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Ozone is a molecule that contains three oxygen atoms and exists in two layers of the atmosphere: the stratosphere and the troposphere. The stratosphere, located about 9 to 18 miles above the Earth's surface, contains the ozone layer, which absorbs most of the sun's harmful ultraviolet (UV) radiation, protecting life on Earth. In contrast, the troposphere, or ground-level ozone, is considered a harmful air pollutant due to its negative impacts on human health and ecosystems. Tropospheric ozone is formed through the interaction of various pollutants, including those emitted by vehicles, power plants, and industrial processes, and has been linked to increased respiratory diseases and crop production losses. While ozone in the stratosphere is beneficial, human activities have led to its depletion, causing a hole in the ozone and potential health and environmental risks.
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What You'll Learn
The ozone layer is found in the stratosphere
The ozone layer is primarily found in the stratosphere, the second layer of the Earth's atmosphere, which sits between 10 and 50 kilometres (6 to 31 miles) above the Earth's surface. This layer contains about 90% of the Earth's ozone, with the remaining 10% found in the troposphere, the layer closest to the Earth.
The stratosphere gets warmer at higher altitudes due to the absorption of ultraviolet radiation by ozone. This warming effect creates a stable environment with little vertical mixing, as warm air remains in the upper stratosphere, and cooler air remains in the lower stratosphere. The ozone layer is often referred to as "good ozone" as it forms a protective barrier that shields the Earth from the sun's harmful ultraviolet rays, specifically the UV-B band of ultraviolet radiation with wavelengths from 280-320 nanometers.
Ozone molecules are constantly formed and destroyed in the stratosphere, and this dynamic process has kept the total amount of ozone relatively stable over the decades it has been measured. However, human activities have led to ozone depletion, creating an infamous "ozone hole" that was first discovered in the 1980s. This depletion is caused by ozone-depleting substances (ODS) such as chlorofluorocarbons (CFCs) and other human-made chemicals. The largest historical extent of the ozone hole was 28.4 million square kilometres in September 2000, and it has been slowly recovering since, with the 2024 hole being the smallest since 2020.
Natural events such as volcanic eruptions and wildfires can also influence stratospheric ozone levels. For example, the 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano injected large amounts of water vapour into the stratosphere, potentially contributing to ozone depletion. Additionally, smoke-charged vortices (SCVs) from wildfires can transport aerosols into the stratosphere, leading to both depletion and increase in the ozone layer due to different chemical reactions.
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Ozone pollution is caused by ODS
Ozone (O3) is a molecule composed of three oxygen atoms. It is found in the stratosphere, about 9 to 18 miles (15 to 30 km) above the Earth's surface. This "good ozone" forms a protective layer that shields us from the sun's harmful ultraviolet rays. However, ozone at ground level is a harmful air pollutant and is the main ingredient in "smog".
Ozone pollution, or smog, is caused by ozone-depleting substances (ODS). ODS include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, hydrobromofluorocarbons, chlorobromomethane, and methyl chloroform. These substances are very stable and do not dissolve in rain, so there are no natural processes to remove them from the lower atmosphere. When ODS break down under intense ultraviolet light in the stratosphere, they release chlorine or bromine atoms that deplete ozone molecules. This depletion leads to higher UVB levels, causing increased skin cancers, cataracts, and potential damage to marine organisms, plants, and plastics.
ODS are used in refrigeration, air conditioning, packaging, insulation, solvents, and aerosol propellants. They are released into the atmosphere through various human activities, such as emissions from cars, power plants, industrial boilers, refineries, and chemical plants. ODS can also be naturally produced, such as by large volcanic eruptions, but their effect on ozone depletion is usually short-lived.
The production and import of ODS are controlled by the Montreal Protocol on Substances that Deplete the Ozone Layer. Many countries have taken actions to reduce ODS emissions and mitigate their impact on the ozone layer. Despite these efforts, ozone depletion continues to occur over latitudes that include North America, Europe, Asia, and parts of Africa, Australia, and South America.
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Ground-level ozone is harmful to human health
Ozone is a gas composed of three oxygen atoms. It exists in two layers of the atmosphere: the stratosphere (upper layer) and the troposphere (ground level). Stratospheric ozone forms a protective layer that acts as a shield against the sun's harmful ultraviolet rays. Ground-level ozone, on the other hand, is considered harmful to human health and the environment.
Ground-level ozone, also known as tropospheric ozone, is a major air pollutant and a key component of smog. It is not directly emitted into the atmosphere but is formed through chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs). These reactions occur when pollutants from cars, power plants, industrial boilers, refineries, and chemical plants interact in the presence of sunlight. Ground-level ozone pollution is more prevalent in urban areas, particularly during hot summer days.
The negative health effects of ground-level ozone pollution are well-documented. Even relatively low levels of ozone in the air can have detrimental consequences for human health. People with asthma are particularly vulnerable to the impacts of ground-level ozone. Long-term exposure to ozone can aggravate asthma symptoms and is likely a contributing factor in asthma development. The airway lining becomes inflamed, similar to skin inflammation caused by sunburn. Additionally, ozone can cause the muscles in the airways to constrict, trapping air in the alveoli, further compromising respiratory function.
Children are considered to be at high risk from exposure to ground-level ozone. Their developing lungs and increased likelihood of outdoor activity during periods of high ozone concentrations put them at a disadvantage. Older adults and people who are active outdoors, including outdoor workers, are also more susceptible to the harmful effects of ground-level ozone. Furthermore, individuals with certain genetic characteristics and those with reduced intake of specific nutrients, such as vitamins C and E, face greater risks from ozone exposure.
The World Health Organization (WHO) has established air quality guidelines that provide global guidance on acceptable levels of air pollutants, including ozone. These guidelines have been updated to drastically reduce the thresholds for safe levels of air pollution. Unfortunately, the vast majority (99%) of the world's population currently breathes air that is considered unsafe. To address this issue, the Environmental Protection Agency (EPA) in the United States has implemented national and regional rules to reduce emissions of pollutants that contribute to ground-level ozone. These efforts are aimed at helping state and local governments meet the Agency's national air quality standards and protect the health of their citizens.
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The ozone layer protects against UV radiation
Ozone is a molecule that contains three oxygen atoms. It is a colourless and highly reactive gas that can be found throughout all layers of the Earth's atmosphere. Ozone is one of the six common air pollutants identified in the Clean Air Act. The EPA calls these "criteria air pollutants" because their levels in outdoor air need to be limited based on health criteria.
Most ozone (about 90%) is found in the stratosphere, which is the second layer of the Earth's atmosphere, about 10-16 kilometres above Earth's surface. The stratosphere gets warmer at higher altitudes due to ozone absorbing ultraviolet radiation. The ozone layer absorbs 97 to 99 percent of the Sun's medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which otherwise would potentially damage exposed life forms near the surface. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface.
In the troposphere, the lowest layer of the atmosphere, ozone is a powerful greenhouse gas and air pollutant that harms human health and ecosystems. It is also a major component of smog. Tropospheric ozone is the product of the atmospheric reaction of a number of precursor pollutants, which have both natural and human sources. Human activities have damaged the ozone layer, causing what is sometimes called a "hole in the ozone". This hole is diminishing, but ozone depletion is not limited to the area over the South Pole. Research has shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America.
Ozone depletion is caused by ozone-depleting substances (ODS) such as chlorofluorocarbons (CFCs) which are released by industry. ODSs affect stratospheric ozone concentrations worldwide. ODSs are very stable and do not dissolve in rain, so there are no natural processes that remove them from the lower atmosphere. Gaseous CFCs can deplete the ozone layer when they slowly rise into the stratosphere, are broken down by strong ultraviolet radiation, release chlorine atoms, and then react with ozone molecules. One chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere.
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Ozone depletion is a global issue
Ozone (O3) is a gas that exists in two layers of the Earth's atmosphere: the stratosphere and the troposphere. The stratosphere, located about 9 to 18 miles (15 to 30 km) above the Earth's surface, contains the ozone layer, which plays a crucial role in protecting life on Earth by absorbing the sun's harmful ultraviolet (UV) radiation. The troposphere, on the other hand, extends from the ground level up to 10 km. While ozone in the stratosphere is beneficial, ozone in the troposphere is considered a pollutant and a strong greenhouse gas that negatively impacts human health and ecosystems.
Ozone depletion refers to the reduction in the total amount of ozone in the Earth's upper atmosphere, particularly in the stratosphere. This depletion has been a significant global concern since it was first discovered in 1982. The issue gained widespread attention in 1985 when a substantial "ozone hole" over Antarctica was reported. This ozone hole is not a literal hole but a region with extremely low ozone concentrations. While the Antarctic ozone hole has a relatively small impact on global ozone levels, it sparked fears of potential severe consequences, such as increased skin cancer rates, cataracts, damage to plants, and harm to marine life.
The primary cause of ozone depletion is human activity, specifically the release of ozone-depleting substances (ODS). ODS include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and halons, which are commonly found in refrigerants, solvents, propellants, and foam-blowing agents. These chemicals are transported into the stratosphere, where they release chlorine atoms that react with and destroy ozone molecules. Natural processes, such as large volcanic eruptions, can also indirectly contribute to ozone depletion by producing aerosols that enhance the destructive capacity of CFC-based chlorine.
International efforts to address ozone depletion have been underway since the adoption of the Montreal Protocol in 1987, which banned the production of CFCs and other ozone-depleting chemicals. As a result, ozone levels have stabilized and are projected to recover fully by mid-century. However, the complex interaction of chemical and meteorological factors continues to influence the evolution of the ozone hole, and the increasing frequency and intensity of wildfires driven by global warming pose additional risks to the fragile ozone layer.
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Frequently asked questions
The troposphere, or ground-level layer, contains ozone pollution.
The troposphere is the lower atmosphere, from the ground up to 10km.
Ozone (O3) is a gas that exists in the troposphere as a powerful greenhouse gas and air pollutant.
The stratosphere is the upper layer of the atmosphere, where ozone forms a protective layer that shields Earth from the sun's harmful ultraviolet rays.
