Ozone can be harmful or beneficial depending on its location. Low-level ozone is formed by reactions between vehicles and volatile organic compounds, causing health effects and air pollution. Ozone in small concentrations can be beneficial as a disinfectant. In the upper atmosphere, ozone acts as a shield against harmful UV rays. CFCs caused a decline in the ozone layer, leading to the creation of the Montreal Protocol to phase them out. They have been replaced by compounds with little or no chlorine. Research is being done on using flammable and non-flammable gases as alternatives.
Ozone is a blue-colored toxic gas made up of three oxygen molecules (O3), which can be hazardous to health or beneficial to life on Earth depending on where it is observed in the atmosphere. At lower levels in the atmosphere, ozone concentrations above minute amounts can cause health effects, affect plant growth, and cause air pollution and damage to buildings. In the upper atmosphere 10-20 miles (20-30 kilometers) above the ground, ozone acts as a shield to prevent some of the sun’s harmful ultraviolet rays from reaching the ground.
Low-level ozone is formed by the reactions of petroleum-fueled vehicles with volatile organic compounds (VOCs) found in gasoline and paint thinners. As the compounds build up in the atmosphere, they react with normal oxygen (O2) molecules and create ozone and other compounds that contribute to smog or air pollution. Ozone is chemically active and, if breathed in, can react with lung tissue and cause damage. It is also corrosive and can cause building damage due to reactions with exterior building products.
Ozone in small concentrations can be beneficial in controlled uses, because it can act as a disinfectant to remove germs. Ozone generators can be used for water treatment plants and in some air purification systems for germ removal. This is deliberately kept at low concentrations to minimize potential adverse health effects. An example of ozone as an air purifier is when lightning strikes during thunderstorms and the air smells fresher afterward. The high electrical energy in lightning can create ozone from oxygen molecules, which will react with air pollution and temporarily clean the air.
In the upper atmosphere, ozone forms naturally from the reactions of oxygen molecules with high-intensity sunlight. Ozone is an excellent absorber of wavelengths of ultraviolet-B (UVB) radiation known to promote cancer in humans and many animals. Ozone constantly reacts with other particles and then regenerates during the day, maintaining a constant ozone concentration. The amount is very small, measured in a few parts per billion parts of air, but important for UVB protection.
Chlorofluorocarbons (CFCs) were invented in the 1930s as a group of products needed to replace hazardous refrigerants such as ammonia and methyl chloride, which were flammable or toxic. Tests with CFCs have shown that humans and animals could be safely exposed to leaks of small amounts found in homes and small businesses without risk. Within a short time, CFCs were used extensively worldwide in refrigeration, aerosol cans and fire extinguishing agents.
Research that began in the 1960s showed that in parts of the Earth’s upper atmosphere, ozone concentrations were declining. In the 1980s, there was a clear relationship between ozone layer losses and airborne CFCs reaching the upper atmosphere. Scientists have proposed that the extremely stable CFC molecules remained in the Earth’s atmosphere for many years, and eventually air currents and weather conditions allowed them to reach atmospheric heights where ozone concentrations were highest.
The same solar energy that created ozone was also strong enough to break down CFC molecules, releasing chlorine (Cl) molecules. These molecules, together with the high-altitude dust and ice crystals, formed reaction sites that broke down the ozone layer and created normal oxygen molecules. While these reactions occurred everywhere in the atmosphere, very low temperatures and weather conditions found at the South Pole caused a higher rate of reaction there.
Satellite data showed very low ozone concentration over the South Pole in the early polar spring after several months of darkness. Scientists and the media coined the term “ozone hole” at the time to explain the effect. Although the ozone hole was temporary each spring and disappeared relatively quickly, it caused great concern about the long-term effects of CFCs.
In 1987, nearly 200 countries belonging to the United Nations signed the Montreal Protocol and agreed to phase out or stop the production of CFCs within certain expiration years. Changes were made to the agreement over the next several decades as new evidence showed higher ozone depletion than initially thought. CFCs have been replaced by compounds with little or no chlorine in their molecules, called hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs).
Interest developed in using flammable gases such as propane and even ammonia for some applications, because these products do not cause ozone depletion. In the early 21st century, manufacturers were looking for ways to safely incorporate flammable gases into consumer products. The research has also been expanded to include non-flammable gases such as carbon dioxide and other technologies that could cool food without the use of refrigerant gases.
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