THE OZONE LAYER
How dangerous is sunlight?
While most molecules of the element oxygen exist as oxygen
gas, O2, some form molecules of ozone, O3. When O2 molecules absorb energy
from electrical discharge and UV radiation some dissociate to individual
O atoms. If these reactive oxygen atoms bind to a molecule of oxygen gas,
they form ozone. If they bind to a molecule of ozone, they form produce
two molecules of oxygen gas.
An ozone layer is necessary to protect terrestrial life
from harmful UV rays. After a day in the sun, a typical epidermal cell
has between 100,000 and 1 million damaged DNA regions as a result of UV
light. Unfortunately, some molecules can destroy ozone without being changed
in the reaction (characteristic of molecules known as catalysts) and thus
one molecule of a catalyst can destroy many ozone molecules. There are
several such catalysts including free radicals resulting from water and
NO. Nuclear explosions inject NO into the stratosphere and thus can decrease
ozone levels. Many feel that the nuclear testing that occurred during
the 1960s had an adverse effect on the ozone layer and that a large scale
nuclear war would drastically reduce the ozone layer.
Another catalyst of ozone degradation are molecules known as CFCs (chloroflourocarbons).
These compounds (which contain chlorine, fluorine, and carbon; freon is
the best known example) were invented in 1930 and, since they are inert,
they made ideal replacements for ammonia and SO2 as refrigerants. They
were later used as propellants in aerosol cans, components of plastic
foams, solvents for oils, sterilizers for medical equipment, and other
uses. Since these molecules are inert, they can reach the stratosphere
(after about 5 years) unchanged. In the stratosphere, UV light reaches
the CFCs and the carbon-chlorine molecules are broken, releasing chlorine
atoms. Chlorine is a catalyst of ozone breakdown and 1 chlorine atom may
destroy 100,000 ozone molecules before winds return it to the lower atmosphere
where they form compounds that are ozone-safe (American Chemical Society,
Concern over the ozone layer intensified in 10/84 when a "hole"
in the ozone layer over Antarctica was detected by British scientists.
NASA confirmed this finding the following year. During winter, permanent
Antarctic clouds exist in which ice crystals serve as a surface for reactions
involving CFCs and chlorine. The reactive forms of chlorine destroy ozone
before they are dispersed. By 1987, ozone levels during winter had dropped
to half the pre-1970 levels. Presently about 60% of the ozone is depleted
from an area 3x the size of the continental U.S. More air movement occurs
at the North Pole and the ozone loss, although serious, is less drastic
than the situation observed over Antarctica. The southern ozone hole is
expanding towards Australia and southern Chile. In 2002, the ozone hole
over Antarctica elongated over time and split into two separate holes
(Varotsos, 2004). The depletion of ozone over the Arctic can cause an
ozone depletion of 30% during much of the year and 70% during spring (Tabazadeh,
2002). Severe ozone depletion in the spring of 2000 led to 2 weeks in
which an area over the Arctic was essentially free of ozone (Bottenheim,
--after Kemp, 2004
The increase in ultraviolet light exposure is blamed for increasing rates of skin cancer. From 1960 to 1986, skin cancer rates increased more than 2x among men and 3.5 times among women in the western U.S. (WHO, 1990a). In 1980, 1 in every 250 Americans contracted melanoma during their lifetime. Today the rate is 1 in 84 (Blatt, 2005).
The increased amounts of UV light can also increase the incidence cataracts
and blindness, not only in humans, but also in animals. This has already
been observed in southern South America. Ultraviolet light can suppress
the immune system and result in a greater susceptibility to infectious
diseases, cancer, and AIDS. UV radiation has an adverse effect on shrimp,
crabs, zooplankton, and fish and it decreases plant and phytoplankton
production. Phytoplankton is the ultimate food source for ocean animals
and is important in the production of oxygen and removal of CO2. Decreases
in phytoplankton of 6-12% have been measured under the Antarctica hole;
corals and other invertebrates may suffer as well. Increased ultraviolet
radiation is increasing plant damage in southern South America and Antarctica
(Day, 1999; Rousseaux, 1999; Last, 1993).