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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.

Image of Ozone Molecule

In the upper atmosphere (stratosphere), oxygen atoms are created as a result of the breakdown of the oxygen molecule (O2) by ultraviolet radiation. 

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).

Stratospheric Ozone Depletion

Under 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 conditions.

The most 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 ozone.

Ground-level Ozone (Smog)

Ozone also 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.

Ozone 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.

The U.S. 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.

Last updated: June 26, 2015


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