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Updated 11 November 2004

Consequences Vol. 1, No. 2, Summer 1995








Impacts of a Projected Depletion of the Ozone Layer

by Frank R. de Gruijl

Life on Earth depends in part on a thin shell of gaseous ozone that stretches from about 10 to 25 miles above our heads, encompassing the planet like an invisible, protective shield. At this altitude, it lies well above the height at which normal commercial aircraft fly, and far beneath the orbital paths of spacecraft. The ozone layer is the main barrier between us and the hazardous ultraviolet radiation that streams toward the Earth, day in and day out, from the burning surface of the Sun. Ozone--a form of oxygen--is selective in what it takes from sunlight: screening out, through a process of atomic absorption, only the more energetic ultraviolet rays while allowing the visible light and the warm infrared to pass through, untouched (Fig. 1).

A History of Ozone

Oxygen itself came into the Earth's atmosphere some two billion years ago as a product of photosynthesis in early forms of plant life. It is now an ever-present component of the air, from the surface of the Earth to the outer reaches of the atmosphere. Toward the top, in the rarefied upper atmosphere, the stream of highly energetic ultraviolet (UV) radiation from the Sun impinges on molecules of ordinary oxygen (O2), splitting them into the two atoms (O) of which they are made. In a perpetual dance the molecules and atoms of oxygen swirl together to form ozone (O3), which reverts in time to more stable O2 molecules. Only to be hit and split apart again. Through this delicate balance a stratospheric ozone layer is maintained. At any time, however, very little ozone is there: enough to form a layer a mere 3 mm (a tenth of an inch) in thickness were it compressed under the conditions that exist at ground level.

The part of the UV spectrum of solar radiation that splits O2 apart in the upper atmosphere lies in what is shown in Fig. 1 as the highly energetic "ultraviolet-C" or UVC range. The ozone that is subsequently created is itself a strong absorber of the remaining UVC and of much of the adjacent region of the UV, closer in wavelength to visible light, that is called the ultraviolet-B or UVB. Sandwiched between the UVB and the violet end of the visible spectrum is the less energetic UVA.

Atmospheric oxygen screens out all of the short wave UVC; the ozone layer--thin as it is--prevents longer wave UVC and most of the solar UVB radiation from reaching the Earth's surface. This is highly beneficial, for UVC and short wave UVB radiations blocked by ozone are particularly damaging to organisms, because these wavelengths are absorbed in living matter by essential molecules, such as DNA, and damage them. Thus, by a magnificent coincidence the biosphere has created its own protective, atmospheric UV filter under which life could evolve and exist on the surface of the planet.

Human intervention and response

In the 1930s commercial laboratories developed compounds of chlorine, fluorine and carbon (called chlorofluorocarbons, or CFCs) and similar bromine-related compounds as convenient, non- corroding, non-toxic and non-flammable gases which subsequently found a wide range of industrial applications, including everyday use in spray cans, refrigerators and air conditioners. In a sense, these fruits of human ingenuity turned out to be too good: by design these so-called halocarbon gases are so stable that they react with almost nothing--until, that is, they slowly rise into the sky: climbing high enough, after a period of years, to be hit by direct UV radiation from the Sun. They then break apart and release chlorine or bromine which can react with the ozone that is there, pushing the delicate balance of natural photochemical reactions in the direction of ozone destruction. Thus, the ozone layer--an evolutionary heritage of a billion years of time--has come under serious threat in but the last few decades of industrial activity.

This startling realization came to light in 1974, based on the laboratory work of Sherwood Rowland and Mario Molina. By then the wheels had been already set in motion for the gradual deterioration of the protective ozone layer. Industry--the world's only source of halocarbon gases--was from a business point of view understandably reluctant to immediately halt production and use of these versatile compounds; nor were there any immediate replacements available. Corrective action awaited wider attention and momentum in public and political arenas. The threat of ozone- depleting gases needed to be substantiated further, and possible adverse effects needed to be weighed against the practical costs of reducing or stopping production. In the process, industry began to capitalize on the growing public concern by producing "ozone friendly" spray cans.

Computer models of atmospheric circulation and chemistry subsequently confirmed the threat of ozone depletion by these compounds, yielding more quantitative estimates of ozone reduction which were continually refined. But although it was well established that UV radiation damages proteins and DNA, very little was known of the actual severity and scale of effects under even natural levels of UV radiation. Forecasting the consequences of increases in ambient UV radiation thus became a formidable task.

The one exception appeared to be the expected increase in skin cancer in light-skinned people. Here increased incidence could be estimated quantitatively, based on higher levels of UVB, and these estimates were also refined by subsequent research. Figures regarding skin cancer and the concern about other potentially grave impacts proved daunting enough to initiate actions to limit the commercial production of CFCs and other ozone-depleting substances.

Something the atmospheric models had not forecast was the dramatic, early springtime depletion of stratospheric ozone over the South Pole of the Earth, later termed the "ozone hole," that was discovered almost through accident in routine measurements taken in Antarctica. Further measurements established that the phenomenon was the result of increased amounts of chlorine in the stratosphere in its most reactive form, intensified by the unique circulation of air near the Earth's poles and the presence of ice crystals in the peculiar stratospheric polar clouds. It was as though Mother Nature were performing a little experiment, in her basement, to emphasize that the consequences of adding chlorine in the ozone layer was not a figment of scientific imagination, and that an anthropogenic increase in gaseous chlorine compounds was not a good idea. This finding brought a sense of urgency to policy makers, and measures were repeatedly stepped up to phase out the production of CFCs and other ozone degrading substances, through a series of unprecedented, international protocols in 1987, 1990, and 1992.

Changes in Atmospheric Ozone

The thin layer of ozone that surrounds the Earth is neither uniform nor constant. It is naturally thinner at the equator than over the poles, and it exhibits substantial temporal variations--as much as +/- 20% at any place, as a result of changing solar intensity and atmospheric circulation. These natural variations can mask any subtle, long-term changes in ozone concentrations, making it necessary, if global trends are sought, to carry out an extended series of reliable measurements that cover all of the Earth.

Spaceborne and other measurements have documented downward trends in stratospheric ozone which are greatest in winter and early spring and strongest in polar regions--especially in the Southern hemisphere. Globally averaged losses have totaled about 5% since the late 1960s. Although the rate of loss has now been slowed, total ozone is expected to continue to drop through the present decade, when the decrease at mid latitudes in the Northern hemisphere in summer and fall should maximize at 6 to 7%. A reduction in ozone of this amount would correspond to a resulting 6 to 12% increase in the average annual dose of biologically-harmful UV radiation. In addition to increases in annual UV dosage, transient depletions in ozone in the spring may cause invisible "UV storms" which could prove particularly harmful to vulnerable young plants and animals in very early developmental stages, such as fish in shallow water. A striking finding was that in the vicinity of the Antarctic ozone hole (64° South latitude) the DNA-damaging UV radiation can exceed the summertime maximum at San Diego (32° N) where because of the lower latitude, the Sun climbs higher in the sky and there is less atmospheric absorption.

Natural events can also affect the pace at which chlorine depletes ozone. Relatively low levels of global ozone documented by satellite measurements during 1992 and 1993 may have been related to the June 1991 eruption of Mt. Pinatubo in the Philippines. The volcanic eruption introduced clouds of dust particles into the stratosphere, thereby enhancing the chemical destruction of ozone by chlorine. In 1994, ozone amounts measured in the stratosphere were relatively normal. In January 1995, however, levels in the northern hemisphere dropped again by 10 to 25%: a change that cannot be attributed to volcanoes. On the whole, one has to be careful and not attach too much weight to ozone levels at any particular time at any particular location, because the ozone layer as a component of the atmosphere has an inherently chaotic behavior. Of greater import are trends in large scale averages of ozone levels over long periods of time.

Good and bad ozone

Ozone is also found in the air near the ground but in amounts that are highly variable from place to place and that constitute at most a tenth of what exists in the stratosphere. In polluted areas, especially where there are high levels of nitrogen oxide in the air--as in congested traffic zones--the residual solar UVB radiation that reaches ground level can cause increased levels of ozone, which is a major constituent of photochemical smog. As in the stratosphere there is a give and take between the production of ozone by the UVB in sunlight and the blocking of the same UV by the ozone itself, and by the sulfur dioxide that is also prevalent in polluted air. Direct exposure to ozone is known to be harmful, and dramatically so in photochemical smog: it causes respiratory complaints and can seriously exacerbate asthma. It is also damaging to plants, as can sometimes be seen in trees that bound busy freeways. Hence, the same ozone that is "good up there" can be "bad down here." Moreover, an ozone depletion in the stratosphere will lead to an increase in the "bad ozone down here" in polluted areas because of the extra UVB radiation that will reach the ground to create it.

Even with full compliance with the now internationally agreed phase-out of ozone-degrading chemicals such as CFCs, the Earth's stratospheric ozone layer is expected to continue to decline, reaching maximum depletion around the turn of the century, and then gradually return to 1970 levels in about 50 years' time. During this period the biosphere will be exposed to higher levels of harmful UV radiation.

Effects on Human Health

Although the ozone layer blocks most of the damaging UVB radiation received from the Sun, a small amount slips by, damaging our skin in the form of sunburns and "suntans." UVB radiation is strongly absorbed in the skin (Fig. 2) and in the outer layers of the eye, and does not penetrate any deeper into the human body.

Like most organisms exposed to sunlight, the human skin has developed various defense mechanisms against the damaging effects of UV radiation. The skin adapts to increased exposure to UV by thickening its outer layer (the epidermis) and by developing pigmentation that serves to shade the more vulnerable and deeper residing dividing cells. Molecular damage is dealt with in the body through repair or replacement. Overly damaged cells will normally self-destruct through a process called apoptosis, and if this fails, the immune system should get rid of any resulting aberrant cells. As explained below, it is when these natural safeguards fail or are overcome that real trouble can ensue.

Quite apart from its damaging effects, UVB radiation can also be beneficial: in the skin it initiates the production of vitamin D3 that helps build and maintain our bones. Very little UVB radiation is needed for this purpose, however, and the process of production is self limiting.

Damage to the eyes

The human eye is less well protected by internal safeguards than the skin, but it is also less exposed thanks to the shielding provided by the brow and eye lids. Under certain circumstances, such as bright reflection from snow-covered ground, the surface of the eyeball can develop a painful "snow blindness" (or photokerato- conjunctivitis). One such episode is, however, usually sufficient for people to take precautions the next time.

Long-term damage to the eyes by UV radiation is more difficult to prevent because there is no feeling of pain or early warning. People usually become aware of the effect when it is too late. Chronic UV exposure can cause pterygium (an outgrowth on the most superficial cell layer of the eyeball) and climatic droplet keratopathy (a degeneration of the fibrous layer that covers the lens); both afflictions can reduce clarity of vision and even cause blindness. Cortical cataracts--one of several forms of the cataracts that cloud the lens of the eye--can also result from UV radiation.

A lack of firm knowledge regarding wavelength and UV dose dependencies make it difficult to estimate the impact of an ozone depletion on any of these long term ocular effects, beyond a tentative estimate of about a 0.5% increase in cataract incidence for every persistent 1% decrease in average ozone concentration. Thus, based on the estimated 6-7% total drop in ozone during summertime, as noted above, the incidence of cataracts might be expected to rise by up to 3% in the early part of the next century.

Skin cancer

As mentioned earlier, UV radiation damages DNA, the molecule that builds the genes that in turn carry the genetic information used by the cell to build up the proteins it needs for its normal functions. After a sunny day on the beach a single, typical exposed cell in the epidermis has developed some 100,000 to 1,000,000 damaged sites in its DNA. In spite of a formidable DNA repair system, some damage may persist, resulting in faulty replication of DNA in a daughter cell. In some instances, a series of such events may cause certain crucial genes (called oncogenes and tumor-suppressor genes) to malfunction, in which case a cell is altered, causing it to grow independently into a tumor or cancer. Recent research is beginning to unravel these steps in more detail.

Non-melanoma skin cancer (mainly squamous and basal cell carcinomas) is among the most frequently diagnosed and most rapidly rising forms of cancer in white populations; in the U.S. alone, about 600,000 cases are diagnosed each year, twice as often in men than in women. Only about 1% of these skin tumors prove lethal: because of their moderate growth rate and early detection (though small, they are cosmetically apparent as red nodules or blotches) they are well treatable. At the same time, adequate treatment can result in mutilation of the skin or of facial features where they are often found.

The connection between non-melanoma skin cancer and exposure to the Sun is well established from both epidemiological data and from experiments with laboratory animals, in which chiefly squamous cell carcinomas have been induced. The types of alterations found in recent studies of the "p53" tumor suppressor gene in human non- melanoma skin cancers clearly identify UV radiation as the cause. Moreover, it has long been known that people with xeroderma pigmentosum (a rare disease that impairs the ability of cells to repair DNA damage induced by UVB radiation) run a 1,000 times higher risk of skin cancer than healthy people.

Such causal relationships are further confirmed by experimentally UV-induced skin cancers in mice. Based on the plausible assumption that the UV-cancer forming processes are basically the same in mice and men, and by combining animal and epidemiological data, quantitative estimates can be made of the consequences of an ozone depletion: the incidence of non-melanoma skin cancer is expected to increase by approximately 2% for every persistent 1% loss in average ozone concentration.

For the U.S. the reduction in ozone--down to as much as -7% in summertime by the end of the century--is expected to result in a steady increase in non-melanoma cancers in ensuing years. By mid- century incidence could rise to as many as 100,000 extra cases per year, when compared to the 1960s. Accumulated over the next century the overall effect of the temporary ozone dip may total 3 to 5 million additional cases in the U.S., and a further delay in the return to 1960 levels of ozone can increase this number substantially. Based on present epidemiological data, the increases would be found most in lower latitudes where the Sun is higher in the sky and where there are more sunny days, as in the American Southwest and Florida. Globally the highest incidence of all forms of skin cancer is found among red-haired and fair-skinned people who have immigrated from more overcast regions of northern Europe to sunnier and lower-latitude Australia.

It is almost impossible to detect the initial increase in non-melanoma skin cancer that might be attributed to the slight decline in ozone over the last decades, because of the incomplete manner in which incidence is customarily reported. Another difficulty in detecting an ozone-related change is the background against which it must be measured: there has been a steep increase in skin cancer, almost worldwide, in the last few decades--most likely due in large part to societal trends in dress and work habits and in the sun-seeking behavior of many modern people. These trends (and the use of sun- tanning parlors) amount to a voluntary increase in risk for those involved, and campaigns have been launched to counter them. An ozone reduction, on the other hand, imposes a population-wide involuntary increase in risk.


In the mid-1980s the relationship between solar UV radiation and skin cancer of the pigment cells (or melanocytes), known as melanoma, was still heavily debated, in part because these more serious cancers are not limited to parts of the body that are the more exposed to direct sunlight. However, epidemiological data from the last decade have substantiated a relationship with solar exposure, and animal experiments have shown that UV radiation can at the very least enhance the development of these more lethal tumors. Although the incidence of melanoma is much lower than that of non-melanoma skin cancer (about 17,000 males and 12,000 females are now diagnosed each year with melanomas in the U.S.), the mortality is much higher, amounting to about 20% of all cases diagnosed.

Interviews with melanoma patients and healthy people have suggested that the risk of melanoma may be related to a history of sporadic over-exposure to UV radiation. Epidemiological studies (and particularly those involving migration) have further demonstrated the particularly insidious relationship between high exposure to the Sun during childhood and a dramatic increase in risk of melanoma skin cancer later in life. Exposure to high levels of UV radiation as a child is also associated with the later development of large numbers of moles, which in the form of dysplastic nevi may sometimes be rather large, very irregularly shaped and pigmented, and reddish due to a higher blood content; a large number of moles (typically over 50 that are more than 2 mm or a tenth of an inch in diameter) is a well established risk factor for melanoma skin cancer.

As yet, however, it is still not possible to produce confident quantitative estimates of the impact of an ozone depletion on the incidence of melanoma. Experiments with laboratory animals have yielded ambiguous information on even the part of the UV that is responsible: a test with opossums, for example, indicates that UVB- induced DNA damage is important, whereas one with fish has shown that solar UVB radiation could be less important than the more prevalent and weaker radiation in the UVA region of the spectrum (Fig. 1). Were UVA radiation, acting independently of UVB radiation, to prove the primary cause of melanoma in humans, a reduction in atmospheric ozone would have very little impact, since ozone--though effective in blocking the UVB- -is almost transparent to the UVA.

Immunological effects

Laboratory experiments have established that a skin tumor removed from a mouse that was chronically exposed to UVB will usually be rejected when the tumor is implanted in a genetically identical mouse that was not subjected to UV radiation . After a series of UVB exposures, however, the second mouse will accept the implanted tumor cells and allow the tumor to grow, well in advance of any subsequent UVB-induced tumors. The obvious conclusion is that exposure to UVB takes away the animal's natural ability to fight off these cancers of the skin. The tumor implants are also accepted when the immune system of the mouse is generally suppressed. In analogy, it has been found in humans that patients on immunosuppressive medication for kidney transplants run a dramatically increased risk of cancer in sun-exposed skin.

Further experiments showed that a prior series of UVB exposures can block attempts to immunize a laboratory animal to react against a foreign compound or infectious agent put in contact with its skin. As is the case for the UV-induced skin tumors, the animal may then develop a particular susceptibility to skin infections from such compounds. While the mechanisms responsible for this kind of selective lack of response to immunization are as yet not clear, the significant finding from animal experiments is that natural immunity to various kinds of infectious diseases is weakened by UVB radiation, including, in some studies, oral infections that do not involve the skin. A special concern is that UVB radiation may also lower the effectiveness of preventive vaccinations, as, for example, against tetanus.

It has also been shown that UV radiation can suppress immune reactions in humans, including blacks, although chiefly through a transient inability to immunize through the part of the skin that was exposed. In 10 to 30% of the cases, however, the immunity failure was not transient. In these instances the subject became lastingly immuno-tolerant, in the sense of being incapable, following a series of UV exposure sufficient to cause mild to notable sunburn, of mounting an adequate immune response against the particular compound with which the trial immunization was carried out. Interestingly, the trait to develop a UV-induced immuno- tolerance appeared to be particularly prevalent among people who had had skin cancers removed. These and other experiments have established that our natural immune system is affected by UV radiation at dosages experienced by many people today; data from other animals have proven that there is a genuine risk for increased severity of infections.

As yet there is no information from medical research on the present impact of ambient UV radiation on infectious diseases and vaccination programs. Acquiring the necessary epidemiological data is no easy task; still, its complete lack should be embarrassing when one considers the potential scale of the impact on people worldwide and the fact that for over a decade such studies have been identified as of high priority among needed research on the effects of an ozone depletion. Today even the crudest quantitative estimates of effects of an ozone depletion on infections and vaccinations seem still a long way off.

Impacts on Ecosystems

All animals and plants and other organisms that are exposed to the Sun, though well shielded by the ozone layer, have developed ways to cope with and protect themselves from the small fraction of solar UVB radiation that normally reaches the Earth's surface. Even a small amount of UVB radiation can have a significant effect on ecosystems. In the tropics, for example, where a thinner ozone layer and a higher Sun result in systematically stronger UV dosage, certain trees have been found to be restricted in their growth by current levels of solar UV radiation. In ecosystem studies, as in medicine, science has not yet reached the point where any practically useful assessments of the consequences of increased dosages can be made. Research has thus far been mainly limited to more rudimentary studies in laboratories and greenhouses that test the sensitivity of different plant species to enhanced UV radiation. Only a few field investigations have been performed on an appreciable scale, and proper ecological studies are still in their infancy.

In general, it appears that plant species can react in widely different ways to increased levels of UVB radiation: some may be clearly limited in their growth; other varieties may be insensitive or rapidly become so by adaptive mechanisms; and still others may even exhibit enhanced growth. Under added stress, as through drought, the differences in UV sensitivity may be completely lost. The majority of plant species that have been tested were agricultural plants; trees appear to run a higher risk of accumulating UV damage over their far longer lifetimes.

In addition to direct effects on photosynthesis and growth, there may also occur more subtle changes, such as a delay in flowering, a shift in the distribution of leaves, a change in leaf structure, or a change in a plant's metabolism. As verified in field studies, such subtle changes may have far-reaching consequences by causing a plant to loose ground to neighboring plants with whom they compete. Thus, dramatic shifts in plant populations and in biodiversity may ensue.

Similar processes can occur in the marine ecosystems that exist at shallow depths in photosynthetically active zones. UV radiation can penetrate tens of meters into clear ocean water. It has been found that phytoplankton--the minuscule, plant-like organisms that float on or near the surface of the ocean and that serve as the base of the entire marine food chain--are sensitive to the levels of UVB radiation that penetrate the ocean's surface. Recent studies have focused particularly on the waters that bound the Antarctic continent, directly under the ozone hole, and rates of phytoplankton production were indeed found to be depressed relative to other similar areas.

These potentially significant disturbances at the basis of terrestrial and marine food webs may have a domino effect that could ultimately affect mankind. Moreover, loss of biodiversity due to enhanced UV radiation may render an ecosystem more vulnerable to the other stresses such as are expected to accompany greenhouse- induced climate change. Higher levels of UVB levels could also reduce the global plant cover that serves as a sink for CO2, thus enhancing climatic change.

Unfortunately, at this time scientific research has produced only limited and widely varying data on possible impacts on single plants or species, and much remains to be done to quantify the possible effects on any marine, terrestrial, or agricultural ecosystem. Ecosystems may be further disturbed by deleterious effects of UV radiation on animals, especially in vulnerable, early stages of life such as larvae or the eggs of frogs in shallow water.

A Personal View

The integrity of the Earth's ozone layer has been a subject of concern for over 20 years. In the U.S. the issue was first raised in about 1970 by what seemed then to be the likely prospects of the intrusion of commercial fleets of supersonic jets (like today's Concorde) that would fly at higher altitudes than conventional aircraft and perturb the natural chemistry of the stratosphere. Later the greater threat of CFCs and bromine-related gases came into the limelight, in time resulting in the international agreements on production limits that now serve as an example of effective worldwide policy response to environmental threats.

As a result of actions taken, ozone-depleting gases in the atmosphere are increasing less rapidly and the ozone layer is probably degrading less rapidly. We should all be encouraged by the collective willingness to take protective measures with significant economic impacts, even in the face of uncertainty regarding the ultimate effects of what is to be avoided, and by the choice that was made to err on the environmentally safe side. Behind the actions taken in the Montreal Protocol of 1987 and the London and Copenhagen amendments of 1990 and 1992 were atmospheric models and atmospheric measurements, including specific campaigns to Antarctica and Greenland. In addition, many countries have in recent years started to monitor the ground-levels UV radiation. Behind all of these actions were perceived impacts on the biosphere, and on human health in particular. Yet we are still not very sure of what the effects would have been had we not phased out the production of these compounds.

Surprisingly little effort has been expended in studies of any of the potential effects when compared to that given to the causes of the loss of ozone. There are several reasons that may explain the imbalance, including the inherent difficulties in funding and accomplishing research that spans the chasms between physical, biological and medical science; the more instant gratification, on the part of policy makers, of regulatory action as compared to longer-term monitoring and research; and the dilemma- -so common to the environmental concerns of today--of long-term problems in a short-term world.

The International Council of Scientific Unions has long sponsored a Scientific Committee on Problems of the Environment (SCOPE) which has emphasized in its recent reports that the impacts of an ozone depletion call for the full utilization of the existing capacities of research, and preferably a major expansion. The UNEP Panel on Environmental Effects of Ozone Depletion concludes in its 1994 report that present reality is far from the goal of defining impacts, and that funding of research on UVB effects is still very low and does not even allow full utilization of the existing research capacity. There may be added hope in a recent decision of the Environmental Research Programme of the European Union to stimulate and support UVB radiation-related environmental research. This may provide a basis for a growing, coherent and well-directed European research program on the effects of solar UV radiation on health and our environment, and serve as an example for other countries of the world.

To justify the initial action to curb ozone-depleting gases it has apparently been sufficient to bring forward and substantiate one or two seriously adverse effects, but full and in-depth knowledge of all effects is needed for a complete and well-balanced assessment, and especially, for subsequent adequate mitigating action. What is clearly needed now is a well-directed, long-term research program aimed at defining possible impacts on human health and on the environment which can provide policy makers with information that is today in very short supply. Hand in hand with such a program must go continued efforts to monitor compliance with the accords. The UNEP committee, pursuant to the Montreal protocol, has the task of providing regular updates (at least every 4 years) on the current knowledge regarding the effects of a depletion of stratospheric ozone, in order to provide the information needed to control the release of ozone degrading substances, and if necessary, to allow well-guided intervention schemes to mitigate the effects of the residual ozone depletion.

The danger at this time is that policy makers and others involved with matters of the environment may come to believe that the existence of formal international agreements marks the end of the troubling subject of ozone layer protection, and with it all concerns regarding possible impacts. In fact, by even the most optimistic scenarios the adverse effects of UVB radiation due to residual ozone depletion will persist far into the next century.

Conclustion: A Summary of Potential Human Health Impacts

Any persistent drop in the amount of protective ozone resident in the stratosphere will increase the amount of solar ultraviolet radiation that reaches the surface of the Earth, at the risk of direct and deleterious effects on human health: that much is known with certainty. Likely effects include permanent clouding of the lens of the eye, which is particularly sensitive to the UVB exposure: a reduction of 7% in the amount of stratospheric ozone (the maximum expected in summer for middle latitudes by the end of the century) could increase the incidence of cataracts in sun-exposed people by 3 to 4%. Even more liable to damage is the sun-exposed skin, in the form of increased skin cancer. A total reduction of 7% in ozone in summertime would probably increase the incidence of non- melanoma skin cancer by about 16%. The incidence of more deadly melanoma could also rise. Chronic exposure to the increased UVB that would accompany any persistent loss in ozone would also affect the natural immune system, with an increased potential for infection and disease.

These changes and associated risks, summarized in Table 1, serve as rough indications of what could follow in the course of the early part of the next century after the presently-projected maximum in man-made ozone depletion. If the ozone layer were indeed restored to a thickness characteristic of the 1960s, the risks to human health would also be reversed, although with a delay in time. On the other hand, were present ozone- protective measures to prove inadequate, the effects may persist and even reach appreciably higher levels.

Reviewed by Edward DeFabo and Richard B. Setlow

Dr. Edward C. DeFabo is a photobiologist in the Dermatology Department of the George Washington University Medical Center in Washington, D.C. who has been involved for more than 20 years in the study of the UVB effects of ozone depletion. His research background is in photoimmunology and he now chairs an international SCOPE Committee on Effects of Ozone Depletion.

Dr. Richard B. Setlow is a biophysicist and photobiologist in the Biology Department of the Brookhaven National Laboratory in Upton, Long Island, New York. He has been involved in research on DNA damage and repair since 1955 and on the problem of the effects of diminished ozone since 1970. Dr. Setlow is a member of the National Academy of Sciences and a past Chairman of the National Research Council Committee on Photobiology.

For Further Reading

Effects of Increased Ultraviolet Radiation on Global Ecosystems, edited by E. De Fabo. Scientific Committee on Problems of the Environment (SCOPE). Paris, 1993.

Environmental Effects of Ozone Depletion: 1994 Assessment, edited by J. C. van der Leun, X. Tang and M. Tevini. United Nations Environment Programme (UNEP). Nairobi, Kenya, 1994. (Available from UNEP, Ozone Secretariat, P.O. Box 30552, Nairobi, Kenya.)

Environmental UV Radiation: Causes-Effects-Consequences, by J. Acevedo and C. Nolan. European Commission, Directorate-General XII for Science, Research and Development. The Environment R&D; Programme. Brussels, Belgium, 1993. (Available from EC Headquarters, Rue de Loi 200, B-1000049 Brussels, Belgium, or by FAX 32 2 296 30 24.)

Scientific Assessment of Ozone Depletion: 1994. World Meteorological Organization, Global Ozone Research and Monitoring Project, Report No. 37, Geneva. (Available from World Meteorological Organization, P.O. Box 2300, 1211-Geneva-2, Switzerland or from NASA, Office of Mission to Planet Earth, Two Independence Square, 300 E Street SW, Washington, D.C. 20546)

"The Need for Scientific Communication with the Public," by F. S. Rowland, in Science, vol. 260, pp 1571-1576, 1993.

UV-B Radiation and Ozone Depletion, edited by M. Tevini. Lewis Publishers, Boca Raton, Florida, 1993.

Dr. Frank R. de Gruijl is a biophysicist on the research staff of the Dermatology Department, University Hospital, University of Utrecht in the Netherlands. Since 1977 he has been involved in studies of effects of ultraviolet radiation on health, and now serves on the committee of the United Nations Environment Programme that deals with effects of an ozone depletion.

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