Ozone: Definition, Composition and Risks for Health

Ozone is a gas made up of three oxygen atoms. It occurs naturally in the Earth's upper atmosphere where it absorbs harmful ultraviolet radiation from the sun. Ozone is essential for life on Earth as it shields us from this radiation. However, ozone present in the lower atmosphere can be harmful to human health.

Ozone is a highly reactive gas due to its molecular structure. The reactivity of ozone makes it useful for industrial applications like water treatment. However, this also means ozone can irritate and damage the human respiratory system.

The ozone molecule is composed of three oxygen atoms bound together (O3). Ozone is an allotrope of oxygen, meaning it has the same chemical composition but a different molecular structure to molecular oxygen (O2).

Ozone is produced in the upper atmosphere when ultraviolet radiation causes an oxygen molecule (O2) to split into two separate oxygen atoms (O). These individual oxygen atoms then combine with intact O2 molecules to form ozone (O3). This is called the ozone-oxygen cycle.

Ozone present in the Earth's lower atmosphere can pose a risk to human health. Ground-level ozone is created by chemical reactions between oxides of nitrogen, carbon monoxide, methane, and other volatile organic compounds in the presence of sunlight.

Inhaling ground-level ozone can damage the lungs and cause respiratory issues. Health effects include:

• Coughing, irritation and pain when taking deep breaths
• Aggravated asthma symptoms
• Increased susceptibility to infections
• Lung inflammation and possible long-term lung damage with repeated exposure

Certain populations like children, the elderly and those with existing respiratory conditions are at higher risk from ozone exposure. Monitoring ground-level ozone and limiting activities during high ozone days can reduce the health impacts.

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What is Ozone?

Ozone is a gas composed of three oxygen atoms bonded together (O3). It is an allotrope of oxygen, meaning it has the same chemical composition as oxygen (O2) but a different molecular structure.

There are two major forms of ozone based on where it is found in the atmosphere. Stratospheric ozone occurs naturally in the upper atmosphere 10-50km above the Earth's surface. Tropospheric ozone is found at ground-level and is produced by chemical reactions driven by human pollution.

Stratospheric ozone plays a vital role absorbing ultraviolet radiation from the sun before it reaches Earth. This ozone layer protects life by blocking harmful UV rays.

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What is Ozone Composed of?

The ozone molecule consists of three oxygen atoms bonded together into a V-shape. The bonding creates an unstable, reactive molecule due to the presence of different oxygen structures within it.

Ozone forms in the upper atmosphere when ultraviolet light splits apart an oxygen molecule (O2) into two individual oxygen atoms (O). These single oxygen atoms then bond with intact O2 molecules to form ozone (O3).

Unlike molecular oxygen which contains a stable double bond (O=O), ozone has a weakened single bond in the middle (O-O) and double bond on the ends. This makes the molecule highly reactive.

Ozone is an allotrope of oxygen along with molecular oxygen (O2) and atomic oxygen (O). Allotropes have the same chemical element but different structures. Ozone's reactivity gives it different properties to O2 despite having the same oxygen composition.

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What are the Major Sources of Ground-Level Ozone?

Ground-level ozone is not directly emitted into the air but is created by chemical reactions between precursor pollutants in the presence of sunlight. This process is known as photochemical smog.

The main sources leading to ozone formation are:

• Vehicle exhaust emissions like nitrogen oxides (NOx) and volatile organic compounds (VOCs).
• Industrial facilities that release NOx, VOCs and carbon monoxide.
• Fossil fuel power stations emitting NOx.
• Biogenic VOCs emitted from vegetation.
• Methane from livestock manure and landfills.

These ozone precursors undergo reactions driven by ultraviolet rays from the sun to produce ground-level ozone pollution.

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What are the Harmful Effects of Ozone?

When inhaled, ozone can irritate and inflame the lining of the lungs. Exposure to ground-level ozone has a number of harmful effects:

• Irritation of the airways causing coughing, throat irritation and pain when breathing deeply.
• Worsening of asthma symptoms like chest tightness and wheezing.
• Increased risk of respiratory infections due to lung inflammation.
• Permanent lung damage with repeated exposures.

Ozone exposure also harms vegetation, reducing plant growth and crop yields. It damages foliage, causing visible foliar injury on sensitive plants. Long-term exposure leads to reduced forest growth and damage to ecosystems.

High ozone levels are toxic to a wide variety of wildlife. It can cause mortality and reduced reproduction in sensitive species.

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What is the Law to Protect the Stratospheric Ozone Layer?

There are international treaties and national laws aimed at protecting the stratospheric ozone layer from ozone-depleting substances (ODS).

The key international agreement is the Montreal Protocol which controls the production and consumption of nearly 100 man-made ODS like chlorofluorocarbons (CFCs), halons, and hydrochlorofluorocarbons (HCFCs). It was agreed in 1987 and has been strengthened over time.

Australia ratified the Montreal Protocol in 1989. The Ozone Protection and Synthetic Greenhouse Gas Management Act 1989 implements Australia's obligations under the Protocol. It phases out ODS, regulates imports/exports, and bans ODS uses.

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What is Australia Doing to Reduce Ozone Pollution?

Australia has implemented various measures to reduce ground-level ozone pollution and meet its Montreal Protocol commitments:

• Phasing out ODS like CFCs, halons and HCFCs in line with Protocol schedules.
• Requiring licences for importing or exporting ODS.
• Banning ODS uses in products like refrigerants, foams and aerosol propellants.
• National vehicle emissions standards to reduce NOx and VOCs from transport.
• Emissions limits for industry and fuel standards to lower NOx and VOC pollution.
• National air quality standards and environment protection policies for ozone.
• Public education campaigns encouraging sun protection and responsible use of ODS.
• Monitoring networks to detect ODS and provide ozone warnings.