Ozone (O3) is a gas composed of
three oxygen atoms. It is formed in a chemical reaction between the
diatomic oxygen molecule (O2) and an oxygen atom (O).
Since there is
abundant O2 in the atmosphere, the key to ozone formation is the availability of
free oxygen atoms. At ground level, these oxygen atoms are primarily
created from the breakup of nitrogen dioxide (NO2) by solar radiation. The
amount of NO2 available is regulated by a complex chemistry involving volatile
organic compounds (VOCs) and other oxides of nitrogen (NOx) in the presence of
heat and sunlight.
In the upper atmosphere (stratosphere), oxygen atoms
are created as a result of the breakdown of the oxygen molecule (O2) by
Ozone has the same chemical structure and properties whether
it occurs miles above the earth or at ground level; however,
ozone has both good and bad effects depending on its location
in the atmosphere. Ozone occurs naturally in the
stratosphere approximately 10 to 30 miles above the earth's
surface and forms a layer that protects life on earth from the
sun's harmful ultraviolet radiation (good effect). In
the lower atmosphere, where natural ozone levels are low,
additional ground-level ozone is formed as a result of human
emissions of VOCs and NOx. Breathing this ozone can
result in damage or irritation to the lungs (bad effect).
natural conditions, ozone in the stratosphere is continuously
produced and destroyed, but at equal rates such that a stable
ozone layer is maintained. However, certain man-made
chemicals, referred to as ozone depleting substances, can
upset this natural balance. These ozone-depleting
substances degrade slowly and can remain intact for many years
as they move through the lower atmosphere until reaching the
stratosphere. There, they are broken down by the
intensity of the sun's ultraviolet rays and release chlorine
and bromine molecules, which destroy the ozone. One
chlorine or bromine molecule can destroy 100,000 ozone
molecules, causing ozone to disappear much faster than nature
can replace it. Presently, satellite observations
indicate a worldwide thinning of the protective ozone layer.
The most noticeable losses occur over the North and South
Poles, because ozone depletion accelerates in extremely cold
common ozone depleting substances are chlorofluorocarbons
(CFCs), once widely used as refrigerants and foam blowing
agents; halons, used in fire extinguishers; and certain
solvents such as carbon tetrachloride. Under the
Montreal Protocol, an international agreement ratified by most
nations in 1987, the production of CFCs, halons, and other
ozone depleting substances has been mostly phased out.
However, it can take years for ozone depleting chemicals to
reach the stratosphere. Therefore, some of the ozone
depleting substances that were released in years past are
still present in the atmosphere and will affect the ozone
layer for many years to come.
As a result
of the thinning of the stratospheric ozone layer, higher
levels of ultraviolet-b (UV-b) radiation from the sun are able
to reach the earth's surface. Increased UV-b can lead to
more cases of skin cancer, cataracts, and impaired immune
systems. Damage to UV-b sensitive crops, such as
soybeans, can reduce yield. Ocean phytoplankton could
decrease, leading to a decline in populations of higher
organisms in the marine food chain. Also, increased UV-b
radiation can be instrumental in forming more ground-level
Ground-level Ozone (Smog)
occurs naturally near the earth's surface. However,
man-made emissions of VOCs and NOx can cause additional ozone,
the primary component of urban smog, to be formed. This
additional ozone, which can more than triple the amount of
natural ground-level ozone, can cause health and environmental
damage. Ozone builds up near the ground through a series
of complex chemical reactions involving VOCs and NOx in the
presence of sunlight. VOCs are produced by natural
sources, such as trees; fuel combustion in engines and
industrial operations; some types of chemical manufacturing
operations; evaporation of solvents in consumer and commercial
products; and evaporation of volatile fuels such as gasoline.
Nitrogen oxides are emitted from motor vehicles; off-road
engines such as aircraft, locomotives and construction
equipment; fossil-fuel burning power plants and other
industrial facilities; and other sources of combustion.
concentrations can reach unhealthy levels when the weather is
hot and sunny with relatively light winds. Even at
relatively low levels, ozone may cause inflammation and
irritation of the respiratory tract, particularly during
physical activity. The resulting symptoms can include
breathing difficulty, coughing, and throat irritation.
Breathing ozone can affect lung function and worsen asthma
attacks. Ozone can also increase the susceptibility of
the lungs to infections, allergens, and other air pollutants.
Medical studies have shown that ozone damages lung tissue, and
complete recovery may take several days after exposure has
ended. In addition, longer-term exposures to moderate
levels of ozone present the possibility of irreversible
changes in the lung structure which could lead to premature
aging of the lungs and worsen chronic respiratory illnesses.
Groups that are sensitive to ozone include children and adults
who are active outdoors, and people with respiratory disease
such as asthma. Sensitive people who experience effects
at lower ozone concentrations are likely to experience more
serious effects at higher concentrations.
Environmental Protection Agency has established a health-based
air quality standard for ozone. The Florida Department
of Environmental Protection, in cooperation with several
county air pollution control agencies, monitors ozone air
quality in Florida's major urban areas.
August 04, 2014