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Continuing medical education Thinning of the ozone layer: Facts and consequences Brett M. Coldiron, MD Cincinnati, Ohio The ozone layer is showing small but definite signs of depletion. Despite this, significantly increased UV radiation transmission at ground level has been found only in the Antarctic and Arctic regions. The potential for increased transmission of UV radiation will exist for the next several hundred years. Although little damage from increased UV radiation has occurred so far, the potential for long-term problems is great. The natural history of ozone and the causes and consequences of, and possible solutions to ozone depletion are examined in this article. (J AM ACAD DERMATOL 1992;27:653-62.)

Several questions come to mind in any discussion of the ozone layer. Is the ozone layer thinning? What is the cause of this thinning if it is present, and what are the consequences of a thinner ozone layer? How can ozone depletion by stopped? How can we minimize adverse effects from ozone depletion? Pinning down exact numbers and figures is difficult, partly because the study of ozone depletion is a new science, and any conclusions must draw on many fields, including biology, chemistry, photoNology, photochemistry, gas phase chemistry, meteorology, cutaneous oncology, engineering, epidemiology, economics, and politics, to name a few. Many variables cannot be directly measured and these variables are interacting in ways never seen before. Most predictions of ozone depletion and associated risks from ozone depletion are not based on "hard" data and should be judged accordingly. ORTHO

The C M E articles are made possible through an educational grant from the Dermatological Division, Ortho Pharmaceutical Corporation.

FromtheDepartment&Dermatology,University&Illinoisat Chicago, and the Departmentof Otolaryngology,Universityof Cincinnati MedicalCenter. Reprintrequests:BrettM. Coldiron,MD, 3024BurnerAve.,Cincinnati, OH 45219. 16/2/41371

BACKGROUND

The troposphere is that part of the atmosphere from the surface of the earth to 10 miles (15 kln) up. The stratosphere ranges from 10 to 30 miles up ( 15 to 50 km). Ozone is found mostly in the stratosphere.I Ozone and oxygen in the stratosphere absorb the vast majority of UV radiation (UVR) transmitted to the earth. This generates significant heat and results in the stratosphere being significantly warmer than the layer below it, the upper troposphere. 2 There is little vertical movement of air between the troposphere and stratosphere because of this heat stratification. Stratospheric ozone depletion from chlorofluorocarbons (CFCs) was first hypothesized by Molina and Rowland 3 in 1974. CFCs are short carbon chains saturated with predominantly chlorine, but also fluorine. Halons are short carbon chains saturated with bromine. It is difficult for molecules such as CFCs or halons (jointly termed halocarbons) to gain access to the stratosphere; this usually requires a major weather disturbance such as a hurricane or exceptionally powerful warm air updrafts. This protects the ozone from human activities in the troposphere to a degree but is also its undoing. When a molecule containing reactive chemical species reaches the stratosphere, it is photodisassociated into 653

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Table I. Important stratospheric reactions Removal of odd oxygen

NO + 03 = NO2 + 02* NO2 + O = NO + 02* C1 + 03 = C10 + O = Br + O3 = BrO + O =

CIO + O21C1 + 02]" BrO + O2:~ Br + O2:~

Temporary removal radicals

NO2 + OH = C1 + CH4 = C10 + NO2 = BrO + NO2 =

HNO3 (nitric acid) HC1 + CH3 C1NO3 (chlorine nitrate) BrNO2 (bromine nitrate)

The reactions at left catalyze the destruction of ozone. Reactions to the right are unstable temporary radicals that disassociate spontaneously or under the influence of UV radiation. *Most important. "~Important at higher altitudes. .]:Less stable than CINO3 and HCt but less common.

multiple reactive molecules that can catalyze the destruction of ozone. Once a reactive molecule (chlorine, bromine, nitrogen oxide) is in the stratosphere, there are few molecules that will react with it to produce a stable end product. When these reactive molecules do combine with nitrogen dioxide or methane to form less reactive molecules (temporary removal radicals), they are usually reactivated by U V C into reactive species (Table I). Intact halocarbons are water insoluble, and it is difficult for these compounds to be washed out of the troposphere or stratosphere in water vapor. 1 In fact, the only natural "sink" for halocarbons is in the stratosphere, in which U V C breaks them down by photolysis. This partially accounts for the exceptionally long haft-lives of halocarbons and other reactive species in the stratosphere. For our purposes, U V C is defined as having a wavelength of 200 to 280 nm, UVB 280 to 320 nm, and U V A 320 to 400 nm. This division is based on the biologic activity of the various wavelengths, with U V R above 320 nm having much less activity (Fig. i). U V R makes up 5% of the total energy that reaches the surface of the earth. 4 However, only about 10% of the U V R that reaches the surface is UVB and has great carcinogenic potential. 4 All U V C and 90% of UVB radiation is blocked by the stratosphere. U V A is little affected by ozone. U V R output by the sun varies with sunspot activity over an 11-year cycle. W e are now 4 years past minimal U V R output in this I 1-year cycle.5 There is also evidence that U V R levels decrease disproportionately to total solar energy output during the solar cycle.5 Stratospheric ozone levels vary directly with U V R output by the sun. 2 As is discussed later,

this may partially account for some decline in the ozone layer and lack of increased levels of UVB at monitoring stations. NATURAL HISTORY OF OZONE Ozone is naturally created by the absorption of high-energy solar UVR by doublet oxygen molecules. This energy splits oxygen molecules. The majority of these single oxygen atoms recombine with doublet oxygen forming the triplet oxygen, ozone molecule. Ozone is unstable and reacts quickly with anything nearby, including other singlet oxygens, to revert back to doublet oxygen. It may also react with free hydrogen to form hydroxyl groups or water. A small steady-state amount of ozone is maintained in the stratosphere. If all the ozone in the stratosphere was brought down to sea level it would only be about 3 mm thick (the ozone column). The ozone layer is thin, widely scattered, and vulnerable. The ozone layer has a natural cycle that increases with increased UVR from the sun and changes with the season. 6, 7 IS THE OZONE LAYER THINNING? This can be answered, yes, with little hesitation. Accurate ozone measurement instrumentation is currently in use, particularly aboard the Nimbus 7 satellite. 8 A definite decrease in the total ozone column, of 3% worldwide, has occurred since 1978.8,, More recently a 3% to 4% decrease in the ozone column has persisted over North America until late May. This decline, for the first time, has persisted *Executive summary. Scientific assessment of stratospheric ozone, 1991. United Nations Environmental programme. United Nations. New York, Oct 22, 1991.

Volume 27 Number 5, Part 1 November 1992

Ozone depletion 655

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Fig. 1. Relative biologic sensitivity of DNA and amount of energy received from sunlight measured after passage through atmosphere. Filtered sunlight and absorption spectrum of DNA cross at 296 nm. Above 330 nm there is little absorption of UVR by DNA, and there is little UVR below 290 nm found in sunlight. Erythema curve demonstrates relative effectiveness of different wavelengths at inducing erythema. Note that lower range UYB is much more readily absorbed by DNA and more effective at inducing erythema. Ozone depletion will theoretically lead to greater penetration of lower range UVB. This is the basis for the disproportionate biologic effect proposed with ozone depletion. Adapted from Setlow (1974).

from winter into spring and summer, in both hemispheres, at middle and high latitudes.* There has been no ozone decline in the tropics. More recently, estimates from the Environmental Protection Agency (EPA) place peak ozone depletion at 8% per decade. 9 Ground and satellite-based observations show a persistent decline in the ozone column. Most impressive are the decreased ozone levels that were first noted over the Antarctic in 1983 l~ (Fig. 2). These declines were not predicted by any of the many computer models of ozone depletion.I, 2 Since 1983 levels have dropped more each year until at least 50% of the ozone is gone in the Antarctic spring. 11A decrease in the ozone column of 6.3% has been noted at latitudes greater than 40 degrees north or south. 12 Although the cause and effect of ozone

depletion may be debated, the fact of depletion, although mild so far, is not. Some of this decline can probably be attributed to the normal "ozone cycle" as already discussed, but a portion is definitely due to human activity.12 These declines in ozone column are deceptive because these numbers are yearly average ozone levels. The greatest decrease in ozone is in the winter when less UVR is transmitted. 4 It must be emphasized that a decrease in ozone does not necessarily translate into an increase in UVR reaching the earth's surface. CONSEQUENCES OF A THINNER OZONE LAYER

Ozone effectively screens almost all of the most dangerous short-wave UVB, from 290 to 300 nm.

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Fig. 2. Ground measurements of ozone column have shown dramatic decline in past 20 years. More recent and more accurate satellite measurements confirm this decline.---Springtime ozone measurements over Antarctica (1957-1986). Ground observations shown as horizontal lines (mean values) through vertical bars (1 standard deviation). NASA sateUite data shown as crosses. (Data from Farman et al. 1~and Stolarski et al. 8)

Under a scenario of ozone depletion, most of the increased transmission of U V R will be short-wave UVB. This results in a disproportionate biologic effect because more biologically active UVB transmission is increased. UVB of 290 n m is 1000 to 10,000 times more effective at producing cellular damage than UVR over 330 nm. 13, 14 It has been estimated that each 1% decrease in the total ozone column will increase the biologically effective UVB 1.3% to 1.5% based on the McKinlay and Diffey erythema action spectrum: If no controls are instituted on CFC production and release into the atmosphere, an overall 40% decrease by 2075 has been predicted, l, 12Even if serious ozone depletion occurs, transmission of UVC, which would be disastrous, is unlikely. UVC is screened effectively by minimal amounts of ozone and under 240 nm by doublet oxygen.7 A thinner ozone layer makes little difference to U V A transmission because it is only slightly affected by ozone. Of great concern is possibly serious worldwide biologic effects, including the killing of ocean plankton and decreased crop production. Plant life largely evolved after the establishment of an ozone layer. UVB causes damage to plankton, which includes juvenile fish, plants, and other microscopic life essential to the marine food chain. Adverse effects

include decreased growth, reproduction, and survival. Experimental evidence indicates that a small increase in UVB could result in significant ecosystem changes. 15 UVB will penetrate several meters into the ocean.:6 Effects of UVB radiation have been measured to 20 meters in clear water and 5 meters in unclear water. :6 However, it now appears that microscopic life forms adapt to increased UVB radiation by increasing pigment production. Ecologic damage may be less than expected because of this. Two thirds of more than 200 terrestrial plant species and cultivars (mostly crop plants), screened for UVB sensitivity at relatively high projected ozone depletion levels (25%), were found to be sensitive. :6 The most sensitive plant groups included peas, beans, melons, cabbage, mustard, and related species. Generally UVB causes reduced leaf and stem growth, lower total dry weight, and lower photosynthetic activity. Plants of the grass family were less sensitive, especially common weeds. Certain cultivats of tomato, potato, sugar beet, and soybean were noticed to have reduced quality under increased UVB. In addition, limited data on coniferous species suggest some adverse effects on growth and photosynthesis. If more short-wave UVB is transmitted, there may be an increased number of skin cancers and cataracts during the next 10 to 30 years. Accurate predictions of an increase in skin cancer are hampered by lack of an animal model for UVB-induced basal cell carcinoma and melanoma. 17 However, it is well known that short-wave UVB in the 295 to 300 nm range is important in producing animal squamous cell carcinoma. ~8 The EPA projects that there could be as many as 550,000 to 2,800,000 additional cataracts worldwide by the year 2075.1 It is estimated that a 1% decrease in ozone will be accompanied by a 0.6% to 0.8% increase in cataracts. UVB can also cause retinal damage, although this effect is less well quantiffed. [9 EPA estimates from 1988, over the lifetimes of the existing population, show that a 1% decrease in ozone will cause a 4% to 6% increase in the incidence of both basal cell carcinoma and squamous cell carcinoma in U.S. citizens born before 2070.: All these predictions are based on a dose response curve extrapolated from a 1981 survey of reported sun exposure and nonmelanoma skin cancer incidence compared with a similar survey from 1970 to 1971. This survey showed a 15% to 20% increase in inci-

Volume 27 Number 5, Part 1 November 1992

Ozone depletion 657

Table II. UVB levels from 1974 to 1985 with Robertson-Berger meters at 297 nm Monthly average UVB level

E1 Paso, Tex.* Florida* Albuquerque, N.M.* Oakland, Calif.* Minnesota* Fort Worth, Tex.* Philadelphia, Pa.* Bismarck, N.D.* Mauna Loa, Hawaii]"

11% decrease Slight decrease Slight decrease Slight decrease Slight decrease No change No change No change No change

*From Scotto JG, Cotton F, Urbach F, et al. Science 1988;239:762-4. tFrom Urbach F. Photochem Photobiol 1989;50:507-13.

dence rate of nonmelanoma skin cancer in 1981 compared with the 1971 survey, which correlated with increased sun exposure. Obviously, extrapolations from such surveys cannot be considered exact. Others calculate that skin cancer incidence will increase 2% worldwide for each 1% decrease in ozone. 2~Kripke et al. 21 estimate that a 0.3% to 2.0% increase in melanoma may be seen for each 1% loss of ozone. With no controls on halocarbons, there may be 163 million to 310 million additional nonmelanoma skin cancers, and 840,000 to 1.4 million additional melanomas by 2075, in the United States. If halocarbon production is held at an annual growth rate of 2.5% (currently 5%), there may be an additional 1 million nonmelanoma skin cancers and 20,000 additional deaths from nonmelanoma skin cancer. An additional 31,000 to 126,000 melanomas are estimated with 7000 to 30,000 melanoma fatalities. These estimates are projected over the lifetimes of the existing population of U.S. citizens born before 2075. Worldwide, a 5% ozone depletion, which we should easily reach, may result in a 5% to 8% increase in malignant melanoma, a 10% increase in basal cell carcinoma, and a 20% increase in squamous cell carcinoma. HAVE UVB LEVELS INCREASED?

Paradoxically, ground measuring stations in the United States reported an actual decrease in UVR between 1974 and 198522 (Table II). These same stations have not reported an increase from 1985 to the present (personal communication, David Finkelstein, PhD, August 1991). These ground metering stations use RobertsonBerger (R-B) meters weighted for 297 nm, the most effectiveUVB wavelength for producing skin erythe-

ma. 22 R-B meters integrate weighted amounts of UVB and provide counts in "sunburn units. ''2~ However, R-B meters are sensitive to cloud cover and aerosols and are unable to be directly calibrated. They are also slightly inaccurate (+3%). R-B meters also probably understate the true amount of shorter wave UVB because their filters do not preferentially screen out higher wavelength energy, which is not as erythrogenic.22 It has been argued that the lack of change in the R-B meters may be due to atmospheric pollution because most R-B meters are stationed at airports near cities. However, an R-B meter on Mauna Loa, Hawaii, 3400 meters in elevation, and in a sparsely populated area, shows the same lack of effect.4 Had a significant increase in UVB occurred, it should have been noted. Measurements with the Jobin-Yvon double monochromator, which may be more accurate, show little increase in UVB at 70 to 80 degrees of latitude. 24 The Jobin-Yvon double monochromator can be directly calibrated and does not have the bias toward longer wavelength UVB as do the R-B meters. Blumthaler and Ambach 25, using an R-B meter, have measured a 1% increase in UVB in the Swiss Alps at 47 degrees north. They corrected their measurements for total solar flux and measured only on cloudless days.

However, for the first time, large increases (100% over normal) in ground-level UVB were measured in Antarctica in 1990.* This increase was measured by monochrometers and R-B meters and correlated well with the measured decrease in ozone column. This suggests that existing instrumentation for mea*Executive summary. Scientific assessment ol" stratospheric ozone, 1991. United Nations Environmental programme. United Nations. New York, Oct 22, 1991.

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suring U V B is accurate, at least at the wavelengths measured. It is a large step to move from ozone depletion to the prediction of increases in skin cancer and cataracts. Our current monitoring system for U V R may be inadequate and the proper instruments for direct measurement of U V R across the spectrum have not yet been built. We do have the technology to do so and this should probably be done. WHAT IS CAUSING OZONE DEPLETION?

The largest ozone-depleting reactions involve nitrous oxide and nitrogen dioxide. These chemically catalyze the conversion of ozone to doublet oxygen in a number of chemical reactions. 6 Nitrogen gases are derived from supersonic aircraft, microorganisms, fertilizers, and h u m a n air pollution, t7 Microbially produced nitrous oxide is probably the most important source. 6 Nitrous oxide can undergo repeated photolysis to generate radicals capable of thinning the ozone. Most of the bromine in the stratosphere is believed to come from methyl bromine, the origin of which is uncertain but possibly from seawater.* Certainly, these are not new sources and we must look further for the cause of the current ozone decline. Ozone depletion is at least partly due to halocarbons such as Freon (CFCs) and bromide compounds. C F C s have m a n y uses, including air conditioning, foam extrusion, and industrial solvents. Halons, such as certain bromide compounds, are uniquely effective in lightweight fire extinguishers. 6 Halocarbons are disassociated by U V C in the 200 to 227 nm range. 18 As already explained, U V C does not, and is unlikely to, reach down to the troposphere, even under conditions of severe ozone depletion. The U V R that penetrates the stratosphere is UVB. Unfortunately, the wavelength of UVB is not short enough to disassociate halocarbons in the troposphere in which the reactive intermediates would be quickly inactivated. 3, 7 Photodisassociated chlorine quickly joins with a free oxygen to form chlorine monoxide, which is the form in which it is usually found in the stratosphere. The chlorine monoxide acts as a catalyst to destroy ozone, creating an ordinary oxygen molecule and *Executive summary. Scientific assessment of stratospheric ozone, 1991. United Nations Environmentalprogramme. United Nations. New York, Oct 22, 1991.

Journal of the American Academy of Dermatology

free chlorine, which repeats the cycle. The chlorine is not consumed in the reaction. Bromine radicals act in a similar fashion. Bromine is believed to be 10 to 100 times more destructive than chlorine. Bromine is a more effective catalyst and less is kept in nonparticipating "reservoir species" because of chemical instability. 6 Other sources of halocarbons include carbon tetrachloride, which is used as dry cleaning fluid, and methylchloroform, which is used to clean computer boards and jet engine parts. All these halocarbons are photedisassociated in the stratosphere. The radicals produced then react with ozone and break it down to oxygen much faster t h a n normal. This results in a lower steady-state ozone level. WHY DON'T WE STOP USING THESE CHEMICALS? Few compounds have the unique physical attributes of CFCs. These attributes include stability (except to U'VC), low thermal conductivity, and low vaporization temperatures for heat transfer applications. CFCs are nontoxic and relatively inexpensive. There are chemical substitutes for some of the functions of CFCs, an others are under development, but these substitutes are generally more expensive, more toxic, less efficient, and less readily available. Producers and users of CFCs are both reluctant to give them up. CFC production is an annual $700 million industry for Dupont. 26 Users of CFCs are naturally reluctant to give up the use of these chemically unique and inexpensive compounds. S o m e countries are reluctant to agree to restrictions until they have developed their economies to the point at which they can afford CFC substitutes. M a n y countries have just entered production of CFCs and will find it prohibitively expensive to build new facilities for production of CFC substitutes. If C F C s are banned, these countries will be forced to buy substitutes from only one or two manufacturers, at great expense. Irrationally, the largest source of atmospheric CFCs (approximately 50%) continues to come f r o m use as propellant in aerosol cans. An estimated 224,000 tons was released into the atmosphere in this manner last year. 26 There are non-ozone-depleting CFC substitutes, such as compressed air, or butane/propane mixtures can be used as aerosol can propellant. However, these substitutes are not suitable for most industrial applications because of their

Volume 27 Number 5, Part 1 November 1992

flanmaability. Another use for which CFCs are unlikely to be replaced is in inhalers for patients with respiratory disease. Butane and propane are not ideal replacements because they contribute to smog formation. Many people, including Prince Charles of England, have publicly given up the use of all aerosol cans. No CFCs have been used in aerosol cans in the United States, Canada, or Scandinavia for more than a decade. The use of CFCs as aerosol propellants was eliminated in Great Britain in 1989. 27 Worldwide use of CFCs parallels economic development. The United States is the largest user followed by Western Europe, Japan, and the Pacific rim. Developing countries use little in comparison. C F C SUBSTITUTES

The current planned substitutes, primarily for heat transfer technology, are hydrochlorofluorocarbons or hydrofluorocarbons (HFCs). These have similar physical properties to CFCs but a hydrogen atom substitutes for one of the chlorine or fluorine atoms, which allows for quicker breakdown in the troposphere. T h e most important of these is H F C 134a, touted as a substitute for C F C 12.27 However, H F C 134a is by no means a "drop in" substitute for CFC 12. T h e lubricant used with CFC 12 is not compatible with H P C 134a. A lubricant that is compatible with both has not been found and is unlikely to exist. The switch from CFC 12 to H P C 134a will require a switch of equipment. In general, all CFC substitutes are inferior to CFCs. They will not function as efficiently, will be more dangerous (flammable or toxic), and will be expensive to purchase and to exchange. T h e haft-life of the CFC substitutes is 7 to 12 years, and most of these should break down before they reach the stratosphere. However, there is some evidence that these gases will cause some ozone depletion, 26 and they are potent greenhouse gases (as are CFCs). As already mentioned, they are more toxic than CFCs and their breakdown in the troposphere will release reactive radicals that can be destructive. Only 25% replacement of current production of CFCs with CFC substitutes is planned. Other possible substitutes, which are rarely mentioned, include ammonia, helium, nitrogen, and carbon dioxide. These chemicals were used for heat transfer and other functions in the past, although they are gener-

Ozone depletion 659 ally more difficult to work with. CFCs were originally developed to replace these compounds. W H Y ARE T H E R E O Z O N E H O L E S OVER THE POLES?

T h e ozone holes over the poles are largely due to unusual conditions, particularly the extremely large, cold land mass that supercools the air. This air slowly circulates without outside mixing. For uncertain reasons, chlorine monoxide levels are 100 times higher in this air mass than they are in the more temperate stratosphere. The air is so cold that it allows the formation of water and nitric acid crystals, which can act as a catalytic surface for chlorine and ozone to combine. In addition, the "freezing out" of the nitric acid results in the loss of a large reservoir species for chlorine, effectively elevating the chlorine concentration. 6 This also occurs over the north pole, to a lesser extent. These ozone holes were generally not perceived to be a threat because they were only present in the polar winter and early spring, when there is the least amount of light. Recently, satellite measurements show an ozone column of only 105 Dobson units (normal 300 to 400) during the Antarctic summer. 28 It is believed that the ozone-poor air reduces ozone levels at lower latitudes when the "spring breakup" occurs. 29 The Antarctic ozone hole has migrated intact over New Zealand and Australia, which could dramatically increase UVB levels. When this occurred in 1987, however, no increased UVB readings were found by R-B meters. This finding was believed to be due to heavy cloud cover during the danger period, a~ AGGRAVATING FACTORS Multiple aggravating factors make solutions to the ozone depletion problem particularly difficult. Stratospheric CFCs have half-lives of 75 to 120 years. These calculations are estimated from data obtained from the troposphere and indeed their haft-lives may be even longer in the stratosphere. It appears that the only "chemical sink" on earth for halocarbons may be the stratosphere. Only occasionally do cloud structures involve the stratosphere, which would allow some of these radicals to be taken out in water vapor. Equally rare is the combination of radicals with other molecules to create a species stable enough to resist repeated photolysis. 3 In addition, there are tremendous quantities of halocarbons latent in the environment. These in-

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clude functioning and abandoned coolant equipment, as well as fire extinguishers that have not yet been used. Chlorine (or chlorine monoxide) is an effective catalyst. Each chlorine radical in the stratosphere will destroy approximately 100,000 ozone molecules in its stratospheric life span. Each C F C molecule breaks into two to four chlorine radicals. Worse, the bromine radicals, from the halons used in fire extinguishers, are believed to be catalysts that are 10 to 1IN) times more effective than chlorine. However, as pointed out earlier, the majority of bromine in the stratosphere is believed to be of natural origin with at most, only 20% being man-made in origin.* In addition, there is new evidence that chlorine and bromine radicals may act synergistically. 31 The half time for recovery of stratospheric ozone, in the absence of continued destruction, is at least 3 to 4 years. 16,32 Other threats to the ozone layer include proposed modernization of developing countries, including China's goal to have a refrigerator in every household by the year 2000. 26 In addition, a new fleet of stratospheric airplanes is being designed that would make world commuting much faster, but will also add to the damage of the ozone layer because of nitrous oxide in their jet exhaust. In addition, there is an 11-year sunspot cycle that restarts in 1991, during which the sun generates significantly more UVR. We have been receiving significantly less U V R from the sun during the past 6 years. 5 This could significantly aggravate problems caused by an already depleted ozone layer. The normal increase of the thickness of the ozone layer, in response to increases in solar UVR, could be interfered with. The recent eruption of Mount Pinatubo, which blew millions of tons of gas and ash 12 to 18 miles into the atmosphere, may accelerate ozone depletion. The sulfate aerosols are theorized to act as a platform, enhancing halocarbon catalysis of ozone by a factor of two or three at middle latitudes. Alternatively, the sulfate aerosols could deflect U V R and interfere with transmission to the ground. The sum effect of the Mount Pinatubo eruption is unknown at this time. CFClevels are increasing by 5 % annually, whereas *Executive summary. Scientific assessment of stratospheric ozone, 1991. United Nations Environmentalprogramme. United Nations. New York, Oct 22, 1991.

halon levels are increasing at 12% annually. It takes 7 to 15 years for CFCs to reach the stratosphere. There will be a long lag time of 10 to 15 years before any of our efforts at limiting halocarbons will have any effect. We may well be examining ozone depletion for the next 150 to 200 years. MODERATING FACTORS

Fortunately, there are some moderating factors relevant to U V R penetration and ozone depletion. The increased U V R penetration increases ozone formation lower in the atmosphere. In fact, ozone levels in the upper troposphere have increased 10% during the last 2 decades.* Many air pollutants such as ozone, carbon dioxide, and methane help block UVR. In addition, increased aerosols in the air, which mankind creates abundantly, also block UVR. These aerosols also lengthen the path U V R must take through the troposphere to reach the ground. It has been calculated that total UVB levels may actually decline despite stratospheric ozone depletion. 33 No one is sure how marked the increase of U V R penetration will be in the temperate latitudes, where most people live, although a great increase at the poles seems inevitable. Other moderating factors include cloud cover, which decreases UVB penetration 20% to 60%. Cloud cover may increase with greenhouse warming. Latitudes greater than 40 degrees north or south will receive the greatest increase in UVB; however, this increase is misleading. High latitude areas receive very little UVB at present, so the percentage increase means less. Currently UVB levels at 60 to 70 degrees north are one eighth to one ninth levels at the equator. 24 In addition, ozone depletion varies with the season, with the greatest decline in the winter, when less UVB reaches the earth. It must be remembered that all risks are relative. It has been calculated that a northern European indoor worker, who takes a 2-week vacation in a sunny location, doubles his annual biologically effective dose of UVB. 34 To produce this doubling at home, the ozone layer would have to be depleted by 50%. SOLUTIONS TO OZONE DEPLETION What are the solutions to ozone depletion? T h e Montreal Protocol, which was signed by 42 nations in August 1988, and has been amended several times since, proposes to stop using ozone-destroying t h e m -

Volume 27 Number 5, Part 1 November 1992 icals and use less harmful substitutes. 35 As of June 1990 the amended protocol calls for a total phaseout of all ozone-damaging halocarbons (including carbon tetrachloride and methylchloroform) by the year 2000. The Montreal Protocol includes subsidies for developing nations to catch up technologically as well as for the sharing of research results and training. Since then, the United States has moved to stop CFC production by 1995. Certainly, the use of CFC as propellants for aerosol cans should be banned. Effective substitutes have been used for many years in the United States, Canada, and Scandinavia. Scavengers of the reactive compounds would be ideal; however, there are no promising leads at present. Compounds capable of inactivating halocarbons are also capable of inactivating ozone. Airlifting great quantities of propane and butane into the stratosphere has been proposed and has merit. These compounds should combine with halocarbon radicals to make less reactive intermediate compounds. However, the amount of material needed to be lifted is tremendous and heavy cargo planes have trouble reaching even the lower stratosphere. Another proposal calls for ground-based UV lasers to blast the halocarbons apart while they are still in the troposphere and can be easily washed out. The practicality of this has not yet been established. UV-resistant crops can be developed. Most plant species have varieties that are more UV resistant, although they may not be as productive. There should be an intense public education effort to warn of the risks of U V R and the folly of sunbathing. Although UVB levels, paradoxically, have not yet increased at temperate latitudes, if ozone depletion continues as projected, increased ground transmission seems inevitable. Physicians, particularly dermatologists, are already leading this effort. W e need to develop new, accurate instruments for the monitoring of UVB and UVC above 240 nm. Currently, there is no monitoring of UVC, and only the R-B meters, weighted for 297 nm, routinely measure UVB. In addition, because of the possible inaccuracies with the R-B meters, we could be receiving an increase in UVB, particularly lower wavelength UVB, and not be aware of it. All concerned parties need to support funding for the EPA or the National Oceanic and Atmospheric Administration to develop new metering instru-

Ozone depletion 661 ments. These instruments need to be installed across the world. Currently there is no established network for UVB measurement by wavelength, and this is needed. 36 Recapture of existing CFCs and halons should and will become mandatory. As of April 1992 home and automobile air conditioning servicers are required to use recycling equipment (called "vampires") when replacing or draining the CFCs from a unit. Customers should insist that this be used when having their cars serviced. Cryodynamics of New Jersey makes a heliumcooled refrigerator and Albers Air Conditioning of Arizona makes an air conditioning unit based on lithium bromide crystals. Natural gas refrigerators use no halocarbons. Recently, a new method of cooling based on sound waves has been developed, although it is not commercially available. On a more individual basis, lifestyles can be changed and excessive sun exposure can be avoided. Sunglasses that block U V R should be worn outdoors as well as a sunscreen. Physicians play a key role in educating the public about the hazards of increased UVB.

REFERENCES

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