Ozone – Friend or Foe?
Let’s find out…


Ozone has been a big buzzword over the years. We hear about the ozone layer, the fact that it is getting thinner, and the importance of protecting it and us. We hear about the “hole in the ozone” and the fact that it is heavily depleted. While the depletion has stopped recently thanks to regulations put in place, it will still take at least another 50 years to replenish. We hear about ozonation and its benefits in our surrounding environment and industrial use.  

Nowadays though, especially in summerwith rising temperatures, we hear in the weather reports about issued ozone alarms. Unhealthy air quality raises red flags, and ozone emissions need to be limited, because it can be dangerous to our health.

So how can ozone be both, good and bad, friend and foe? Lets take a closer look.

What exactly is ozone?

Ozone-oxygen cycle | Renewal of ozone in the ozone layer

Oxygen makes up about 21% of our atmosphere. A single atom of oxygen is unstable – it wants to combine with something else. That is why oxygen is almost always found in pairs of O2. Under certain conditions however, a triple molecule with the chemical formula Omay form: Ozone. It is a pale blue gas with a distinctively pungent smell. Most people can detect a concentration of about 0.02 ppm (40 μg / m³) or less in the air. It has a very specific sharp odor, somewhat resembling chlorine bleach.  The total mass in the atmosphere is about 3 billion metric tons. That may seem like a lot, but it is only 0.00006 percent of the atmosphere. 2

Ozone is a highly reactive gas, and while its unstable at high concentrations, it can have valuable benefits. Its half-life varies with conditions such as temperature, humidity, and air movement. 

What determines if ozone is friend or foe? Location.

Ozone is both a natural and a man-made product. It occurs in the Earth’s upper atmosphere (the stratosphere) and lower atmosphere (the troposphere).  Depending on where it is in the atmosphere, it affects life on Earth in either good or bad ways. 

Variation of ozone concentration with altitude
The concentration of ozone varies with altitude. Peak concentrations, an average of 8 molecules of ozone per million molecules in the atmosphere, occur between an altitude of 30 and 35 kilometers. Source:NASA Ozone Watch3

Ozone in the stratosphere (our friend)

Ninety percent of Earth’s ozone is found in the stratosphere. This is the second, so-called ozone layer of the Earth’s atmosphere, between 10 and 50 km above the Earth’s surface. It is partially responsible for the deep blue-violet beauty of the twilight sky. 

Ozone in the stratosphere is formed naturally in a two-step reactive process. Solar ultraviolet (UV) radiation interacts with molecular oxygen (O2). In the first step sunlight breaks apart an oxygen molecule to form two separate oxygen atoms. In the second step, each atom then undergoes a binding collision with another oxygen molecule to form an ozone molecule. These reactions occur continually. As a result, the largest production occurs in the tropical stratosphere. Chemical reactions with reactive gases such as hydrogen, nitrogen oxides, as well as chlorine and bromine provide a natural balance.

Ozone Layer

Like a sponge, the stratospheric ozone layer absorbs about 98% of the harmful ultraviolet radiation from the sun. Thereby it is shielding the Earth’s surface. It screens most energetic, UV-C, radiation, most of the UV-B radiation, and about half of the UV-A radiation.  UV-B and UV-C are so energetic that they can damage life on earth. Consequences of UV-B or UV-C exposure are sunburns. UV-B also leads to an increased risk of cancer and has a variety of other effects on our health.4 At this altitude and function, ozone is vital to life on earth.

Absorbation of solar ultraviolet radiation by ozone in the atmosphere
Solar ultraviolet radiation is largely absorbed by the ozone in the atmosphere—especially the harmful, high-energy UV-a and UV-b. The graph shows the flux (amount of energy flowing through an area) of solar ultraviolet radiation at the top of the atmosphere (top line) and at the Earth’s surface (lower line). The flux is shown on a logarithmic scale, so each tick mark on the y-axis indicates 10 times more energy. Source: NASA Ozone Watch3

Ozone Hole

Despite that fact that ozone in the stratosphere is continuously produced and removed by natural processes, industrial “man-made” pollutants add “new leaks to the bucket,” thereby reducing stratospheric ozone levels. Ozone-depleting compounds contain various combinations of the chemical elements chlorine, fluorine, bromine, carbon, and hydrogen and are often described by the general term halocarbons. The compounds that contain only chlorine, fluorine, and carbon are called chlorofluorocarbons, usually abbreviated as CFCs. CFCs, carbon tetrachloride, and methyl chloroform are important human-produced ozone-depleting gases that have been used in many applications including refrigeration, air conditioning, foam blowing, cleaning of electronics components, and as solvents.  

Although CFCs are nonreactive in the troposphere, they can be slowly transported to the stratosphere. There they break down into molecules such as chlorine monoxide (ClO), which depletes ozone by transforming it back into oxygen gas. Another important group of human-produced halocarbons is the halons, which contain carbon, bromine, fluorine, and (in some cases) chlorine and have been mainly used as fire extinguishants.4

What happens when the ozone layer is degraded and thus considerably thinner in certain places is, unfortunately, all too well known and scientifically documented, since the ozone hole has formed over the Antarctic. The name is a bit misleading in that the ozone layer does not have a real hole in it, but has become much thinner, depleted.

View of the south polo from NASA's TOMS
Image above: View of the South Pole from NASA’s TOMS (Total Ozone Mapping Spectrometer) satellite. Blue and green indicate relatively large amounts of ozone. Red and yellow mark the “ozone hole,” an area of decreased ozone. Credit: NASA5

Prohibitions of various chemicals, especially the group of CFCs, have led to the fact that it is now said that the ozone layer is slowly regenerating, with the emphasis on slow to lay. Current forecasts assume more than fifty years before conditions could again be reached as in the middle of the last century.6

The reduced ozone levels have increased the amount of harmful ultraviolet radiation reaching the Earth’s surface. When scientists talk about the ozone hole, they are talking about the destruction of stratospheric, the “good”, ozone.

So if this is the good one, where do we find the bad?

Ozone in the troposphere (our foe)

At the Earth’s surface, ozone comes into direct contact with life forms and displays its destructive side (hence, it is often called “bad ozone”). Because it reacts strongly with other molecules, high levels are toxic to living systems. 

In the lower atmosphere near earth’s surface (the troposphere), ozone is produced by chemical reactions involving naturally occurring gases and gases from pollution sources, primarily from photochemical reactions between two major classes of air pollutants, volatile organic compounds (VOC) and nitrogen oxides (NOx). These reactions partially depend upon the presence of heat and sunlight, resulting in higher ambient ozone concentrations in summer months and we typically experience it as “smog” or haze. This can be incredible dangerous to our health and can cause a range of physical ailments and significantly affect performance. This is when ozone alarms are active.

Health risk evaluation

Ozone toxicity occurs in a continuum in which higher concentrations, longer exposure duration and greater activity levels during exposure cause greater effects. Short-term acute effects include respiratory symptoms, pulmonary function changes, increased airway responsiveness and airway inflammation. Ozone exposure has been reported to be associated with increased hospital admissions for respiratory causes and exacerbation of asthma. 6

Ozone and how we use it for our benefit

Even though ozone is toxic, it is being used in various ways to our benefit. Perhaps less well known, it is one of the strongest and most environmentally friendly oxidants, thus one of most powerful disinfectants in the world, second only to Fluorine. It is three thousand times more potent than chlorine. For the past 100 years, Ozone has been utilized commercially for odor reduction (think of ozonation in your car to get rid of unwanted smells) and water purification (drinking water treatment for sterilization). This substance is a very effective anti-bacterial, germicidal and fungicidal agent, converting organic material into their base compounds. Its high oxidation potential makes ozone very useful and can be widely used for cleaning clothes, as well as cleaning agent and killing mold and bacteria in air streams.

Everything in moderation

Ozone, as many other things, can be very useful and valuable to human health, depending on its location and its concentration. In low-level concentrations, it is found everywhere in our environment. It is generated naturally, even in the troposphere, for example in the vicinity of strongly moving water such as a waterfall or in a thunderstorm formed by the electric discharge of the lightning. It is no coincidence that the air near a waterfall or after a thunderstorm is perceived as fresher, because ozone has the aforementioned strong cleaning effect and is able to break down pollutants (odors) and microorganisms.

However, ozone remains a toxic gas with vastly different chemical and toxicological properties from oxygen. For this reason, health standards and recommendations to limit human exposure have been established, to help draw the line between beneficial detergent and health effecting irritant.

How much ozone is too much?

The EU currently defines the following benchmarks: From 0.09 ppm (180 μg / m³) the population should be informed. Sensitive individuals should avoid physical exertion in this environment as of this concentration. The threshold for an actual alert is 0.12 ppm (240μg / m³). From this value, all people should avoid physical exertion in this environment. For longer stays in closed spaces, such as at work, the EU provides a benchmark at currently 0.06 ppm (120 μg / m³). Any concentrations below this level are considered safe for human health.

Because the human sense of smell can detect ozone even at much lower concentrations – depending on sensitivity even at 0.02 ppm (40 μg / m³) and less, humans have a natural warning mechanism.

What is your local air quality index today? Find out here!

Bipolar ionization and ozone

Prior to a thunderstorm there is a very high concentration of contaminants in the air, such as bacteria, allergens and VOC’s. A thunderstorm releases high voltage electrical discharges and, by doing that, emits high concentrations of positively and negatively charged ions. Lightning will produce ozone as well through electrical excitation of oxygen molecules. The ions created in this process play a major role to lower the amount of the pollutants and are the key player for providing us with fresh and clean air in nature. 

Dielectric barrier discharge

This principle of air purification achieved by bipolar ionization is comparable to this natural phenomenon. Our devices are treating the air as if under the influence of a purifying thunderstorm. Charged ions and a small amount of ozone form “activated oxygen” – which bind the contaminants in the space where you breathe – just as nature does. Activated oxygen initiates an oxidation process, which chemically alters the odor molecules and germs. New harmless substances are formed. Activated oxygen damages the cell structure of mold spores as well as bacteria, so that they become inactive. The activated air acts as a natural cleaning agent in the room and restores and re-balances indoor air in closed environments.

In the process of bipolar ionization ozone is formed, however, we are talking about concentrations in the range of 0.01 ppm. By comparison, the current EU Directive specifies a value of 0.06 ppm (120 μg / m³) for longer stays – eg. 8 hours a day, 5 days a week. In that case there are no health risk for humans. In addition, we offer sensor-controlled controls in our ionization systems, in which ozone sensors are used for protection, to avoid any danger or risks evolving.10

In nature, humans are always exposed to a certain low ozone concentration, which, unless it leads to increased accumulation, does not affect their health. 

In conclusion, friend or foe is always subjective to amount and location. Know your facts before making a judgement.


1 Wikipedia: Ozone–oxygen cycle

2 Wikipedia: Ozone

3 NASA Ozon Watch

4 United Nations Environment Programme, Environmental Effects Assessment Panel. Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2016 | Photochem Photobiol Sci. 2017 Feb 15;16(2):107-145. doi: 10.1039/c7pp90001e | PubMed PMID: 28124708; PubMed Central PMCID: PMC6400464

5 NOAA’s Earth System Research Laboratory – Chemical Sciences Division 

NASA Study: First Direct Proof of Ozone Hole Recovery Due to Chemicals Ban, 2018

7 WHO Air Quality guidelines for Europe

8 ScienceDirect: Ozonation

9 Ozonation of municipal wastewater effluents | Panagiota Paraskeva, Nigel J. D. Graham | Water Environ Res. 2002 Nov-Dec; 74(6): 569–581

10 Blitze im Luftkanal: Der Einsatz bipolarer Ionisation schafft neue Möglichkeiten für geruchsfreie Luft

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