One of the issues motivating the environmental movement since its inception has been a concern for human health. Recently, worries about the harmful effects of a possible increase in the penetration of UV radiation into the biosphere have received much attention. This FAQ summarizes the causes and consequences of the issue that is at the root of these concerns - stratospheric ozone depletion.
What is ozone?
Ozone is a form of oxygen. The oxygen we breathe is in the form of oxygen molecules (O2) - two atoms of oxygen bound together. Ozone, on the other hand, consists of three atoms of oxygen bound together (O3). Most of the atmosphere's ozone occurs in the stratosphere. Ozone is colourless and has a very harsh odour.
Where is ozone found?
Approximately 90 per cent of all ozone is produced naturally in the stratosphere. While ozone can be found through the entire atmosphere, the greatest concentration occurs at an altitude of about 25 km. This band of ozone-rich air is known as the "ozone layer".
Ozone also occurs in very small amounts at ground level. It is produced at ground level through a reaction between sunlight and volatile organic compounds (VOCs) and nitrogen oxides (NOx), some of which are produced by human activities. Ground-level ozone is a component of urban smog - a serious air pollutant.
Even though both types of ozone are exactly the same molecule, their presence in different parts of the atmosphere has very different consequences. Stratospheric ozone blocks harmful solar radiation - all life on Earth has adapted to this filtered solar radiation. Ground-level ozone, in contrast, is simply a pollutant. It will absorb some incoming solar radiation, but it cannot make up for stratospheric ozone loss.
How is stratospheric ozone formed?
Ozone is created in the stratosphere when highly energetic solar rays strike molecules of oxygen (O2) and cause the two oxygen atoms to split apart. If a freed atom bumps into another O2, it joins up, forming ozone (O3).
Ozone is also naturally broken down in the stratosphere by sunlight and by a chemical reaction with various compounds containing nitrogen, hydrogen and chlorine. These chemicals all occur naturally in the atmosphere in very small amounts.
In an unpolluted atmosphere there is a balance between the amount of ozone being produced and the amount of ozone being destroyed. As a result, the total concentration of ozone in the stratosphere remains relatively constant.
Why is the ozone layer important?
Ozone's unique physical properties allow the ozone layer to act as our planet's sunscreen, providing an invisible filter to help protect all life forms from the sun's damaging ultraviolet (UV) rays. Most incoming UV radiation is absorbed by ozone and prevented from reaching the Earth's surface. Without the protective effect of ozone, life on Earth would not have evolved the way it has.
Is the ozone layer evenly distributed around the Earth?
No. The amount of ozone above a location on the Earth varies naturally with latitude, season, and from day-to-day. Under normal circumstances, the ozone layer is thickest over the poles and thinnest around the equator. The ozone layer over Canada is normally thicker in winter and early spring; it can vary naturally by about 25 per cent between January and July. Weather conditions can also cause considerable daily variations.
What is ultraviolet radiation?
Ultraviolet radiation is one form of radiant energy coming from the sun. The various forms of energy, or radiation, are classified according to wavelength, measured in nanometres (one nm is a millionth of a millimetre). The shorter the wavelength, the more energetic the radiation. In order of decreasing energy, the principal forms of radiation are gamma rays, X rays, UV (ultraviolet radiation), visible light, infrared radiation, microwaves, and radio waves. There are three categories of UV radiation:
| UV-A, between 320 and 400 nm |
| UV-B, between 280 and 320 nm |
| UV-C, between 200 and 280 nm |
How harmful is UV?
Generally, the shorter the wavelength, the more biologically damaging UV radiation can be if it reaches the Earth in sufficient quantity.
UV-A, although it is the least energetic form of UV radiation, reaches the Earth in greatest quantity. Most UV-A rays pass right through the ozone layer.
UV-B radiation is potentially very harmful. Fortunately, most of the sun's UV-B radiation is absorbed by ozone in the stratosphere.
UV-C radiation is potentially the most damaging because it is very energetic. Fortunately, all UV-C is absorbed by oxygen and ozone in the stratosphere and never reaches the Earth's surface.
Do factors other than stratospheric ozone affect the amount of UV radiation that reaches the earth?
| Yes. | Although the ozone layer is the one constant defense against UV penetration, several other factors can have an effect:
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| Latitude. | Since the sun's rays impact the Earth's surface at the most direct angle over the equator they are the most intense at this latitude.
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| Season. | During winter months, the sun's rays strike at a more oblique angle than they do in the summer. This means that all solar radiation travels a longer path through the atmosphere to reach the Earth, and is therefore less intense.
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| Time of day. | Daily changes in the angle of the sun influence the amount of UV radiation that passes through the atmosphere. When the sun is low in the sky, its rays must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components. The greatest amount of UV reaches the Earth around midday when the sun is at its highest point.
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| Altitude. | The air is thinner and cleaner on a mountaintop - more UV reaches there than at lower elevations.
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| Cloud cover. | Clouds can have a marked impact on the amount of UV radiation that reaches the Earth's surface; generally, thick clouds block more UV than thin cloud cover.
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| Rain. | Rainy conditions reduce the amount of UV transmission.
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| Air pollution. | Much as clouds shield the Earth's surface from UV radiation, urban smog can reduce the amount of UV radiation reaching the Earth.
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| Land Cover. | Incoming UV radiation is reflected from most surfaces. Snow reflects up to 85 per cent; dry sand and concrete can reflect up to 12 per cent. Water reflects only five per cent. Reflected UV can damage people, plants, and animals just as direct UV does.
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What is ozone depletion?
Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction. Observations of an antarctic ozone "hole" and atmospheric records indicating seasonal declines in global ozone levels provide strong evidence that global ozone depletion is occurring. Although natural phenomena can cause temporary ozone loss, chlorine and bromine released from synthetic compounds are now accepted as the main cause of this depletion.
How long has ozone depletion been occurring?
Based on data collected since the 1950's, scientists have determined that ozone levels were relatively stable until the late 1970's. Severe depletion over the Antarctic has been occurring since 1979 and a general downturn in global ozone levels has been observed since the early 1980's.
How much of the ozone layer has been depleted around the world?
Global ozone levels declined an average of about 3 per cent between 1979 and 1991. This rate of decline is about three times faster than that recorded in the 1970's. In addition to Antarctica, ozone depletion now affects almost all of North America, Europe, Russia, Australia, New Zealand, and a sizable part of South America.
Short term losses of ozone can be much greater than the long term average. In Canada, ozone depletion is usually greatest in the late winter and early spring. In 1993, for example, average ozone values over Canada were 14% below normal from January to April.
What human activities cause ozone depletion?
Emissions of chlorine and bromine containing synthetic compounds known as industrial halocarbons are the cause of stratospheric ozone depletion.
Why are industrial halocarbons so effective at destroying ozone?
Industrial halocarbons are effective ozone-depleters for two reasons. The first is that they are not reactive, which means they survive long enough in the atmosphere to drift up into the stratosphere. The second is that they help the natural reactions that destroy ozone.
Unlike most chemicals released into the atmosphere at the Earth's surface, industrial halocarbons are not "washed" back to Earth by rain or destroyed in reactions with other chemicals. They simply do not break down in the lower atmosphere and they can remain in the atmosphere from 20 to 120 years or more.
Once they reach the stratosphere, UV-C radiation breaks up these molecules into chlorine (from CFCs, methyl chloroform, carbon tetrachloride) or bromine (from halons, methyl bromide) which, in turn, break up ozone (O3). Both chlorine and bromine activate and speed up the ozone destruction reactions without being altered or destroyed themselves. Thus, a single chlorine atom can destroy up to 100,000 ozone molecules before it finally forms a stable compound and diffuses out of the stratosphere.
Did you know?
The term ozone "hole" refers to a large and rapid decrease in the abundance of ozone molecules, not the complete absence of them. The Antarctic ozone "hole" occurs during the southern spring between September and November.
What are the most widely used ozone-depleting halocarbons?
CFCs are widely used as coolants in refrigeration and air conditioners, as solvents in degreasers and cleaners, and as a blowing agent in the production of foam. Emissions of CFCs account for roughly 80 percent of total stratospheric depletion. HCFCs (Hydrochlorofluorocarbons) contain chlorine but, unlike CFCs, they also contain hydrogen (the H) which causes them to break down in the lower atmosphere. They are called transition chemicals because they are considered an interim step between strong ozone-depleters and replacement chemicals that are entirely ozone-friendly.
Carbon tetrachloride is used as an industrial solvent, an agricultural fumigant, and in many other industrial processes including petrochemical refining. It accounts for less than 8 per cent of total ozone depletion.
Methyl chloroform is a versatile, all-purpose industrial solvent used primarily to clean metal and electronic parts. Methyl chloroform accounts for roughly 5 per cent of total ozone depletion.
Halons are used primarily as fire suppressants. Halons account for only about 5 per cent of global ozone depletion, but the atmospheric concentration of these potent, long-lived ozone destroyers is rising by an estimated 11 - 15 per cent annually.
Methyl bromide has been used as a pesticide since the 1960's. Today, scientists estimate that human sources of methyl bromide are responsible for approximately 5 to 10 per cent of global ozone depletion.
How does UV-B exposure affect people?
Exposure to UV-B radiation causes skin cancer, hastens skin aging, and can cause eye damage. The human immune system can also be weakened by exposure to UV-B.
It is important to note, however, that UV-B radiation has always had these effects on humans. In recent years these effects have become more prevalent because Canadians are spending more time in the sun and are exposing more of their skin in the process. An increase in the levels of UV-B reaching the Earth as a result of ozone depletion may compound the effects that sun worshipping habits have already created.
Although fair-skinned, fair-haired individuals are at highest risk for skin cancer, the risk for all skin types increases with exposure to UV-B radiation. The effects of UV-B on the human immune system have been observed in people with all types of skin.
How does UV-B exposure affect plants and animals?
Excessive UV-B inhibits the growth processes of almost all green plants. There is concern that ozone depletion may lead to a loss of plant species and reduce global food supply. Any change in the balance of plant species can have serious effects, since all life is interconnected. Plants form the basis of the food web, prevent soil erosion and water loss, and are the primary producers of oxygen and a primary sink (storage site) for carbon dioxide.
UV-B causes cancer in domestic animals similar to those observed in humans. Although most animals have greater protection from UV-B because of their heavy coats and skin pigmentation, they cannot be artificially protected from UV-B on a large scale. Eyes and exposed parts of the body are most at risk.
How much more ozone depletion will occur?
No one knows for certain. Even if all nations meet their international commitments to phase out ozone-depleting substances, the levels of these chemicals in the stratosphere will remain near peak values for the next 10 to 20 years.
To the late 1990's, the ozone layer over southern Canada thinned by an
average of about 6 per cent. There is some concern that radiative effects
of increased greenhouse gas concentrations may result in delayed ozone
layer recovery. Eventually, however, a slow recovery is expected to begin,
with a return to pre-1980 values after the mid-portion of the twenty-first century.
It is important to note that scientific knowledge of the atmosphere and the processes that deplete the ozone layer is not complete. The sudden and unexpected appearance of the Antarctic ozone hole reveals that the ozone layer does not respond predictably to the quantities of industrial chemicals we are dumping into it.
Can ozone depletion be stopped and reversed?
Yes. If concentrations of ozone-destroying chemicals are reduced, the natural balance between ozone creation and destruction can be restored. However, this might require the complete elimination of CFCs, halons, carbon tetrachloride, methyl chloroform, HCFCs, and methyl bromide. In late 1991, scientists estimated that even with the current global schedule to eliminate ozone-destroying substances, the ozone layer would not return to 'normal' (pre-1980 chlorine levels) until the middle of the next century.
What can I do to protect myself and my family from the effects of ultraviolet radiation?
A few simple steps will help protect us from the sun's harmful rays:
- Keep sun exposure to a minimum, especially between the hours of 10:00 a.m. and 3:00 p.m. when the sun's rays are the most intense.
- Wear wide-brimmed hats, UV-B blocking sunglasses, and long-sleeved shirts and pants.
- Wear sunscreen with a Sun Protection Factor (SPF) of 15 or greater on any exposed skin. Reapply every hour or after swimming or strenuous activity.
Action
The best strategy for ozone protection is to avoid purchasing products containing ozone-depleters. Ask before you purchase fire extinguishers, foam products, refrigerators and air conditioners. Refuse to purchase products containing ozone-depleters if alternatives are available. Write companies still using these chemicals and voice your concerns.
In some cases, however, consumer products containing ozone-depleters are already in use in our homes and offices and cannot be easily replaced. A second strategy, therefore, involves proper care and maintenance of equipment to ensure that the CFCs they contain are never released to the stratosphere.
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