Ozone – from atmosphere to health and technology

Ozone is a chemical compound that plays a crucial role for life on Earth. It occurs naturally in the atmosphere and acts as a protective layer against the sun's harmful ultraviolet rays. At the same time, ground-level ozone can be a health hazard and an important indicator of air pollution. Ozone is thus a dual-natured substance – a lifesaver in the stratosphere and a potential health hazard in the troposphere.

Ozone's chemical properties and formation processes

Ozone (O₃) is an allotropic form of oxygen – a molecule consisting of three oxygen atoms, in contrast to the more common diatomic oxygen molecule (O₂) that we breathe. The extra oxygen atom gives ozone unique chemical properties, particularly its high reactivity. It is this reactivity that makes ozone both a life-saving and potentially harmful gas, depending on its concentration and location in the atmosphere.

Molecular structure and stability

The ozone molecule is bent, with an angle of approximately 117°, which gives it dipolar properties. This structure imparts a polarity that makes the molecule highly reactive with other substances. Ozone is unstable at room temperature and degrades relatively quickly into oxygen, especially in the presence of heat or catalytic substances.

The formula O₃ shows that ozone has a resonance system: the electrons are delocalized across the molecule, which somewhat stabilizes the structure, but not enough to make ozone particularly long-lived. In practice, ozone breaks down within minutes to hours depending on the surrounding environment.

Formation of ozone in the stratosphere (the good ozone)

In the stratosphere, between approximately 15 and 35 kilometers altitude, ozone is formed through photochemical reactions involving the sun's ultraviolet (UV) light. The process is called the Chapman Cycle, named after the British chemist Sydney Chapman. It can be simply described in three steps:

• Photolysis of oxygen molecules (O₂): UV light with wavelengths below 240 nm breaks down oxygen molecules:
  O₂ + UV radiation → 2O
• Reaction between atomic oxygen and oxygen molecules: The free oxygen atoms react with other oxygen molecules:
  O + O₂ → O₃
• Breakdown of ozone by UV light: Ozone absorbs UV light and breaks down:
  O₃ + UV radiation → O₂ + O

This creates a dynamic equilibrium where ozone is constantly created and destroyed, but in sufficient quantity to form the so-called ozone layer. This layer absorbs 97–99% of the sun's UVB radiation, protecting living organisms from genetic damage and skin cancer.

Ozone in the troposphere (the harmful ozone)

At ground level, in the troposphere (0–15 km), ozone is primarily formed by human activity. Here, it is a secondary pollutant that arises in photochemical smog processes – a complex mixture of chemical reactions involving nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and sunlight. The main reactions occur in several steps:

1. Emissions of NO and VOCs: From cars, industries, and combustion.
2. Reaction between NO₂ and sunlight: NO₂ breaks down by UV light:
   NO₂ + UV → NO + O
3. Formation of ozone: The free oxygen reacts with oxygen molecules:
   O + O₂ → O₃

Unlike stratospheric ozone, this ground-level ozone has no protective functions. On the contrary, it is harmful to the respiratory tract and negatively affects plants by impairing photosynthesis.

Natural sources of ozone

Even without human influence, a certain amount of ozone is formed in the troposphere through:

• Thunderstorms: Lightning generates high temperatures that break down nitrogen and oxygen, creating NOₓ and enabling ozone formation.
• Biogenic VOCs: Plants naturally emit volatile organic compounds that can participate in photochemical reactions.
• Transport from the stratosphere: Ozone can be transported down to the troposphere via convective and turbulent processes.

Lifespan and dispersion

The lifespan of ozone varies greatly depending on altitude and surroundings. In the stratosphere, ozone can exist for several weeks, while ground-level ozone often breaks down within hours. However, tropospheric ozone can be transported over long distances via atmospheric currents, making it a regional and global environmental problem.

Catalytic degradation – the mechanism of the ozone hole

Chlorofluorocarbons (CFCs) in the stratosphere: During the 1980s, it was discovered that CFCs release chlorine into the stratosphere. These chlorine atoms act as catalysts and destroy ozone through cyclical reactions.

First reaction – ozone degradation:
Cl + O₃ → ClO + O₂

Second reaction – re-formation of chlorine atom:
ClO + O → Cl + O₂

A single chlorine atom can destroy up to 100,000 ozone molecules before it is removed. This is what caused the famous ozone hole over Antarctica, which is, however, shrinking thanks to measures like the Montreal Protocol.

The importance of ozone for the environment and ecosystems

Ozone has a profound impact on the Earth's environment and ecosystems, but the effects vary depending on where in the atmosphere the ozone is located. In the stratosphere, ozone acts as a protective barrier against harmful ultraviolet radiation. In the troposphere, however, ozone acts as a powerful plant toxin and contributes to climate change. This duality makes ozone a substance that both supports and threatens the ecological system.

The ozone layer's role in protecting ecosystems

The stratospheric ozone layer is an essential component in preserving life on Earth. By absorbing most of the sun's ultraviolet (UV-B and UV-C) rays, it prevents genetic mutations, skin cancer, eye damage, and damage to plant DNA.

Without an intact ozone layer, the following environmental effects would be inevitable:

• Increased UV radiation harms phytoplankton, which are the base of marine food chains.
• Photosynthesis is inhibited in crops and trees, which can reduce global food production.
• Amphibians and other UV-sensitive species suffer from skin damage, egg mortality, and impaired reproduction.

Reductions in the ozone layer, such as the ozone hole over Antarctica, therefore have direct effects on marine and terrestrial ecosystems. In the Arctic and Antarctic, researchers have observed reduced plankton growth during periods of high UV levels, threatening the entire marine ecosystem's food webs.

Effects of ground-level ozone on plant life

Unlike the protective ozone layer, ground-level ozone is a serious environmental pollutant. It is formed by photochemical reactions between nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of sunlight. This type of ozone has been shown to be directly toxic to plants.

Damage to plant life from ozone includes:

• Visible damage to leaf surfaces: Necrosis, yellowing, and spots that reduce the plant's photosynthetic capacity.
• Impaired growth: Ozone disrupts cellular respiration and inhibits the function of stomata, leading to reduced carbon dioxide uptake.
• Reduced agricultural yields: Particularly sensitive crops include soybeans, wheat, potatoes, and corn. Yields can decrease by up to 20–30% in ozone-rich environments.

Field studies have shown that ozone can affect natural ecosystems by altering competitive relationships between plant species. For example, ozone-sensitive species tend to be displaced by more tolerant ones, which can eventually change the composition of entire ecosystems.

Effects on wildlife

Ozone affects animals in several indirect ways:

• Loss of food sources: If vegetation decreases due to ozone damage, grazing animals such as deer, rabbits, and insects are affected.
• Respiratory damage: Animals, especially small mammals and birds that live near the ground, may experience respiratory problems at high ozone levels.
• Chain effects in the food web: If ozone affects primary producers like plants and plankton, the entire trophic structure – from insects to predators – is affected.

Even in aquatic environments, ozone plays a role. Elevated UV levels during ozone depletion can negatively affect fish eggs and larval development. Some fish species, especially those that spawn in shallow waters, are particularly vulnerable to UV damage.

Ozone as a climate-influencing substance

Ground-level ozone is also a potent greenhouse gas. Although its global warming potential (GWP) is lower than carbon dioxide, its effect per molecule is significantly stronger. Ozone contributes to the warming of the troposphere and therefore affects the global climate.

According to the IPCC (Intergovernmental Panel on Climate Change), ozone is one of the most important short-lived climate-forcing agents, along with methane and black carbon. Reducing ground-level ozone would thus have dual benefits: improved air quality and climate benefits.

Ozone and ecosystem function and biodiversity

Ecosystems are dynamic systems based on sensitive interactions between species, climate, and chemical cycles. Ozone disrupts these systems at several levels:

• Disruption of mutualistic relationships: For example, ozone damage to plants can lead to pollinating insects no longer being attracted by the scents or colors of flowers.
• Disruption of the nitrogen cycle: Ozone affects soil organisms involved in nitrogen fixation and mineralization, which in turn affects soil fertility.
• Changes in ecosystem stability: Ozone-related species loss can reduce ecosystems' resilience to other stressors such as drought, disease, and temperature changes.

Research shows that long-term ozone exposure can lead to so-called ecosystem shifts – where entire habitats are replaced by others, often less biodiverse, systems.

Interaction between ozone and other environmental toxins

Ozone also interacts with other air pollutants and environmental toxins. For example:

• In the presence of sulfur oxides (SOₓ), ozone can contribute to the formation of acid rain.
• Together with particles from combustion, ozone can form smog, which has serious effects on both the environment and health.
• Ozone can oxidize metals and organic compounds, which in turn affects water bodies and soil quality when these substances are removed from the atmosphere.

Ozone's impact on human health and societal consequences

Ozone is an invisible but powerful air pollutant that significantly affects human health, especially at ground level. Even at low concentrations, ozone can cause or worsen a range of health problems – from respiratory irritation to chronic lung diseases. The societal effects are equally serious: increased healthcare costs, reduced work productivity, and the need for political and technological measures.

How people are exposed to ozone

Exposure occurs primarily through inhalation. Since ozone is a gas, it comes into direct contact with mucous membranes in the eyes, nose, throat, and lungs. Ozone levels can become high particularly on sunny, warm days with low air circulation – especially in urban environments and areas with heavy traffic or industrial activity.

Groups with high exposure include:

• Urban residents in densely populated areas
• Children playing outdoors
• Athletes and people who work physically outdoors
• The elderly and people with lung diseases such as asthma or COPD

Measurements show that ozone levels are often highest in the afternoon, especially during the summer months. This means that schoolchildren, commuters, and the elderly in cities run the highest risk of exposure.

Health risks associated with ozone

Ozone is a powerful oxidant that reacts with cells in the respiratory tract. It causes inflammation, irritation, and cellular damage. Exposure can lead to both acute and chronic health effects.

Acute effects:

• Coughing, difficulty breathing, and throat irritation
• Impaired lung function during physical activity
• Eye irritation and runny nose
• Increased sensitivity to allergens

Chronic effects:

• Exacerbation of asthma and other lung diseases
• Increased risk of pneumonia and bronchitis
• Long-term reduction in lung capacity, especially in children
• Possible effects on the cardiovascular system (research ongoing)

Studies have shown that even short-term exposure to ozone levels above 70 ppb (parts per billion) can affect lung function in sensitive groups. Long-term exposure has been linked to increased mortality from cardiopulmonary diseases.

Health effects in particularly vulnerable groups

Children: Because their lungs are still developing, children are particularly sensitive. They also breathe faster than adults, which increases exposure.

Elderly: Often with weakened immune systems and underlying diseases, making them more susceptible to ozone damage.

Asthmatics: Studies show that ozone exposure can significantly increase the number of asthma attacks, especially among children.

Physically active individuals: Ozone levels in the lungs increase with breathing rate. People who exercise or work outdoors on ozone-rich days therefore run a higher risk.

Economic and societal consequences

Ozone pollution has significant economic costs. According to the OECD and WHO, the global costs related to ground-level ozone amount to hundreds of billions of dollars annually.

Direct costs:

• Increased burden on healthcare (visits, medications, hospital stays)
• Loss of productivity due to sick leave
• Loss of quality of life and premature death

Indirect costs:

• Reduced life expectancy and work performance
• Strain on social security systems
• Investments in air quality monitoring and action programs

In the EU, several hundred thousand premature deaths each year are estimated to be related to air pollution, with ozone being a significant component. According to the European Environment Agency (EEA), ozone annually leads to up to 20,000 deaths in the EU, in addition to millions of cases of illness.

Measures to protect the population

Authorities in many countries have introduced guidelines, limit values, and warning systems for ozone. Examples:

• Environmental zones in cities: Restrict traffic during periods of high ozone levels.
• Ozone warnings: Published in real-time via apps, news media, and signs.
• Recommendations: Such as avoiding outdoor exercise on warm afternoons or closing windows.

The WHO's guideline value for an 8-hour average ozone concentration is 100 µg/m³. Many countries, like Sweden, have slightly lower limit values to protect sensitive groups.

Future risks and trends

Climate change risks exacerbating the ozone problem. Increased temperatures and more solar radiation increase the production of ground-level ozone. At the same time, more heatwaves and longer summer seasons can mean longer exposure times for the population.

Furthermore, urbanization, increased traffic, and global economic growth lead to higher emissions of ozone-forming substances – especially in rapidly growing developing countries.

Ozone's industrial applications and technological uses

Despite its negative effects at ground level, ozone is also a very useful gas in several industrial and technical contexts. Thanks to its strong oxidizing capacity, ozone is an effective agent for disinfection, odor removal, sterilization, and chemical conversion. It is used in everything from water treatment plants to the food industry, healthcare, and advanced manufacturing processes.

Water purification and wastewater treatment

Ozone is widely used in water purification, where it acts as a powerful oxidizing and disinfecting agent. It can replace chlorine in many contexts, with several advantages:

• Kills bacteria, viruses, and parasites more effectively than chlorine.
• Oxidizes organic pollutants, including pesticides, pharmaceutical residues, and phenols.
• Improves smell and taste in drinking water by breaking down malodorous substances.
• Reduces byproduct formation, as ozone does not form trihalomethanes (THMs) as chlorine does.

Facilities in Europe and Japan have long used ozone in municipal water treatment plants. In Sweden, among others, the water treatment plants in Gothenburg and Uppsala use ozone in their purification processes.

Ozone is also used in advanced wastewater treatment, particularly to break down micropollutants that are not captured in traditional biological stages.

Food industry

In the food industry, ozone is an approved and effective method for disinfecting both food and equipment. Since ozone quickly breaks down into oxygen, it leaves no residue – an important advantage from both food safety and environmental perspectives.

Applications include:

• Disinfection of fruits and vegetables to remove bacteria and mold spores.
• Extension of shelf life by reducing microbial growth.
• Sanitation of surfaces and production equipment in dairies, slaughterhouses, and bakeries.
• Oxygen enrichment of cold rooms, which reduces mold growth.

In the USA, ozone is classified as GRAS (Generally Recognized As Safe) by the FDA for use in food production.

Odor removal and air quality

Ozone is often used in systems to remove odors in:

• Hotel rooms
• Smoke-damaged residences
• Garbage rooms
• Vehicles (especially in car detailing)

By reacting with odor molecules and breaking their chemical bonds, the source of the odor is eliminated. It is used both as permanently installed systems and as portable "ozone generators". However, caution is required here, as high concentrations of ozone are hazardous to health.

Medical use of ozone

Ozone therapy is a controversial but growing method in alternative medicine and, to some extent, conventional medicine, particularly in countries such as Germany, Italy, and Russia. The applications are many:

• Sterilization of surgical instruments and surfaces
• Disinfection of wounds (especially difficult-to-heal infections)
• Ozone injections for joint problems and pain relief
• Ozonated water for mouthwash and dental treatment

Ozone's antimicrobial properties make it particularly effective in dental surgery and the treatment of periodontitis. It is also used in hyperbaric therapy, where ozone is mixed with oxygen and applied in various ways to the body. However, this use is controversial, as scientific support varies depending on the method and diagnosis.

Textile and paper industry

In the textile industry, ozone is used to:

• Bleach cotton and other natural fibers without the use of chlorine-based chemicals.
• Remove dyes from wastewater in dyeing processes.

The paper industry uses ozone in so-called TCF (Totally Chlorine Free) bleaching, where it replaces chlorine in pulp and paper production. This significantly reduces environmental impact.

Electronics and semiconductor industry

In the manufacturing of microchips and other advanced components, ozone is used for:

• Cleaning of surfaces at a molecular level
• Oxidation of silicon layers
• Disinfection of process environments in clean rooms

Since ozone is highly reactive but rapidly degradable, it can be used for extremely thorough cleaning without leaving chemical residues.

Energy sector and environmental technology

Ozone technology is used in combination with other methods, such as UV light and activated carbon, in advanced environmental technology to:

• Break down pharmaceutical residues in water
• Oxidize persistent industrial chemicals
• Pre-treat biogas and exhaust gases from combustion

There are also experimental uses of ozone in exhaust gas purification from diesel vehicles and as part of CO₂ reduction strategies.

Ozone regulation, international agreements, and environmental policy

The regulation of ozone-related issues is one of the most comprehensive and successful examples of global environmental cooperation. Both the protection of the stratospheric ozone layer and the control of ground-level ozone have required coordination between science, politics, industry, and civil society. This section outlines the history, key agreements, national legislation, and future policy challenges.

Discovery of the ozone problem

In the 1970s, scientists such as Paul Crutzen, Mario Molina, and Sherwood Rowland began to raise alarm that chlorofluorocarbons (CFCs) could deplete ozone in the stratosphere. Their research showed that CFCs, used in refrigerants, spray cans, and foam plastics, rose into the atmosphere where they released chlorine that destroyed ozone molecules.

When the so-called ozone hole over Antarctica was discovered by British scientists in 1985, it led to global attention and pressure for action.

The Montreal Protocol – a milestone in environmental history

The Montreal Protocol, signed in 1987, is the most influential agreement for the protection of the ozone layer. It entered into force in 1989 and has since been modified several times to cover more substances and technologies.

The agreement aims to:

• Phase out ozone-depleting substances, such as CFCs, halons, HCFCs, and brominated solvents.
• Support developing countries with technology transfer and financial assistance.
• Monitor and report emissions and levels of ozone-depleting substances.

Results:

• More than 99% of regulated substances have been phased out.
• The ozone layer has shown clear signs of recovery.
• The Montreal Protocol has prevented millions of cases of skin cancer and eye damage.

Scientists estimate that the ozone layer could be restored to 1980 levels around 2050–2065 if the protocol is strictly followed.

Kigali Amendment – the link between ozone and climate

In 2016, the Kigali Amendment to the Montreal Protocol was adopted. It focuses on phasing out HFCs (hydrofluorocarbons), which have replaced CFCs but are also potent greenhouse gases. Although HFCs do not deplete ozone, they contribute to climate change. The Kigali Amendment is thus an example of how ozone policy and climate policy are beginning to converge.

Regulation of ground-level ozone – a more difficult problem

Unlike stratospheric ozone, where specific chemicals could be identified, the problem of ground-level ozone is more complex. Ozone is formed as a secondary pollutant, meaning it arises from reactions between other emissions – primarily nitrogen oxides (NOₓ) and VOCs – in the presence of sunlight.

EU's Air Quality Directive includes limit values for ozone:

• Target value: 120 µg/m³ as an 8-hour average (not to be exceeded more than 25 times per calendar year).
• Information threshold: 180 µg/m³ – the public should be informed.
• Alert threshold: 240 µg/m³ – specific measures must be taken.

The US EPA (Environmental Protection Agency) has a national limit value of 70 ppb (parts per billion) as an 8-hour average.

National strategies – examples from Sweden

Through the Swedish Environmental Protection Agency and the Environmental Code, Sweden has introduced several policy instruments to reduce ozone-forming emissions:

• Environmental zones in cities where older vehicles are prohibited.
• Requirements for purification technology in industries and energy facilities.
• Taxes and fees on VOC emissions and NOₓ emissions.
• Air quality monitoring through environmental monitoring programs such as Luftweb.

Despite this, ozone limit values are still regularly exceeded in certain parts of the country, especially during warm summers.

International challenges and justice aspects

The ozone problem is global in nature, but its consequences and solutions are not evenly distributed:

• Developing countries have less technical and economic capacity to implement clean technology.
• Export of old vehicles from richer to poorer countries increases emissions in the Southern Hemisphere.
• Climate change risks further worsening the ozone situation, especially in tropical areas.

The Montreal Protocol has been successful precisely because it includes mechanisms for justice: technical assistance, financial support, and flexibility for low-income countries. This model is now seen as a role model for other global environmental agreements.