Ozone Layer Depletion
The Importance Of Stratospheric Ozone
Stratospheric ozone is biologically important because it selectively absorbs much of the incoming solar electromagnetic radiation within the ultraviolet (UV) range. Ozone is very effective within the UV-C wavelength range of 200–280 nm, somewhat less so in the UV-B range of 280–320 nm, and it is rather ineffective in absorbing UV-A at 320–400 nm. However, UV-A is not very damaging to organisms. Although UV-C is extremely damaging, virtually none of this radiation penetrates through Earth's upper atmosphere. Therefore, the greatest anxiety in terms of biological damages caused by ultraviolet radiation concerns the relatively variable exposures to UV-B, which are directly influenced by concentrations of stratospheric ozone. Note, however, that fluxes of UV-B to Earth's surface are also related to certain conditions in the troposphere, such as the thickness of cloud cover, concentrations of particulates and certain chemicals, and changes in the angle of the sun, which influences the thickness of atmosphere that must be penetrated by UV-B rays before the earth's surface is reached.
Because ozone selectively absorbs these deleterious wavelengths of solar radiation, it serves as an ultraviolet shield. As such, stratospheric ozone helps to protect humans and other organisms on Earth's surface from some of the harmful effects of exposure to this high-energy electromagnetic radiation. In fact, without the protective action of the stratospheric ozone layer, it is likely that terrestrial life would not be possible on Earth, and that oceanic life would be restricted to relatively greater depths than those at which it can now comfortably occur.
If not intercepted, ultraviolet radiation is capable of damaging genetic material. The genetic molecules deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) and many proteins and other biochemicals are effective absorbers of ultraviolet radiation. DNA and RNA are especially efficient at absorbing wavelengths shorter than 320 nm, but these important chemicals are damaged by this absorption. Damages to genetic materials could result in an increased incidence of skin cancers. Basal carcinomas account for about 75% of human skin cancers, and squamous cell carcinomas about 20%. These are both serious diseases, but they can usually be successfully treated if detected early enough. The other skin cancer is malignant melanoma, a deadly disease that accounts for about 5% of total skin carcinomas, and which is often fatal soon after it is diagnosed.
It is well known that people living in relatively sunny places have increased risks of all of these skin cancers, and that individual behaviors that increase exposures to UV-B also carry higher risks of developing these diseases (for example, sunbathing, or occupation exposures related to working outdoors, such as in agriculture, fishing, or construction). Compared with light-skinned people, individuals with relatively dark skin are much more tolerant of exposure to UV-B, because they are protected by the skin pigment melanin.
Within skin-color types, there are well-established, statistical relationships between exposures to UV-B and risks of developing skin cancers, most notably malignant melanoma. Using these relationships, predictions of increased rates of skin cancers that could be caused by depletions of stratospheric ozone have been made. For example, the U.S. Environmental Protection Agency has suggested that a 1% decrease in stratospheric ozone could result in a 2% increase in exposure to UV-B, and a 3–6% increase in skin cancers. In fact, many countries have reported increased incidences of all skin cancers. Usually this phenomenon is attributed to human behaviors that influence exposure to UV-B, such as sunbathing. However, increased exposures related to depletions of stratospheric ozone may also be important. Because melanoma takes 10–20 years to develop, there has not been enough time for ozone depletion to play a significant role in the rate of occurrence of this cancer. Research over the next decade will indicate whether the incidence of this cancer is rising and whether this rise may be due to the depletion of the ozone layer.
Other human-health effects of ultraviolet exposure include increased risks of developing cataracts and other damage to the cornea, damage to the retina, a suppressed immune system, sunburns of exposed skin, skin allergies, and an accelerated aging of the skin.
Domestic and wild animals are subject to the same sorts of increased risks of diseases and damages associated with increased UV-B exposure as are humans. Unlike humans, however, these animals do not wear protection to diminish those risks.
Other potential ecological damages associated with increased ultraviolet exposures include decreases of plant productivity in regions stressed by UV-B radiation, caused by the degradation of photosynthetic pigments. All terrestrial plants are at risk, as are plants occurring in shallow waters. The most exposed plants occur at high altitudes, for example in alpine tundra, or at high latitudes, such as polar seas and tundras. However, few data are now available that allow a general evaluation of the importance of these potential decreases in plant productivity.
In addition, some stratospheric ozone makes its way to the lower atmosphere, where it contributes to ozone pollution. Ozone is an important pollutant in the lower troposphere where it damages agricultural and wild plants, weakens synthetic materials, and causes discomfort to humans. During events of great turbulence in the upper atmosphere, such as thunderstorms, stratospheric ozone may enter the troposphere. Usually this only affects the upper troposphere, although observations have been made of stratospheric ozone reaching ground level for short intervals of time. On average, stratospheric incursions account for about 18% of the ozone in the troposphere, while photochemical reactions within the lower atmosphere itself account for the remaining 82% of tropospheric ozone.
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