Frequently Asked Questions About The Ozone Hole
- What is the ozone hole?
- Is the ozone hole related to global warming?
- How big was the 2003 ozone hole, and is it getting bigger?
- Has there been ozone loss in places besides Antarctica?
- Has ozone destruction increased levels of UV-B light at the surface?
- What are the human health effects of increased UV-B light?
- Has ozone loss contributed to an observed increase in sunburns and skin cancer in humans?
- What is the effect of increased UV-B light on plants?
- What is the effect of increased UV-B light on marine life?
- Is the worldwide decline in amphibians due to ozone depletion?
- Are sheep going blind in Chile?
- What do the skeptics say about the ozone hole?
- What are the costs and savings of the CFC phaseout?
- What steps have been taken to save the ozone layer? Are they working?
- What replacement chemicals for CFCs have been found? Are they safe?
- When was the ozone hole discovered?
- How do CFCs destroy ozone?
- Are volcanos a major source of chlorine to the stratosphere?
- What is ozone pollution?
- Where can I go to learn more about the ozone hole?
- Related Blogs
What is the ozone hole?
The "ozone hole" is a loss of stratospheric ozone in springtime over Antarctica, peaking in September. The ozone hole area is defined as the size of the region with total ozone below 220 Dobson units (DU). Dobson Units are a unit of measurement that refer to the thickness of the ozone layer in a vertical column from the surface to the top of the atmosphere, a quantity called the "total column ozone amount." Prior to 1979, total column ozone values over Antarctica never fell below 220 DU. The hole has been proven to be a result of human activities--the release of huge quantities of chlorofluorocarbons (CFCs) and other ozone depleting substances into the atmosphere.
Is the ozone hole related to global warming?
Global warming and the ozone hole are not directly linked, and the relationship between the two is complex. Global warming is primarily due to CO2, and ozone depletion is due to CFCs. Even though there is some greenhouse gas effect on stratospheric ozone, the main cause of the ozone hole is the harmful compounds (CFCs) that are released into the atmosphere.
The enhanced greenhouse effect that we're seeing due to a man-made increase in greenhouse gases is acting to warm the troposphere and cool the stratosphere. Colder than normal temperatures in this layer act to deplete ozone. So the cooling in the stratosphere due to global warming will enhance the ozone holes in Arctic and Antarctic. At the same time, as ozone decreases in the stratosphere, the temperature in the layer cools down even more, which will lead to more ozone depletion. This is what's called a "positive feedback."
How big was the 2010 ozone hole, and is it getting bigger?
Every four years, a team of many of the top scientists researching ozone depletion put together a comprehensive summary of the scientific knowledge on the subject, under the auspices of the World Meteorological Organization (WMO). According to their most recent assessment, (WMO, 2006), monthly total column ozone amounts in September and October have continued to be 40 to 50% below pre-ozone-hole values, with up to 70% decreases for periods of a week or so. During the last decade, the average ozone hole area in the spring has increased in size, but not as rapidly as during the 1980s. It is not yet possible to say whether the area of the ozone hole has maximized. However, chlorine in the stratosphere peaked in 2000 and had declined by 3.8% from these peak levels by 2008, so the ozone hole may have seen its maximum size. Annual variations in temperature will probably be the dominant factor in determining differences in size of the ozone hole in the near future, due to the importance of cold-weather Polar Stratospheric Clouds (PSCs) that act as reactive surfaces to accelerate ozone destruction.
The 2010 hole was the tenth smallest since 1979, according to NASA. On September 25, 2010, the hole reached its maximum size of 22 million square kilometers. The 2010 hole was slightly smaller than North America, which is 25 million square kilometers. Record ozone holes were recorded in both 2000 and 2006, when the size of the hole reached 29 million square kilometers. The graph below, taken from NOAA's Climate Prediction Center, compares the 2010 ozone hole size with previous years. The smaller size of the 2010 hole compared to most years in the 2000s is due to the fact that the jet stream was more unstable than usual over Antarctica this September, which allowed very cold air in the so-called "polar vortex" over Antarctica to mix northwards, expelling ozone-deficient air and mixing in ozone-rich air. This also warmed the air over Antarctica, resulting in the formation of fewer Polar Stratospheric Clouds (PSCs), resulting in fewer locations for the chemical reactions needed to destroy to occur.
Has there been ozone loss in places besides Antarctica?
Yes, ozone loss has been reported in the mid and high latitudes in both hemispheres during all seasons (WMO, 2006). Relative to the pre-ozone-hole abundances of 1980, the 2002-2005 losses in total column ozone were:
- About 3% in the Northern Hemisphere south of 60°N
- About 6% in the Southern Hemisphere north of 60°S
Other studies have shown the following ozone losses:
- About 12% at Punta Arenas, Chile, the southernmost city in the world (Abarca and Casiccia, 2002)
- About 8% in summer in southern Australia (Manin et. al., 2001)
- About 10-15% in summer in New Zealand (McKenzie et. al., 1999)
In 2011, the Arctic saw record ozone loss according to the World Meteorological Organization (WMO). Weather balloons launched in the Arctic measured the ozone loss at 40%—the previous record loss was 30%. Although there has been international agreement to reduce the consumption of ozone-destroying chemicals, effects from peak usage will continue because these compounds stay in the atmosphere long after they're released. The WMO estimates it will take several decades before we see these harmful compounds reach pre-1980s levels.
Ozone loss in the Arctic is highly dependent on the meteorology, due to the importance of cold-weather Polar Stratospheric Clouds (PSCs) that act as reactive surfaces to accelerate ozone destruction. Some Arctic winters see no ozone loss, and some see extreme loss like that in 2011.
A future Arctic ozone hole similar to that of the Antarctic appears unlikely, due to differences in the meteorology of the polar regions of the northern and southern hemispheres (WMO, 2002). However, a recent model study (Rex et. al., 2004), indicates that future Arctic ozone depletion could be much worse than expected, and that each degree Centigrade cooling of the Arctic may result in a 4% decrease in ozone. This heightened ozone loss is expected due to an increase in PSCs. The Arctic stratosphere has cooled 3°C in the past 20 years due the combined effects of ozone loss, greenhouse gas accumulation, and natural variability, and may cool further in the coming decades due to the greenhouse effect (WMO, 2002). An additional major loss of Arctic (and global) ozone could occur as the result of a major volcanic eruption (Tabazadeh, 2002).
Has ozone destruction increased levels of UV-B light at the surface?
Yes, ozone destruction has increased surface levels of UV-B light (the type of UV light that causes skin damage). For each 1% drop in ozone levels, about 1% more UV-B reaches the Earth's surface (WMO, 2002). Increases in UV-B of 6-14% have been measured at many mid and high-latitude sites over the past 20 years (WMO, 2002, McKenzie, 1999). At some sites about half of this increase can be attributed to ozone loss. Changes in cloudiness, surface air pollution, and albedo also strongly influence surface UV-B levels. Increases in UV-B radiation have not been seen in many U.S. cities in the past few decades due to the presence of air pollution aerosol particles, which commonly cause 20% decreases in UV-B radiation in the summer (Wenny et al, 2001).
Source: World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1998, WMO Global Ozone Research and Monitoring Project - Report No. 44, Geneva, 1998.
What are the human health effects of increased UV-B light?
From the outset it should be pointed out that human behavior is of primary importance when considering the health risks of sun exposure. Taking proper precautions, such as covering up exposed skin, using sunscreen, and staying out of the sun during peak sun hours is of far greater significance to health than the increased UV-B due to ozone loss is likely to be.
A reduction in ozone of 1% leads to increases of up to 3% in some forms of non-melanoma skin cancer (UNEP, 1998). It is more difficult to quantify a link between ozone loss and malignant melanoma, which accounts for about 4% of skin cancer cases, but causes about 79% of skin cancer deaths. Current research has shown that melanoma can increase with both increased UV-B and UV-A light, but the relationship is not well understood (UNEP, 2002). In the U.S. in 2003, approximately 54,200 persons will have new diagnoses of melanoma, and 7,600 will die from the disease, and more than 1 million new cases of the other two skin cancers, basal cell carcinoma and squamous cell carcinoma, will be diagnosed (American Cancer Society, 2002) . Worldwide, approximately 66,000 people will die in 2003 from malignant melanoma, according to the World Health Organization. However, the significant rises in skin cancer worldwide can primarily be attributed human behavioral changes rather than ozone depletion (Urbach, 1999; Staehelin, 1990).
On the positive side, UV light helps produce vitamin D in the skin, which may help against contraction of certain diseases. Multiple sclerosis has been shown to decrease in the white Caucasian population with increasing UV light levels. On the negative side, excessive UV-B exposure depresses the immune system, potentially allowing increased susceptibility to a wide variety of diseases. And in recent years, it has become apparent that UV-B damage to the eye and vision is far more insidious and detrimental than had previously been suspected (UNEP, 2002). Thus, we can expect ozone loss to substantially increase the incidence of cataracts and blindness. A study done for Environment Canada presented to a UN meeting in 1997, estimated that because of the phase-out of CFCs and other ozone depleting substances mandated by the 1987 Montreal Protocol, there will be 19.1 million fewer cases of non-melanoma skin cancer, 1.5 million fewer cases of melanoma, 129 million fewer cases of cataracts, and 330,000 fewer skin cancer deaths worldwide.
Has ozone loss contributed to an observed increase in sunburns and skin cancer in humans?
Yes, Punta Arenas, Chile, the southernmost city in the world (53°S), with a population of 154,000, has regularly seen high levels of UV-B radiation each spring for the past 20 years, when the Antarctic ozone hole has moved over the city (Abarca, 2002). Ozone levels have dropped up to 56%, allowing UV-B radiation more typical of summertime mid-latitude intensities to affect a population unused to such levels of skin-damaging sunshine. Significant increases in sunburns have been observed during many of these low-ozone days. During the spring of 1999, a highly unusual increase in referrals for sunburn occurred in Punta Arenas during specific times when the ozone hole passed over the city. And while most of the worldwide increase in skin cancer rates the past few decades has been attributed to people spending more time outdoors, and the use of tanning businesses (Urbach, 1999), skin cancer cases increased 66% from 1994-2000 compared to 1987-1993 in Punta Arenas, strongly suggesting that ozone depletion was a significant factor.
What is the effect of increased UV-B light on plants?
UV-B light is generally harmful to plants, but sensitivity varies widely and is not well understood. Many species of plants are not UV-B sensitive; others show marked growth reduction and DNA damage under increased UV-B light levels. It is thought that ozone depletion may not have a significant detrimental effect on agricultural crops, as UV-B tolerant varieties of grains could fairly easily be substituted for existing varieties. Natural ecosystems, however, would face a more difficult time adapting. Direct damage to plants from ozone loss has been documented in several studies. For example, data from a spring, 1997 study in Tierra del Fuego, at the southern tip of Argentina, found DNA damage to plants on days the ozone hole was overhead to be 65% higher than on non-ozone-hole days (Rousseaux et. al., 1999).
What is the effect of increased UV-B light on marine life?
UV-B light is generally harmful to marine life, but again the effect is highly variable and not well understood. UV-B radiation can cause damage to the early developmental stages of fish, shrimp, crab, amphibians and other animals (UNEP, 2002).. Even at current levels, solar UV-B radiation is a limiting factor in reproductive capacity and larval development, and small increases in UV-B radiation could cause significant population reductions in the animals that eat these smaller creatures. One study done in the waters off Antarctica where increased UV-B radiation has been measured due to the ozone hole found a 6-12% decrease in phytoplankton, the organism that forms the base of the food chain in the oceans (Smith et. al., 1990). Since the ozone hole lasts for about 10-12 weeks, this corresponds to an overall phytoplankton decrease of 2-4% for the year.
Is the worldwide decline in amphibians due to ozone depletion?
No. The worldwide decline in amphibians is just that--worldwide. Ozone depletion has not not yet affected the tropics (-25° to 25° latitude), and that is where much of the decline in amphibians has been observed. It is possible that ozone depletion in mid and high latitudes has contributed to the decline of amphibians in those areas, but there are no scientific studies that have made a direct link.
Are sheep going blind in Chile?
Yes, but not from ozone depletion! In 1992, The New York Times reported ozone depletion over southern Chile had caused "an increase in Twilight Zone-type reports of sheep and rabbits with cataracts" (Nash, 1992). The story was repeated in many places, including the July 1, 1993 showing of ABC's Prime Time Live. Al Gore's book, Earth in the Balance, stated that "in Patagonia, hunters now report finding blind rabbits; fishermen catch blind salmon" (Gore, 1992). A group at Johns Hopkins has investigated the evidence and attributed the cases of sheep blindness to a local infection ("pink eye") (Pearce, 1993).
What do the skeptics say about the ozone hole?
Ever since the link between CFCs and ozone depletion was proposed in 1974, skeptics have attacked the science behind the link and the policies of controlling CFCs and other ozone depleting substances. We have compiled a detailed analysis of the arguments of the skeptics. It is interesting to note how the skeptics are using the same bag of tricks to cast doubt upon the science behind the global warming debate, and the need to control greenhouse-effect gases.
What are the costs and savings of the CFC phaseout?
The costs have been large, but not as large as initially feared. As the United Nations Environment Programme (UNEP) Economic Options Committee (an expert advisory body) stated in 1994: "Ozone-depleting substance replacement has been more rapid, less expensive, and more innovative than had been anticipated at the beginning of the substitution process. The alternative technologies already adopted have been effective and inexpensive enough that consumers have not yet felt any noticeable impacts (except for an increase in automobile air conditioning service costs)" (UNEP, 1994). A group of over two dozen industry experts estimated the total CFC phaseout cost in industrialized counties at $37 billion to business and industry, and $3 billion to consumers (Vogelsberg, 1997). A study done for Environment Canada presented to a UN meeting in 1997, estimated a total CFC phaseout cost of $235 billion through the year 2060, but economic benefits totaling $459 billion, not including the savings due to decreased health care costs. These savings came from decreased UV exposure to aquatic ecosystems, plants, forests, crops, plastics, paints and other outdoor building materials.
What steps have been taken to save the ozone layer? Are they working?
In 1987, the nations of the world banded together to draft the Montreal Protocol to phase out the production and use of CFCs. The 43 nations that signed the protocol agreed to freeze consumption and production of CFCs at 1986 levels by 1990, reduce them 20% by 1994, and reduce them another 30% by 1999. The alarming loss of ozone in Antarctica and worldwide continued into the 1990's, and additional amendments to further accelerate the CFC phase-out were adopted. With the exception of a very small number of internationally agreed essential uses, CFCs, halons, carbon tetrachloride, and methyl chloroform were all phased out by 1995 in developing countries (undeveloped countries have until 2010 to do so). The pesticide methyl bromide, another significant ozone-depleting substance, was scheduled to be phased out in 2004 in developing countries, but a U.S.-led delaying effort led to a one-year extension until the end of 2005. At least 183 counties are now signatories on the Montreal Protocol.
The Montreal Protocol is working, and ozone depletion due to human effects is expected to start decreasing in the next 10 years. Observations show that levels of ozone depleting gases at a maximum now and are beginning to decline (Newchurch et. al., 2003). NASA estimates that levels of ozone-depleting substances peaked in 2000, and had fallen by 3.8% by 2008. Provided the Montreal Protocol is followed, the Antarctic ozone hole is expected to disappear by 2050. The U.N. Environment Program (UNEP) said in August 2006 that the ozone layer would likely return to pre-1980 levels by 2049 over much of Europe, North America, Asia, Australasia, Latin America and Africa. In Antarctica, the agencies said ozone layer recovery would likely be delayed until 2065.
What replacement chemicals for CFCs have been found? Are they safe?
Hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and "Greenfreeze" chemicals (hydrocarbons such as cyclopentane and isobutane) have been the primary substitutes. The primary HFC used in automobile air conditioning, HFC-134a, costs about 3-5 times as much as the CFC-12 gas it replaced. A substantial black market in CFCs has resulted.
HCFCs are considered a "transitional" CFC substitute, since they also contribute to ozone depletion (but to a much less degree than CFCs). HCFCs are scheduled to be phased out by 2030 in developed nations and 2040 in developing nations, according to the Montreal Protocol. HCFCs (and HFCs) are broken down in the atmosphere into several toxic chemicals, trifluoroacetic acid (TFA) and chlorodifluoroacetic acid (CDFA). Risks to human health and the environment from these chemicals is thought to be minimal (UNEP/WMO, 2002).
HFCs do not cause ozone depletion, but do contribute significantly to global warming. For example, HFC-134a, the new refrigerant of choice in automobile air conditioning systems, is 1300 times more effective over a 100-year period as a greenhouse gas than carbon dioxide. At current rates of HFC manufacture and emission, up to 4% of greenhouse effect warming by the year 2010 may result from HFCs.
"Greenfreeze" hydrocarbon chemicals appear to be the best substitute, as they do not contribute to greenhouse warming, or ozone depletion. The hydrocarbons used are flammable, but the amount used (equivalent to two butane lighters of fluid) and safety engineering considerations have made quieted these concerns. Greenfreeze technology has captured nearly 100% of the home refrigeration market in many countries in Europe, but has not been introduced in North America yet due to product liability concerns and industry resistance.
When was the ozone hole discovered?
Ozone depletion by human-produced CFCs was first hypothesized in 1974 (Molina and Rowland, 1974). The first evidence of ozone depletion was detected by ground-based instruments operated by the British Antarctic Survey at Halley Bay on the Antarctic coast in 1982. The results seemed so improbable that researchers collected data for three more years before finally publishing the first paper documenting the emergence of an ozone hole over Antarctica (Farman, 1985). Subsequent analysis of the data revealed that the hole began to appear in 1977. After the 1985 publication of Farman's paper, the question arose as to why satellite measurements of Antarctic ozone from the Nimbus-7 spacecraft had not found the hole. The satellite data was re-examined, and it was discovered that the computers analyzing the data were programmed to throw at any ozone holes below 180 Dobson Units as impossible. Once this problem was corrected, the satellite data clearly confirmed the existence of the hole.
How do CFCs destroy ozone?
CFCs are extremely stable in the lower atmosphere, only a negligible amount are removed by the oceans and soils. However, once CFCs reach the stratosphere, UV light intensities are high enough to break apart the CFC molecule, freeing up the chlorine atoms in them. These free chlorine atoms then react with ozone to form oxygen and chlorine monoxide, thereby destroying the ozone molecule. The chlorine atom in the chlorine monoxide molecule can then react with an oxygen atom to free up the chlorine atom again, which can go on to destroy more ozone in what is referred to as a "catalytic reaction":
Cl + O3 -> ClO + O2
ClO + O -> Cl + O2
Thanks to this catalytic cycle, each CFC molecule can destroy up 100,000 ozone molecules. Bromine atoms can also catalytically destroy ozone, and are about 45 times more effective than chlorine in doing so.
For more details on ozone depletion chemistry, see the Usenet Ozone FAQ.
Are volcanos a major source of chlorine to the stratosphere?
No, volcanos contribute at most just a few percent of the chlorine found in the stratosphere. Direct measurements of the stratospheric chlorine produced by El Chichon, the most important eruption of the 1980's (Mankin and Coffey, 1983), and Pinatubo, the largest volcanic eruption since 1912 (Mankin et. al., 1991) found negligible amounts of chlorine injected into the stratosphere.
What is ozone pollution?
Ozone forms in both the upper and the lower atmosphere. Ozone is helpful in the stratosphere, because it absorbs most of the harmful ultraviolet light coming from the sun. Ozone found in the lower atmosphere (troposphere) is harmful. It is the prime ingredient for the formation of photochemical smog. Ozone can irritate the eyes and throat, and damage crops. Visit the Weather Underground's ozone pollution page, or our ozone action page for more information.
Where can I go to learn more about the ozone hole?
We found the following sources most helpful when constructing the ozone hole FAQ:
- Environmental Protection Agency ozone depletion pages (best source of info on the Internet)
- World Meteorological Organization 2002 scientific assessment
- Latest satellite measurements from NOAA
- United Nations Environment Programme 2002 Assessment
- Usenet FAQ
- Ozone Hole Tour from the University of Cambridge
Dr. Jeff Masters' Recent Ozone Layer Blogs
- Unprecedented Arctic ozone hole in 2011; a Florida tropical storm next week? - October 4, 2011
- Record Arctic ozone loss this Spring - May 21, 2005
- Aurora activty and ozone loss - May 16, 2005
Dr. Ricky Rood's Recent Climate Change Blogs
- Turn Off the Carbon Dioxide Emissions – It Still Gets Warmer - December 12, 2014
- California Drought, El Niño, Warm Earth (One more time) - December 5, 2014
- Essay on Need to Adapt - December 1, 2014
- El Niño, California Drought, and Predictions - October 28, 2014
- Just What Is the New York Climate Summit? - September 24, 2014
Abarca, J.F, and C.C. Casiccia, "Skin cancer and ultraviolet-B radiation under the Antarctic ozone hole: southern Chile, 1987-2000," Photodermatology, Photoimmunology & Photomedicine, 18, 294, 2002.
American Cancer Society. Cancer facts & figures 2002. Atlanta: American Cancer Society, 2002.
Farman, J.C., B.D. Gardner and J.D. Shanklin, "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx Interaction, Nature, 315, 207-210, 1985.
Gore, A., "Earth in the Balance: Ecology and the Human Spirit", Houghton Mifflin, Boston, 1992.
Manins, P., R. Allan, T. Beer, P. Fraser, P. Holper, R. Suppiah, R. and K. Walsh. "Atmosphere, Australia State of the Environment Report 2001 (Theme Report)," CSIRO Publishing and Heritage, Canberra, 2001.
Mankin, W., and M. Coffey, "Increased stratospheric hydrogen chloride in the El Chichon cloud", Science, 226, 170, 1983.
Mankin, W., M. Coffey, and A. Goldman, "Airborne observations of SO2, HCl, and O3 in the stratospheric plume of the Pinatubo volcano in July 1991", Geophys. Res. Lett., 19, 179, 1992.
McKenzie, R., B. Connor, G. Bodeker, "Increased summertime UV radiation in New Zealand in response to ozone loss", Science, 285, 1709-1711, 1999.
Molina, M.J., and F.S. Rowland, Stratospheric Sink for Chlorofluoromethanes: Chlorine atom-catalyzed destruction of ozone, Nature, 249, 810-812, 1974.
Nash, N.C., "Ozone Depletion Threatening Latin Sun Worshipers", New York Times, 27 March 1992, p. A7.
Newchurch, et. al., "Evidence for slowdown in stratospheric ozone loss: First stage of ozone recovery", Journal of Geophysical Research, 108, doi: 10.1029/2003JD003471, 2003.
Pearce, F., "Ozone hole 'innocent' of Chile's ills", New Scientist, 1887, 7, 21 Aug. 1993.
Rex, M. et. al., "Arctic ozone loss and climate change", Geophys. Res. Lett., 31, L04116, 2004.
Rousseaux, M.C., C.L. Ballare, C.V. Giordano, A.L. Scopel, A.M. Zima, M. Szwarcberg-Bracchitta, P.S. Searles, M.M. Caldwell, S.B. Diaz, "Ozone depletion and UVB radiation: impact on plant DNA damage in southern South America", Proc Natl Acad Sci, 96(26):15310-5, 1999.
Smith, D.A., K. Vodden, L. Rucker, and R. Cunningham, "Global Benefits and Costs of the Montreal Protocol on Substances that Deplete the Ozone Layer", Applied Research Consultants report for Environment Canada, Ottawa, 1997.
Smith, R., B. Prezelin, K. Baker, R. Bidigare, N. Boucher, T. Coley, D. Karentz, S. MacIntyre, H. Matlick, D. Menzies, M. Ondrusek, Z. Wan, and K. Waters, "Ozone depletion: Ultraviolet radiation and phytoplankton biology in Antarctic waters", Science, 255, 952, 1992.
Staehelin, J., M. Blumthaler, W. Ambach, and J. Torhorst, "Skin cancer and the ozone shield", Lancet 336, 502, 1990.
Tabazadeh, A., K. Drdla, M.R. Schoeberl, P. Hamill, and O. B. Toon, "Arctic "ozone hole" in a cold volcanic stratosphere", Proc Natl Acad Sci, 99(5), 2609-12, Mar 5 2002.
United Nations Environmental Programme (UNEP), "1994 Report of the Economics Options Committee for the 1995 Assessment of the Montreal Protocol on Substances that Deplete the Ozone Layer", UNEP, Nairobi, Kenya, 1994.
United Nations Environmental Programme (UNEP), "Environmental Effects of Ozone Depletion: 1998 Assessment", UNEP, Nairobi, Kenya, 1998.
United Nations Environmental Programme (UNEP), "Environmental Effects of Ozone Depletion and its interactions with climate change: 2002 Assessment", UNEP, Nairobi, Kenya, 2002.
Urbach, F. "The cumulative effects of ultraviolet radiation on the skin: Photocarcinogenesis," In: Hawk J, , ed. Photodermatology. Arnold Publishers, 89-102, 1999.
Vogelsberg, F.A., "An industry perspective - lessons learned and the cost of CFC phaseout", HPAC Heating/Piping/AirConditioning, January 1997, 121-128.
Wenny, B.N., V.K. Saxena, and J.E. Frederick, "Aerosol optical depth measurements and their impact on surface levels of ultraviolet-B radiation", J. Geophys. Res., 106, 17311-17319, 2001.
World Meteorological Organization (WMO), "Scientific Assessment of Ozone Depletion: 2002 Global Ozone Research and Monitoring Project - Report #47", WMO, Nairobi, Kenya, 2002.
Young, A.R., L.O. Bjorn, J. Mohan, and W. Nultsch, "Environmental UV Photobiology", Plenum, N.Y. 1993.