Ozone is a gas with a dual nature, not unlike Dr. Jekyll and Mr. Hyde. The ozone layer of our atmosphere is critical for life on Earth, and ozone layer depletion is an important environmental issue. But, paradoxically, the same ozone in the lower atmosphere is a pollutant with serious health effects. Here are the facts on ozone.
What is Ozone?
Ozone is a gas that's made up of three oxygen molecules; it's sometimes referred to as O3 or "trioxygen." An unstable gas, it easily degrades into atmospheric oxygen, or O2, which is made up of two oxygen molecules. Ozone is formed in the Earth's atmosphere from O2 through the action of ultraviolet radiation from the sun.
Pure ozone is light blue in color and has a sharp, fresh smell like freshly cut hay or chlorine -- the fresh, clean scent that follows a thunderstorm is largely due to increased ozone in the air. Because it oxidizes easily, it's used in many industrial processes like sterilizing equipment or treating water and wastewater. This same oxidizing ability, however, makes it dangerous to plants and animals, since it can damage living tissue like lung tissue and plant fibers.
Ozone: The Dr. Jekyll Side
Ozone is produced naturally in the stratosphere -- from about 6 to 30 miles above the ground -- when the sun's UV radiation interacts with oxygen, or O2. But because it's so unstable, ozone is easily broken down in the presence of oxygen and other gases. There's a constant turnover of ozone in the stratosphere, as oxygen, O2 and ozone are being created, then broken down, then re-created.
This upper-atmosphere ozone, by interacting with UV radiation, absorbs a significant amount of the radiation that would otherwise strike the Earth. UV radiation is what causes sunburns; in greater quantities, it's powerful enough to destroy living tissue and cause cancer.
UV radiation at the top of the atmosphere is hundreds of millions times more deadly than it is at the Earth's surface, largely thanks to ozone. Without the UV-absorbing action of ozone, life on Earth would be almost impossible.
The Ozone Hole
This important ozone layer is under attack by man-made chemicals that break down ozone into O2. Gaseous compounds like chlorofluorocarbons (CFCs), halons, methyl bromide, carbon tetrachloride and methyl chloroform all cause ozone layer depletion, and are released by human activities.
These gaseous substances were once used as coolants, foaming agents, fire extinguishers, solvents, pesticides and aerosol propellants, according to the EPA. (Some, unfortunately, still are used in some countries.) Once released into the air, these ozone-depleting substances degrade very slowly and can remain intact for years.
Even though regulations have reduced or eliminated most uses of these compounds -- laws now make it nearly impossible to use CFCs as refrigerants -- their past use is still destroying the ozone layer, causing a large ozone hole to form over different regions of the world. The ozone hole is particularly large over Antarctica and the South Pole (see image, above). There's some evidence that UV radiation is killing off the ocean's population of plankton, which are critical to all life on Earth.
Ozone: The Mr. Hyde Side
While ozone in the upper atmosphere plays a vital role in protecting life on Earth, ozone near ground level is a very different story. In the presence of sunlight, volatile organic compounds -- released when fuels like gasoline are burned -- and nitrous oxide combine to form ozone. This is a key component of smog, the noxious and deadly form of air pollution that's so common in areas like Mexico City and Houston.
When animals inhale ozone at or near ground level, it can cause severe respiratory reactions and other health problems like chest pain, coughing, throat irritation and congestion. It can greatly worsen bronchitis, emphysema and asthma. Ozone also can reduce lung function and inflame the linings of the lungs; repeated exposure may permanently scar lung tissue.
Healthy people also experience difficulty breathing when exposed to ozone pollution. Because ozone forms in sunny weather, anyone who spends time outdoors in the summer may be affected, particularly children, outdoor workers and people exercising. Millions of Americans live in areas where the national ozone health standards are exceeded on a regular basis.
Ground-level ozone also damages plants, leading to reduced agricultural and commercial forest yields, reduced growth and survivability of young plants, and increased susceptibility to plant diseases, pests and other stresses like harsh weather. In the United States alone, ground-level ozone is responsible for an estimated $500 million in reduced crop yields each year.
Fortunately, organizations and agencies like the EPA have made significant strides in reducing ozone pollution in recent years, though much more work needs to be done on an international level to reduce the harmful effects of ground-level ozone and the very real threat of ozone layer depletion.