Center for International Earth Science Information Network (CIESIN) http://www.ciesin.org/TG/HH/ozhlthhm.html
Eye: Absorption of UV radiation by the cornea (that absorbs most of the UV-B) and by the lens the UV radiation rarely reaches the retina of the eye. However, acute short-term exposure of the eye to UV-B radiation can lead to increased incidence of eye decease ranging from simple "snow blindness" (photokeratitis) to squamous cell carcinoma (SCC). Strongest correlation between eye diseases and exposure to solar radiation is observed for the condition pterigyium and climatic droplet keratopathy. With SCC, a recent epidemiological study found a strong relationship between the incidence of the disease and the latitude of exposure  (with 40-50 percent increase in incidence per 10° change in latitude).
Exposure can also lead to certain types of cataract formation increasing the risk by up to 3-fold . Epidemiological data linking the cataracts and exposure to solar radiation has been reviewed . Cataract is the leading cause of blindness worldwide requiring surgical removal of the affected lens, involving very significant public health costs worldwide.
Skin: Sunburn is the most common result of exposure to solar UV radiation. UV-B is well known to be about 1000 times more efficient than UV-A in causing sunburn. Even with the most sensitive skin types with no pigmentation sunscreens and controlled exposure offer a convenient means of controlling the condition.
Skin cancer is the most common form of cancer among fair-skinned people in the US and the incidence rates of common forms of skin cancer increases with the dose of UV-B radiation. The incidence of NMSC (the predominant form of which is the non-lethal basal cell carcinoma) in US correlates well with available UV-B levels. A relatively higher level of risk exists for fair-skinned populations  exposed to increased solar UV-B contents. The role of UV-B in promoting the more serious malignant melanoma remains unresolved. Factors such as genetics, avoidance behavior and diet can modify the impacts of exposure in both humans and in animal species.
One clear benefit of increased UV-B exposure of the human skin, however, is the increased production of vitamin D!
Immune reactions: The role of UV radiation in modulating immune reactions in humans is well established and it is likely that increased UV-B radiation will impact the incidence of infectious diseases. Risk factors for certain cancers (skin cancer and non-Hodgkin's lymphoma) include immunosup-pression. UV-induced immunosuppression is therefore likely to promote such cancers, but no epidemiological supporting data are available at the present time.
Crop species: Increases in solar UV-B radiation affect crop plants (as well as other plants) in several different ways including direct damage, alteration of patterns of gene activity, and changes in their physiological cycles. These changes can significantly impact crop yields, with the direction of the response depending on the species and even varieties of the same species. This is particularly true of economical important crops59 such as soybean  and rice  where different varieties were found to either increase or decrease in yield as a result of increased UV-B content in sunlight. About half of all species investigated, however, show some effect to increased UV-B radiation, with some adapting fairly quickly and others very slowly to the exposure. Selective breeding of the UV-resistant varieties might be considered to avoid loss in crop yield under such exposure conditions. Changes in UV-B levels also affect the interaction between species in a community and can affect the susceptibility of species to pathogens and insects.
Marine environment: Depending on the clarity of water UV-B radiation could penetrate 5-20 m into the water column, and has a direct influence on the phyto- and zooplankton species in the worlds ocean. This potentially impacts the capacity for primary production and carbon dioxide fixation in these systems, . UV radiation affects the growth, reproduction and the physiology of phytoplanktons (including their photosynthetic pigment content)  . A particular concern is the effect of increased UV-B on the marine species that constitute the base of the food pyramid accounting for almost a third of the global productivity. If the species are able to sense and react to the radiation by moving away to deeper waters, their photsynthetic production will tend to decrease60. As with crop species the UV-B tolerance case of marine species also varies widely.
The main threats of UV-B to zooplankton species is the reduction of their food supply, the phytoplankton, and stresses at various states of developmental stages of their life cycle, possibly causing mass mortalities . While planktons seem to survive well in tropical regions with already high UV-B levels, species adapted to low UV levels in the Polar Regions might be particularly susceptible to any increase in UV-B. An important consequence of any significant reduction in the plankton levels is its adverse impact on the world's fisheries. Reduced fish stocks can potentially alter the diet of a large part of the population; fish constitute about 40 percent of the dietary protein intake in Asia (where a majority of the global population is found in).
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