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Introduction

Planet Earth has its own natural sunscreen that shields us from the sun's damaging ultraviolet radiation. It's called the ozone layer: a fragile band of gases beginning 15 kilometres above our planet, and reaching up to the 40-kilometre level. Human activities have caused a substantial thinning of this protective covering not only over the North and South Poles, but right over our heads. Stopping ozone layer depletion is one of the major challenges facing the world today. The stakes are incredibly high. For the ozone layer is truly a "conserver of life," essential to the survival of all living things.

The Stratospheric Ozone Layer

The ozone layer lies in the stratosphere, in the upper level of our atmosphere. Stratospheric ozone filters out most of the sun's potentially harmful shortwave ultraviolet (UV) radiation. This ozone has become depleted, due to the release of such ozone-depleting substances as chlorofluorocarbons (CFCs). When stratospheric ozone is depleted, more UV rays reach the earth. Exposure to higher amounts of UV radiation could have serious impacts on human beings, animals and plants

Depletion of the Stratospheric Ozone Layer (Ozone Depletion)In 1985, a group of scientists made an unsettling discovery: a marked decrease in stratospheric ozone over the South Pole, in the Antarctic. The depletion appeared during the southern hemisphere's spring (October and November) and then filled in. Soon after the Antarctic hole was found, Canadian scientists discovered that the ozone layer above the Arctic is also thinning significantly.

The highest latitudes — the north and south poles — experience the greatest amount of ozone loss, during their spring. Ozone depletion is most pronounced in the Antarctic. But ozone depletion, to a lesser degree, now occurs in the mid-latitudes. For example, the amount of stratospheric ozone over the northern hemisphere has been dropping by 4% per decade.

What does this mean for life on earth? Even the smallest reduction in stratospheric ozone can have a noticeable impact by increasing the amount of UV radiation that reaches the planet. Studies show, for example, that a decrease in stratospheric ozone could cause additional deaths from skin cancer. Even a 1% global reduction in ozone is expected to cause a significant drop in crop yields, in a world that is already struggling to feed itself.

The Causes of Ozone Depletion

Scientific evidence indicates that stratospheric ozone is being destroyed by a group of manufactured chemicals, containing chlorine and/or bromine. These chemicals are called "ozone-depleting substances" (ODS).

ODS are very stable, nontoxic and environmentally safe in the lower atmosphere, which is why they became so popular in the first place. However, their very stability allows them to float up, intact, to the stratosphere. Once there, they are broken apart by the intense ultraviolet light,

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releasing chlorine and bromine. Chlorine and bromine demolish ozone at an alarming rate, by stripping an atom from the ozone molecule. A single molecule of chlorine can break apart thousands of molecules of ozone.

What's more, ODS have a long lifetime in our atmosphere — up to several centuries. This means most of the ODS we've released over the last 80 years are still making their way to the stratosphere, where they will add to the ozone destruction.

The main ODS are chlorofluorocarbons (CFCs), hydrochlorofluorcarbons (HCFCs), carbon tetrachloride and methyl chloroform. Halons (brominated fluorocarbons) also play a large role. Their application is quite limited: they're used in specialized fire extinguishers. But the problem with halons is they can destroy up to 10 times as much ozone as CFCs can. For this reason, halons are the most serious ozone-depleting group of chemicals emitted in British Columbia.

Hydrofluorocarbons (HFCs) are being developed to replace CFCs and HCFCs, for uses such as vehicle air conditioning. HFCs do not deplete ozone, but they are strong greenhouse gases. CFCs are even more powerful contributors to global climate change, though, so HFCs are still the better option until even safer substitutes are discovered.

The Main Ozone-Depleting Substances (ODS)

Chlorofluorocarbons (CFCs) o The most widely used ODS, accounting for over 80% of total stratospheric ozone

depletion. o Used as coolants in refrigerators, freezers and air conditioners in buildings and cars

manufactured before 1995.o Found in industrial solvents, dry-cleaning agents and hospital sterilants. o Also used in foam products — such as soft-foam padding (e.g. cushions and mattresses)

and rigid foam (e.g. home insulation). Halons

o Used in some fire extinguishers, in cases where materials and equipment would be destroyed by water or other fire extinguisher chemicals. In B.C., halons cause greater damage to the ozone layer than do CFCs from automobile air conditioners.

Methyl Chloroform o Used mainly in industry — for vapour degreasing, some aerosols, cold cleaning,

adhesives and chemical processing. Carbon Tetrachloride

o Used in solvents and some fire extinguishers. Hydrofluorocarbons (HCFCs)

o HCFCs have become major, “transitional” substitutes for CFCs. They are much less harmful to stratospheric ozone than CFCs are. But HCFCs they still cause some ozone destruction and are potent greenhouse gases.

The Impacts of Ozone Depletion

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Stratospheric ozone filters out most of the sun's potentially harmful shortwave ultraviolet (UV) radiation. If this ozone becomes depleted, then more UV rays will reach the earth. Exposure to higher amounts of UV radiation could have serious impacts on human beings, animals and plants, such as the following:

Harm to human health: o More skin cancers, sunburns and premature aging of the skin. o More cataracts, blindness and other eye diseases: UV radiation can damage

several parts of the eye, including the lens, cornea, retina and conjunctiva. o Cataracts (a clouding of the lens) are the major cause of blindness in the world. A

sustained 10% thinning of the ozone layer is expected to result in almost two million new cases of cataracts per year, globally (Environment Canada, 1993).

o Weakening of the human immune system (immunosuppression). Early findings suggest that too much UV radiation can suppress the human immune system, which may play a role in the development of skin cancer.

Adverse impacts on agriculture, forestry and natural ecosystems: o Several of the world's major crop species are particularly vulnerable to increased

UV, resulting in reduced growth, photosynthesis and flowering. These species include wheat, rice, barley, oats, corn, soybeans, peas, tomatoes, cucumbers, cauliflower, broccoli and carrots.

o The effect of ozone depletion on the Canadian agricultural sector could be significant.

o Only a few commercially important trees have been tested for UV (UV-B) sensitivity, but early results suggest that plant growth, especially in seedlings, is harmed by more intense UV radiation.

Damage to marine life: o In particular, plankton (tiny organisms in the surface layer of oceans) are

threatened by increased UV radiation. Plankton are the first vital step in aquatic food chains.

o Decreases in plankton could disrupt the fresh and saltwater food chains, and lead to a species shift in Canadian waters.

o Loss of biodiversity in our oceans, rivers and lakes could reduce fish yields for commercial and sport fisheries.

Animals: o In domestic animals, UV overexposure may cause eye and skin cancers. Species

of marine animals in their developmental stage (e.g. young fish, shrimp larvae and crab larvae) have been threatened in recent years by the increased UV radiation under the Antarctic ozone hole.

Materials: o Wood, plastic, rubber, fabrics and many construction materials are degraded by

UV radiation.o The economic impact of replacing and/or protecting materials could be

significant.

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What Governments are Doing about Ozone Depletion

194 nations, including Canada, have signed an international agreement to end the production of chlorofluorocarbons (CFCs), halons and other ozone-depleting substances (ODS). The agreement is called the Montreal Protocol on Substances that Deplete the Ozone Layer (1987). The protocol has been amended several times, to speed up ODS phaseout dates and to include more types of ODS. 

The federal and provincial/territorial governments share responsibility for protecting the ozone layer. Under the Montreal Protocol, the federal government is responsible for controlling the import, manufacture, use, sale and export  of ODS. To meet these requirements, Canada has established:

regulations; an ODS-phaseout program: Canada’s Strategy to Accelerate the Phase-Out of CFC and

Halon Uses and to Dispose of the Surplus Stocks (PDF: 211 KB/27 pages); and the National Action Plan for the Environmental Control of Ozone-Depleting Substances

and their Halocarbon Alternatives (PDF: 117KB/42 pages).

For more information, see Environment Canada's Stratospheric Ozone website.

The provincial/territorial governments manage the use and handling of ODS. The B.C. Government passed the Ozone Depleting Substances Regulation in 1993 to control ODS stored in products and equipment, and encourage consumers and industry to use more environmentally safe alternatives.

In Schedule A of the regulation, Class I lists all CFCs and halons, as well as methyl chloroform and carbon tetrachloride. Class I substances are considered to have the most significant impact on ozone layer depletion. Class II of Schedule A lists all hydrochlorofluorocarbons (HCFCs), which are considered the “transitional” substances or alternatives to Class I substances. Both Class I and Class II are ozone-depleting substances.

The regulation was amended in November 1999, mainly to include other halocarbons as Class III substances — e.g., hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). The Class III substances do not contain chlorine or bromine atoms, so they don't deplete the ozone layer. However, they are considered potent greenhouse gases and have a significant global warming potential (GWP).

The amendments in 1999 also strengthened certain requirements, and the regulation was renamed the Ozone Depleting Substances and Other Halocarbons Regulation. The regulation was amended again in 2004, largely to implement Canada’s National Action Plan, which includes phaseout dates for Class I substances (Section 27 of the regulation). 

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These amendments also included additional CFC-refill restrictions for the mobile and commercial refrigeration sectors, refill restrictions for halon fire extinguishers and revised seller take-back provisions for surplus CFC refrigerators. For more information and to download a copy of the regulation, see Amendments to the Ozone Depleting Substances and Other Halocarbons Regulation.

To make sure ODS are recovered correctly, technicians working with these chemicals must be an “approved person” as defined in the regulation. This includes having successfully completed an ODS environmental-awareness course approved by Environment Canada and the ministry. Technicians must also follow the procedures detailed in Environment Canada’s Environmental Code of Practice for Elimination of Fluorocarbon Emissions from Refrigeration and Air Conditioning Systems (PDF: 61 pages) and the Environmental Code of Practice for Halons.

B.C. no longer allows the recharging of a motor vehicle air conditioner (MVAC) with any Class I substance, e.g., CFC-12, the common refrigerant in older air conditioners. When vehicles are scrapped or their air conditioners are repaired, the ODS must be recovered safely, with no leaks. A Class II, III or other alternative substance must be used as the replacement refrigerant. Anyone servicing an MVAC system must have successfully completed an MVAC servicing-and-retrofitting course approved by the Ministry of Environment.

What You Can Do about Ozone Depletion

Help Prevent Further Ozone Depletion

The nations of the world have taken a crucial step in joining together to halt the production and use of ozone-destroying chemicals. But the work can't stop there. Here's what you can do:

Know the rules: It is illegal to recharge refrigerators, freezers and home/vehicle air conditioners with CFCs.

If you have an older vehicle with an air conditioner*, have it serviced by a qualified technician, and make sure the CFC is recaptured and recycled by technician who is specifically certified to do this work. If you don't use your air conditioner — or if the vehicle is about to be scrapped — make sure a qualified technician recaptures and recycles the CFC. *Vehicles of model year 1995 or newer do not use CFCs.

The same rules apply to older refrigerators freezers and home air conditioners, which may contain CFCs.

Don't buy or use portable fire extinguishers that contain halons.

Protect Yourself from Ultraviolet (UV) Radiation

Some ultraviolet (UV) radiation from the sun has always reached the earth, but most of it has been screened out by the ozone layer. There has always been a reason for people to avoid too

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much of the sun's damaging rays. But this is true now more than ever, due to ozone depletion. Be sun safe. Follow these tips:

There's no such thing as a "healthy" tan. Tanning isn't good for you, especially when the ozone layer is depleted. Fair-skinned people are particularly vulnerable to UV radiation, as are infants and children — but everyone should be careful.

Be aware that UV radiation is most intense during the summer, so take extra precautions. Don't overlook all the "innocent" minutes throughout the year when you're outside briefly. They can add up to a lot of radiation.

Sit in the shade, and avoid prolonged exposure when the sun is high: between 10 a.m. and 4 p.m.

Wear protective clothing and a broad-brimmed sunhat. Sunglasses with 100% UV protection are also important.

Use a good sunscreen and apply it liberally. It should have a sun-protection factor (SPF) of 30 or higher, and screen both UV-A and UV-B rays.

Reapply sunscreen after you've been swimming or perspiring a lot. Check Environment Canada's UV Index: It helps Canadians protect themselves from

overexposure to UV radiation, by providing twice-daily forecasts of the amount of radiation expected for different areas of the country.

Taking a holiday in your favourite tropical isle? Have fun, but be very cautious about those UV rays. Though ozone depletion is not as pronounced near the equator, the ultraviolet radiation is extremely intense, mainly due to the angle of the sun.

Keep in mind that you can still get a lot of sun in the winter. Be especially careful when you're doing outdoor sports, such as skiing. Reflection off fresh snow nearly doubles UV radiation.

Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions.[1] The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events.

The details of polar ozone hole formation differ from that of mid-latitude thinning but the most important process in both is catalytic destruction of ozone by atomic halogens.[2] The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbonrefrigerants, solvents, propellants, and foam-blowing agents (CFCs, HCFCs, freons, halons). These compounds are transported into the stratosphere by winds after being emitted at the surface.[3] Both types of ozone depletion were observed to increase as emissions of halocarbons increased.

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CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone generated worldwide concern, leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in sunburn, skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion.

Ozone cycle overview

Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after intaking an ultraviolet photon whose wavelength is shorter than 240 nm. This converts a single O2 into two atomic oxygen radicals. The atomic oxygen radicals then combine with separate O2 molecules to create two O3 molecules. These ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process that terminates when an oxygen atom "recombines" with an ozone molecule to make two O 2

molecules.

2O3→3O2

The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.

Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine atom (Cl·) and bromine atom (Br·). The dot is a common notation to indicate that all of these species have an unpaired electron and are thus extremely reactive. All of these have both natural and man-made sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the levels of chlorine and bromine. These elements are found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g.

CFCl3 + electromagnetic radiation → Cl· + ·CFCl2

The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is:

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Cl·+ O3 → ClO + O2: The chlorine atom changes an ozone molecule to ordinary oxygen

ClO + O3 → Cl· + 2O2: The ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone.

The overall effect is a decrease in the amount of ozone, though the rate of these processes can be decreased by the effects of null cycles. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well.

A single chlorine atom would keep on destroying ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine contribute significantly to overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly bound HF, while organic molecules containing iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities.

On average, a single chlorine atom is able to react with 100,000 ozone molecules before it is removed from the catalytic cycle. This fact plus the amount of chlorine released into the atmosphere yearly by chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) demonstrates how dangerous CFCs and HCFCs are to the environment.

Ozone cycle

Consequences of ozone layer depletion

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Since the ozone layer absorbs UVB ultraviolet light from the sun, ozone layer depletion increases surface UVB levels (all else equal), which could lead to damage, including increase in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer and eye damage in human beings. This is partly because UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and because it is nearly impossible to control statistics for lifestyle changes in the populace.

Increased UV[edit]

Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness and density of the layer. When stratospheric ozone levels decrease, higher levels of UVB reach the Earth’s surface. [1][35] UV-driven phenolic formation in tree rings has dated the start of ozone depletion in northern latitudes to the late 1700s.[36]

In October 2008, the Ecuadorian Space Agency published a report called HIPERION, a study of the last 28 years data from 10 satellites and dozens of ground instruments around the world among them their own, and found that the UV radiation reaching equatorial latitudes was far greater than expected, with the UV Index climbing as high as 24 in some very populated cities; the WHO considers 11 as an extreme index and a great risk to health. The report concluded that depleted ozone levels around the mid-latitudes of the planet are already endangering large populations in these areas. Later, the CONIDA, the Peruvian Space Agency, published its own study, which yielded almost the same findings as the Ecuadorian study.

Biological effects[edit]

The main public concern regarding the ozone hole has been the effects of increased surface UV radiation on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown so large as to affect parts of Australia, New Zealand, Chile, Argentina, and South Africa, environmentalists have been concerned that the increase in surface UV could be significant.[37]

Ozone depletion would magnify all of the effects of UV on human health, both positive (including production of Vitamin D) and negative (including sunburn, skin cancer, and cataracts). In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.

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Basal and squamous cell carcinomas[edit]

The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood—absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that every 1% decrease in long-term stratospheric ozone would increase the incidence of these cancers by 2%.[38]

Malignant melanoma[edit]

Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15–20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet fully understood, but it appears that both UVB and UVA are involved. Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women. [39] A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.[40]

Cortical cataracts[edit]

Epidemiological studies suggest an association between ocular cortical cataracts and UVB exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UVB was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity.[41] In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. Based on these results, ozone depletion is predicted to cause hundreds of thousands of additional cataracts by 2050.[42]

Increased tropospheric ozone[edit]

Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. The risks are particularly high for young children, the elderly, and those with asthma or other respiratory difficulties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.[43]

Increased production of vitamin D[edit]

Vitamin D is produced in the skin by ultraviolet light. Thus, higher UVB exposure raises human vitamin D in those deficient in it. Recent research (primarily since the Montreal Protocol) shows

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that many humans have less than optimal vitamin D levels. In particular, in the U.S. population, the lowest quarter of vitamin D (<17.8 ng/ml) were found using information from the National Health and Nutrition Examination Survey to be associated with an increase in all-cause mortality in the general population.[44] While blood level of Vitamin D in excess of 100 ng/ml appear to raise blood calcium excessively and to be associated with higher mortality, the body has mechanisms that prevent sunlight from producing Vitamin D in excess of the body's requirements.[45]

Effects on non-human animals[edit]

A November 2010 report by scientists at the Institute of Zoology in London found that whales off the coast of California have shown a sharp rise in sun damage, and these scientists "fear that the thinning ozone layer is to blame".[46] The study photographed and took skin biopsies from over 150 whales in the Gulf of California and found "widespread evidence of epidermal damage commonly associated with acute and severe sunburn", having cells that form when the DNA is damaged by UV radiation. The findings suggest "rising UV levels as a result of ozone depletion are to blame for the observed skin damage, in the same way that human skin cancer rates have been on the increase in recent decades."[47]

Effects on crops[edit]

An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV radiation and would be affected by its increase.[48] "Despite mechanisms to reduce or repair the effects of increased ultraviolet radiation, plants have a limited ability to adapt to increased levels of UVB, therefore plant growth can be directly affected by UVB radiation."[49]

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