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Air
Conditioning Definitions
Benefits
of the CFC Phase-out
Case
Studies of Successful CFC Elimination
Decibel Comparisons
Faq Ozone
Depletions
How
Air Conditioning Works
Ozone
Science Phase Outs
Understanding
Air Conditioning
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The Earth's ozone layer protects all life from the sun's harmful
radiation, but human activities have damaged this shield. Less
protection from ultraviolet light will, over time, lead to higher
skin cancer and cataract rates and crop damage. The U.S., in cooperation
with over 140 other countries, is phasing out the production of
ozone-depleting substances in an effort to safeguard the ozone
layer.
The Ozone Layer
The Earth's atmosphere is divided into several layers. The lowest
region, the troposphere, extends from the Earth's surface up to
about 10 kilometers (km) in altitude. Virtually all human activities
occur in the troposphere. Mt. Everest, the tallest mountain on
the planet, is only about 9 km high. The next layer, the stratosphere,
continues from 10 km to about 50 km. Most commercial airline traffic
occurs in the lower part of the stratosphere.
Most atmospheric ozone is concentrated in a layer in the, stratosphere
about 15-30 kilometers above the Earth's surface. Ozone is a molecule
containing three oxygen atoms. It is blue in color and has a strong
odor. Normal oxygen, which we breathe, has two oxygen atoms and
is colorless and odorless. Ozone is much less common than normal
oxygen. Out of each 10 million air molecules, about 2 million
are normal oxygen, but only 3 are ozone.
However, even the small amount of ozone plays a key role in the
atmosphere. The ozone layer absorbs a portion of the radiation
from the sun, preventing it from reaching the planet's surface.
Most importantly, it absorbs the portion of ultraviolet light
called UVB. UVB has been linked to many harmful effects, including
various types of skin cancer, cataracts, and harm to some crops,
certain materials, and some forms of marine life.
At any given time, ozone molecules are constantly formed and
destroyed in the stratosphere. The total amount, however, remains
relatively stable. The concentration of the ozone layer can be
thought of as a stream's depth at a particular location. Although
water is constantly flowing in and out, the depth remains constant.
While ozone concentrations vary naturally with sunspots, the
seasons, and latitude, these processes are well understood and
predictable. Scientists have established records spanning several
decades that detail normal ozone levels during these natural cycles.
Each natural reduction in ozone levels has been followed by a
recovery. Recently, however, convincing scientific evidence has
shown that the ozone shield is being depleted well beyond changes
due to natural processes.
Ozone Depletion
For over 50 years, chlorofluorocarbons (CFCs) were thought of
as miracle substances. They are stable, nonflammable, low in toxicity,
and inexpensive to produce. Over time, CFCs found uses as refrigerants,
solvents, foam blowing agents, and in other smaller applications.
Other chlorine-containing compounds include methyl chloroform,
a solvent, and carbon tetrachloride, an industrial chemical. Halons,
extremely effective fire extinguishing agents, and methyl bromide,
an effective produce and soil fumigant, contain bromine. All of
these compounds have atmospheric lifetimes long enough to allow
them to be transported by winds into the stratosphere. Because
they release chlorine or bromine when they break down, they damage
the protective ozone layer. The discussion of the ozone depletion
process below focuses on CFCs, but the basic concepts apply to
all of the ozone-depleting substances (ODS).
In the early 1970s, researchers began to investigate the effects
of various chemicals on the ozone layer, particularly CFCs, which
contain chlorine. They also examined the potential impacts of
other chlorine sources. Chlorine from swimming pools, industrial
plants, sea salt, and volcanoes does not reach the stratosphere.
Chlorine compounds from these sources readily combine with water
and repeated measurements show that they rain out of the troposphere
very quickly. In contrast, CFCs are very stable and do not dissolve
in rain. Thus, there are no natural processes that remove the
CFCs from the lower atmosphere. Over time, winds drive the CFCs
into the stratosphere.
The CFCs are so stable that only exposure to strong UV radiation
breaks them down. When that happens, the CFC molecule releases
atomic chlorine. One chlorine atom can destroy over 100,000 ozone
molecules. The net effect is to destroy ozone faster than it is
naturally created. To return to the analogy comparing ozone levels
to a stream's depth, CFCs act as a siphon, removing water faster
than normal and reducing the depth of the stream.
Large fires and certain types of marine life produce one stable
form of chlorine that does reach the stratosphere. However, numerous
experiments have shown that CFCs and other widely-used chemicals
produce roughly 85% of the chlorine in the stratosphere, while
natural sources contribute only 15%.
Large volcanic eruptions can have an indirect effect on ozone
levels. Although Mt. Pinatubo's 1991 eruption did not increase
stratospheric chlorine concentrations, it did produce large amounts
of tiny particles called aerolsols (different from consumer products
also known as aerosols). These aerosols increase chlorine's effectiveness
at destroying ozone. The aerosols only increased depletion because
of the presence of CFC- based chlorine. In effect, the aerosols
increased the efficiency of the CFC siphon, lowering ozone levels
even more than would have otherwise occurred. Unlike long-term
ozone depletion, however, this effect is short-lived. The aerosols
from Mt. Pinatubo have already disappeared, but satellite, ground-based,
and balloon data still show ozone depletion occurring closer to
the historic trend.
One example of ozone depletion is the annual ozone "hole"
over Antarcticathat has occurred during the Antarctic Spring since
the early 1980s. Rather than being a literal hole through the
layer, the ozone hole is a large area of the stratosphere with
extremely low amounts of ozone. Ozone levels fall by over 60%
during the worst years.
In addition, research has shown that ozone depletion occurs over
the latitudes that include North America, Europe, Asia, and much
of Africa, Australia, and South America. Over the U.S., ozone
levels have fallen 5-10%, depending on the season. Thus, ozone
depletion is a global issue and not just a problem at the South
Pole.
Reductions in ozone levels will lead to higher levels of UVB
reaching the Earth's surface. The sun's output of UVB does not
change; rather, less ozone means less protection, and hence more
UVB reaches the Earth. Studies have shown that in the Antarctic,
the amount of UVB measured at the surface can double during the
annual ozone hole. Another study confirmed the relationship between
reduced ozone and increased UVB levels in Canada during the past
several years.
Laboratory and epidemiological studies demonstrate that UVB causes
non-melanoma skin cancer and plays a major role in malignant melanoma
development. In addition, UVB has been linked to cataracts. All
sunlight contains some UVB, even with normal ozone levels. It
is always important to limit exposure to the sun. However, ozone
depletion will increase the amount of UVB, which will then increase
the risk of health effects. Furthermore, UVB harms some crops,
plastics and other materials, and certain types of marine life.
The World's Reaction
The initial concern about the ozone layer in the 1970's led to
a ban on the use of CFCs as aerosol propellants in several countries,
including the U.S. However, production of CFCs and other ozone-depleting
substances grew rapidly afterward as new uses were discovered.
Through the 1980s, other uses expanded and the world's nations
became increasingly concerned that these chemicals would further
harm the ozone layer. In 1985, the Vienna Convention was adopted
to formalize international cooperation on this issue. Additional
efforts resulted in the signing of the Montreal Protocol in 1987.
The original protocol would have reduced the production of CFCs
by half by 1998.
After the original Protocol was signed, new measurements showed
worse damage to the ozone layer than was originally expected.
In 1992, reacting to the latest scientific assessment of ozone
layer, the Parties decided to completely end production of halons
by the beginning of 1994 and of CFCs by the beginning of 1996
in developed countries.
Because of measures taken under the Protocol, emissions of ozone-depleting
substances are already falling. Assuming continued compliance,
stratospheric chlorine levels will peak in a few years and then
slowly return to normal. The good news is that the natural ozone
production process will heal the ozone layer in about 50 years.
The EPA Stratospheric Protection Program
In addition to regulating the end of production of the ozone-depleting
substance., the U.S. Environmental Protection Agency (EPA) implement
sseveral other programs to protect the ozone layer under Title
VI of the Clean Air Act. These programs include refrigerant recycling,
product labeling, banning nonessential uses of certain compounds,
and reviewing substitutes.
EPA's Stratospheric Ozone Protection Hotline responds to inquiries
and distributes information about ozone depletion and EPA's programs
to protect the ozone layer. Call 1-800-296-1996 between 10 am
and 4 pm Eastern Standard Time to ask questions or to order free
copies of the following documents:
- Reports to the Nation: Our Ozone
Shield
- Written by the National Oceanic
and Atmospheric Administration (NOAA), this booklet describes
the history and science of ozone depletion.
- Executive Summary to Scientific
Assessment of Ozone Depletion: 1994
- This Assessment represents the consensus
conclusions of nearly 300 atmospheric researchers worldwide.
The Executive Summary includes answers to common questions about
ozone depletion. (Note: an Adobe Acrobat (PDF) version of the
Executive Summary of the 1998 Assessment is available from NOAA's
web site.)
Written by EPA's Stratospheric Protection Division
Last updated on December 24, 1997
http://www.epa.gov/ozone/science/sc_fact.html
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