Table of Contents
Figures & Tables
|Total column ozone global average as a function of latitude.|
|Weighted irradiance as a function of wavelength|
|Ozone, oxygen and appearance of major life forms as a
function of geologic time
|Comparison of Several Common Radiation Risks|
|Comparison of Commonplace Risks|
This document provides: (1) a summary in layman terms of the scientific findings on the biological and human health risks associated with ozone depletion; and (2) an explanation and perspective on the predicted impacts.
Concern about the effect of chlorofluorocarbons (CFCs) on the ozone layer were first raised in 1974 by Drs. Sherwood Rowland and Mario Molina. They hypothesized that CFCs were able to persist in the atmosphere long enough to diffuse upward into the stratosphere. Once there, intense solar radiation would break them up, releasing reactive chlorine atoms which would then destroy ozone. Their theory initially met with skepticism but mounting evidence and the discovery of the Antarctic ozone hole in 1985 galvanized the interest of scientists and policy makers.
Subsequent research has indicated that CFCs, along with certain other chemicals from our industrialized society, are in fact depleting ozone in the stratosphere.
The problem with introducing chlorine atoms into the stratosphere is that a sequence of chemical reactions occur that result in the destruction of ozone and the regeneration of the original chlorine atoms. In other words, the chlorine atoms are not initially used up by the reaction. Rather, they are regenerated by the reaction and therefore are capable of reacting with ozone over and over again. Each chlorine atom can destroy over 100,000 ozone molecules before it ultimately returns to the troposphere as hydrochloric acid and is removed during rain storms.
Relatively recently, human activities have introduced large quantities of chlorine atoms into the atmosphere. Presently, each year, about 400,000 tons of reactive chlorine atoms are released by the breakdown of chlorofluorocarbon (CFC) molecules and other compounds (80% man-made) in the stratosphere. The net effect is to destroy ozone faster than it is naturally created.
Because ozone plays an important role in reducing the amount of ultraviolet radiation that reaches the earth's surface, depletion of ozone brings about an increase in ultraviolet radiation. Since UV radiation is readily absorbed by living tissue, and since light at this wavelength has sufficient energy to break chemical bonds, it can be injurious to both plants, animals and humans.
3. Solar Radiation & "Amplification Factors"
Energy from the sun reaches the earth as infrared, visible, and ultraviolet light. Ultraviolet light is biologically harmful above certain doses, as demonstrated by sunburns. When discussing increases in harmful ultraviolet light due to ozone depletion, it is helpful to distinguish between three different types of ultraviolet light. These are: (1) ultraviolet A (UV-A) with wavelengths between 400 and 320 nanometers (nm)); (2) ultraviolet B (UV-B) with wavelengths from 320 to 290 nm; and (3) ultraviolet C (UV-C) with wavelengths between 290 and 190 nm.
UV-C is completely absorbed in the upper atmosphere by oxygen and ozone. Only UV-A and UV-B reach the surface of the earth. UV-A is not affected by changes in the levels of stratospheric ozone. UV-B is more biologically harmful and is very sensitive to changes in stratospheric ozone.
The level of UV-B striking the earth varies by time of day, season, cloudiness, and latitude. Stratospheric ozone also varies by season and is especially dependent upon latitude. With the exception of the recent phenomena of ozone holes over the poles, ozone is normally thinnest at the equator and thickest at higher latitudes. See Figure 1.
Figure 1. Total column ozone global average as a function of latitude. (From CIAP, 1975)
Because of these variations in UV-B and in the thickness of the ozone layer, it necessary to average all of these factors globally to derive an estimate of the change in the average exposure to UV due to ozone depletion.
Two other factors play a key role in predicting the biological impact of ozone depletion:
Because of these two factors, it would be misleading to estimate biological impacts simply on the basis of the overall change in the radiant energy of the UV-B radiation. Rather, it is necessary to predict the change in the quantity of radiation at each wavelength and to weight the wavelengths according to their physiological impact.
An example of the result of such weighting is graphically shown in Figure 2. Without the weighting, it would appear in our example that the impact of a 50% ozone depletion is small. For each different type of biological effect we wish to predict, a different weighting function must be determined.
Figure 2. Weighted irradiance as a function of wavelength (From NAS, 1979)
For these reasons, scientists use the term "percent change in biologically damaging ultraviolet radiation" or %DUV for short, rather than percent change in UV-B. %DUV is calculated using a weighting function as discussed above, which takes into account the relative physiological impact of different UV-B wavelengths. In epidemiological terms, %DUV is the "percent change in the physiologically effective dose of UV-B".
This relationship between decreased ozone and increased dose of physiologically effective UV-B is quantified by a "Physiological Amplification Factor" (PAF). The PAF is defined as the ratio of the percent change in biologically damaging ultraviolet radiation (%DUV) to the percent change in stratospheric ozone (%O3).
PAF = %DUV / %O3
A second factor must be taken into account to finally calculate the biological effect of a decrease in stratospheric ozone. Very often, the increase in biological damage is not proportional to the increase in the "dose" of a biologically damaging agent. For example, for skin cancer, our best evidence is that a doubling of DUV results in at least a four-fold increase in the incidence of skin cancer, rather than a simple doubling of the incidence of skin cancer. This relationship between biological damage and increased dose of DUV is quantified by a "Biological Amplification Factor" (BAF). The BAF is defined as the ratio of the percent change in biological effect (%Eff) such as skin cancer, to the percent change in DUV.
BAF = %Eff / %DUV
Finally, by multiplying the PAF by the BAF, we can calculate the predicted percent increase in biological damage (%Eff) for each percent decrease in ozone (%O3). This is simply called the "Radiation Amplification Factor" (RAF).
RAF = PAF • BAF = %Eff / %O3
We will use this equation to determine the biological impact of ozone depletion.
4. Effects on Humans
Some of the possible harmful effects of increased UV-B light on humans include:
4.1 Immune Inhibition
The effects of immune inhibition, skin deterioration and cataracts will not be quantified here for the following reasons: Immune inhibition has been demonstrated in laboratory animals but is not well quantified. Also, cancers other than skin cancer do not increase at lower latitudes (where there is greater UV-B).
4.2 Skin deterioration
Skin deterioration due to sunlight is well documented but primarily affects appearance and is not life-threatening.
Cataracts are a major cause of blindness in the world. In countries with good medical facilities, surgery can prevent most cataracts from causing blindness. Nevertheless, even in the U.S., cataracts are a leading cause of blindness. Every year, about 50,000 Americans become blind. Worldwide, there are approximately 17 million people who are blind due to cataracts, accounting for more than 50% of the blindness in the world (UNEP, 1994).
Exposure to UV-A (as opposed to UV-B) is believed to play a significant role in the formation of cataracts and may also affect the immune system, however the quantity of UV-A reaching the earth is not affected by depletion of ozone. Nevertheless, a recent study (Taylor & McCarty, 1996) suggests that UV-B also causes cataracts. However, there is presently inadequate information available on the wavelength dependence of this effect and proper dose-response relationships. Nevertheless, with some reasonable assumptions, estimates based on epidemiological data can be produced. In 1994, the United Nations Environment Programme (UNEP, 1994) sited estimates for the overall radiation amplification factor (RAF) for cataracts of 0.3-0.6 and 0.5. This means that a 1% increase in ozone depletion would be expected to result in approximately a 0.3 to 0.6% increase in the incidence of cataracts. At this point in time, these estimates of the RAF have a high degree of uncertainty.
Assuming an RAF of 0.5 for cataracts, a sustained 10% increase in ozone depletion would eventually bring about an increase of 850,000 blind persons (van der Leun and de Gruijl, 1993). Since cataract induced blindness mostly occurs in the later decades of life, the number of additional blind people would be roughly, 850,000 ÷ 25 or about 34,000 persons per year. Again, it should be kept in mind that these estimates are highly uncertain.
4.4 Skin Cancer
There are two basic types of skin cancer: melanoma and non-melanoma.
Melanoma is the most serious form of skin cancer and is also one of the fastest growing types of cancer in the U.S. If not caught in its early stages, melanoma is often fatal. Melanoma cases in the U.S. have almost doubled in the past two decades with 34,000 cases and 7,200 deaths in 1995 alone (Long et al., 1996). This corresponds to a lifetime cancer risk factor of roughly one per 100 persons.
Non-melanoma skin cancer is the most common form of all cancers, but has a low fatality rate. There were an estimated 800,000 cases and 2,100 deaths in 1995 (Long et al., 1996) in the U.S. The lifetime cancer risk factor for non-melanoma skin cancer in the U.S. is roughly one in five persons.
The evidence for UV being a causative factor in skin cancers are as follows:
In addition, there are additional suggestive factors:
The worldwide incidence of non-melanoma skin cancer can be estimated. As stated above, the current incidence of non-melanoma skin cancer in the U.S. is about 800,000 persons annually with a death rate of about 2,100 per year. This is for a current population of about 270 million. The world population of White people is about 1,095 million (N. America, Europe, and Russia). The Latin and Asian population, with a susceptibility about 1/10th that of Whites, is 3,950 million (S. America, Asia). Blacks are for the most part not susceptible. This gives a race-corrected estimated worldwide incidence for non-melanoma skin cancer of 5.4 million persons per year with a death rate of about 14,000. This number may be low since it assumes the U.S. quality of treatment.
Estimation of Biological Amplification Factor for Skin Cancer:
Estimation of Physiological Amplification Factor for Skin Cancer:
Calculation of overall Radiation Amplification Factor (RAF) for Non-Melanoma Skin Cancer:
Based on the above studies, we have conservatively assumed a BAF of 2.0 and a PAF of 2.0 for non-melanoma skin cancers. The resulting Radiation Amplification Factor for non-melanoma skin cancer due to ozone depletion is 4.0. Similar numbers have been quoted by the popular press (Caldicott, 1992; Corson; 1990, Gore, 1993).
It should also be noted that the estimates of the mean RAF for non-melanoma skin cancer have been decreasing steadily over the past two decades: 6 in '71, 4 in '80, 3 in '89, 2.3 in '91 and 2.0 in '93 (van der Leun et al, 1993).
We have chosen a health-protective (i.e., conservative) RAF of 4.0 to use in our estimation of health impacts in keeping the standard practice of EPA and other agencies concerned with protection of public health, to typically use a 90th percentile estimate, so that there is only a 10% chance that the risks may actually be higher than what are projected (as opposed to a 50% chance).
Estimation of overall Radiation Amplification Factor (RAF) for Melanoma Skin Cancer:
The relationship between increased UV-B exposure and melanoma skin cancer is less clear. For example, unlike non-melanoma skin cancers, the origin of melanoma skin cancers appears to be linked with intermittent, intense exposures (i.e., severe sunburns) and/or exposures in childhood (IARC, 1992). Also, the are very few animal studies, and the one study we do have for a tropical fish indicates a PAF much lower than 1.0.
As stated in the UNEP 1994 Assessment:
"It is conceivable that UV radiation may contribute in various ways to the induction of melanomas, and that the specific mechanisms differ in the two animal models. Although it is difficult to induce melanoma in mice by UV irradiation, it can be done quite efficiently with exposure to chemical carcinogens, and concomitant UV exposure can then promote the melanomagenesis.
How these experimental data should be extrapolated to humans is, of course, very much an open question. CM in humans may well have a multifactorial etiology. Although UV radiation is likely to play a dominant role, (e.g., initiating precursor lesions during youth and suppressing immunity to the tumor cells as a result of a sunburn in the final stage of tumor development), other factors may affect expression of the UV effect."
For these reasons, we will not estimate an RAF for melanoma skin cancer at this time.
5. Effect on Land Organisms
5.1 Cultivated Plants
Cultivated plants make up most of the world's food supply. About 1,500 million tons of wheat, corn, and rice make up the staples of our planetary diet. U.S. exports alone of these products are over $10 billion per year (Johnson, 1997). Therefore, even a small decrease in crop productivity from increased UV-B would be of great economic significance.
A number of studies have been performed. The most recent results suggest that 30-50% of all species are deleteriously affected by UV-B (Teramura and Sullivan, 1994). Field studies have shown that there is a great variability in the impact of UV-B between both species and varieties of the same species. The types of impacts also vary greatly. They may include yield but also may involve changes such as leaf size, photosynthesis rate, and resistance to diseases and insects.
Cucumbers are an example of the high sensitivity of some plants to UV-B. Cucumber growth is only 50-60% as much at the equator as it is at a latitude of 70° North.
Sugar beets, tomatoes, and mustard have also been sound to be sensitive to UV-B levels, while peanuts, peas, potatoes, and sorghum are not sensitive (NRC, 1979). With rice, corn, squash and soybeans, the response depends on the variety (Tevini, 1993).
Based on such studies, it is clear that the environmental response of plants to an increase in UV-B is likely to be complex. Small changes in leaf size may increase the ability of weeds to grow around some crops. Small changes in resistance to insects or disease, or in the length of the growing season, could cause large changes in yield. The most likely thing to happen will be a change in the relative population of the various species. Studies have shown that sometimes the crop wins and sometimes the weeds win (Runeckles and Krupa, 1994).
Because of all these complicating factors, we have not attempted to estimate potential crop loss from increased UV-B.
5.2 Wild Plants
By "wild plants", we mean essentially forest trees, which accounts for 80% of the plant biomass productivity on earth.
One major concern with trees is that any reduction in their productivity due to increased UV would affect their uptake of carbon dioxide from the atmosphere. This would increase CO2 levels significantly. Thus, ozone depletion could exacerbate the greenhouse effect. That in turn, could cause changes to cloud cover, precipitation patterns, temperatures, and so on, which would impact all life on this planet.
Of fifteen species of conifers, 7 were found to have been harmed by UV, 5 were unharmed, and 3 were improved (Teramura, 1990, 1994). Loblolly pine, which is grown in the Southeast U.S. for paper production, is one of the most sensitive to UV radiation. Field studies have shown that after three years, an increase in UV-B corresponding to a 25% decrease in ozone, caused a 17-19% loss of biomass in three out of four seed types. Similar results were seen for a 16% equivalent ozone reduction.
As in the case of the crop plants, the consequences of a small ozone change on the world's natural plants is likely to be complex. We have not attempted to estimate a cost at this time.
The direct effects of increased UV radiation on wild and domesticated animals is not thought to be of great economic significance (NRC, 1982). First, most animals are protected by fur or hair. Second, the few problems seen in cattle and sheep are eye and nose cancers that are infrequent or too late to have an economic impact. It should be noted however that indirect impacts, such as effects on their habitat, or potentially, changes to the biosphere bought about by an exacerbated greenhouse effect, could be more significant.
6. Effects on Marine Organisms - Phytoplankton and Zooplankton
"Plankton" are the collection of small or microscopic organisms, including tiny plants and animals, found in great numbers in fresh or salt water at or near the surface. They are too small to swim any great distance; thus, they drift with the currents and serve as food for fish and other larger organisms.
Phytoplankton, which are tiny aquatic plants, are particularly important because they are the very base of the aquatic food chain:
Phytoplankton -> Krill -> all higher aquatic species
Fish are highly dependent upon phytoplankton as a food source. Since about 30% of all protein for human consumption comes from the sea, the human food supply is also very dependent upon phytoplankton.
In addition, phytoplankton account for roughly half of the worldwide uptake of carbon dioxide (CO2) from the atmosphere. Therefore, any decrease in phytoplankton would have a major impact on global CO2 levels. For example, just a 10% loss would be equivalent to a doubling of all current fossil fuel burning worldwide.
Pure water is transparent to ultraviolet radiation. It is the presence of light scattering materials that cause attenuation. Typically, there is about a 40% attenuation of UV light at 0.5 meters below the surface rising to an 80% attenuation at 1.5 meters (NRC, 1983).
Studies have found that phytoplankton are highly sensitive to UV light and that even existing levels of UV radiation are highly toxic. In fact, the population of phytoplankton at the surface of the ocean is only one fifth of that 1.5 meters below the surface (Haeder, 1993). While phytoplankton appear to have an ability to descend to lower depths during periods of high light intensity, they respond only to visible light, not UV. Therefore, an increase in UV is not something to which they would readily adapt (NCR, 1976).
Declines in phytoplankton production in Antarctic due to the ozone hole have been documented (Smith et al., 1992). Other studies have shown that shrimp, crab larvae, and anchovies are also sensitive to UV-B (Worrest, 1982).
One estimate (Haeder, 1993) is that a 16% loss of ozone would lead to a 5% loss in plankton productivity, which would in turn lead to a 7% loss in fish production. That is about 6 million tons of fish worth roughly $20 billion.
7. Issue of Adaptation to Higher Levels of UV-B
Whether organisms could adapt to a higher level of UV-B radiation remains a controversial issue. Studies have shown that repair mechanisms exist in both plant, animal and human cells for UV-B insults.
However, studies on cells exposed to UV-B have also shown that the genetic elimination of even one repair mechanism increases cell mortality by one to two orders of magnitude and that elimination of two mechanisms increases cell mortality by at least three orders of magnitude (Geise, 1976). Thus, it appears that the existing cellular repair mechanisms are already taking care of 99% to 99.9% of the UV damage, and the harmful effects that we see are due to the occasional failures of the repair mechanisms. The conclusion is that adaptation to UV-B radiation may already be largely optimized and any further improvement would take an extremely long period of time.
Figure 3 shows the long term evolution of the atmosphere, including the ozone layer, and the appearance of the major life forms over time. It also shows that higher life forms did not appear until after the ozone layer was almost fully developed, about 700 million years ago. But simple life developed 3 billion or more years ago. In other words, life remained primitive for 2.3 billion years until the ozone layer developed. It may be that if higher life could not develop and survive without a highly developed ozone layer for 2.3 billion years, it just may not be possible to do so.
Figure 3. Ozone, oxygen and appearance of major
life forms as a function of geologic time
(From R.P. Wayne, 1985)
8. Risk Perspective
The increased health risks due to ozone depletion are not meaningful unless put into the context of other comparable health risks that we encounter on an everyday basis.
Table 1 shows a comparison of several other types of common radiation risks . Note that the incremental lifetime cancer risk due to medical x-rays or even the potassium in our own bodies is about 30 in 100,000. More importantly, non-melanoma skin cancer is rarely fatal while the types of cancers induced by radiation from medical x-rays or potassium are often fatal.
Table 1. Comparison of Several Common Radiation Risks (From Wilson and Crouch, 1987)
Cancers if all U.S. population exposed
Incremental lifetime cancer risk
|Potassium in own body||
|Cosmic radiation at sea level||
|Cosmic radiation at Denver||
|One transcontinental round trip by air||
Other common place risks to which we are exposed everyday are shown in Table 2. Motor vehicle accidents, home accidents, and air pollution, have individual lifetime death risks (as opposed to cancer risks) of approximately 2 in 100, 8 in 1,000, and 1 in 100, respectively.
Table 2. Comparison of Commonplace Risks
(From Wilson and Crouch, 1987)
Incremental lifetime death risk
|Motor vehicle accident (total)||
|Motor vehicle accident (pedestrian only)||
|Air Pollution (eastern US)||
|Cigarette smoking (1 pack/day)||
|Four tablespoons of peanut butter per day
|Alcohol, light drinker||
Yet another way of looking at this is to consider the impact of moving to a lower latitude. Since the amount of protective ozone in the stratosphere is less at lower latitudes, the effects of ozone depletion can be compared to shifting our exposure to the sun further south.
The potential effects of an increase in UV-B radiation on the biosphere due to ozone depletion are very serious. It is fortunate the world governments have united to restrict the production and use of chlorofluorocarbons. Many of the worlds food staples would be adversely impacted by an increase in UV-B light. Because of the adverse impact of UV-B light on the productivity of phytoplankton and zooplankton, marine fisheries would be severely impacted.
Exposure to UV-B radiation is the principal cause of non-melanoma skin cancers in humans. While infrequently fatal (0.26%), the incidence of non-melanoma skin cancer is so high -- about 800,000 new cases in U.S. each year and an estimated 5.4 million cases worldwide -- that there are over 14,000 associated deaths each year worldwide. Also, exposure to UV-B appears to also be a contributing factor in the formation of cataracts which is the leading cause of blindness in the world. For this reason, even modest increases in UV-B radiation due to ozone depletion would be expected to bring about increased cases of skin cancer and blindness in the human population.
In addition, a decrease in the productivity of forests and phytoplankton due to increased UV-B would dramatically reduce the uptake of carbon dioxide by plants. This would not only reduce oxygen production but would contribute to global warming, with attendant changes in cloud cover, precipitation patterns, temperatures, and so on, which would impact all life on this planet.
The complexity of interdependent effects makes it virtually impossible to predict the full consequences of ozone depletion on the biosphere. Fortunately, actions are already being taken by the world community to deal with this problem.
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