This is a brief introduction to the subject of ozone, summarising information about the properties, limit values and regulation, health implications and commercial applications of ozone.
Ozone occurs in nature – partly as a protective layer against ultraviolet radiation in the stratosphere and partly as a ground-level ozone in the atmosphere. Ozone is often associated with environmental hazards, the thinning of the ozone layer and an elevated level of ground-level ozone.
The fact is that ozone is used in many industrial processes (such as the bleaching of paper mass), in order to disinfect air and water. Ozone is a highly reactive molecule that is very much in demand due to its strong oxidising properties. Its use is an adequately tried and tested procedure.
Ozone units and ozone generators are used to eliminate odours in restaurants, kitchens, refuse collection points and sanitary facilities, as well as to purify air in vehicles, hotel rooms and residential spaces.
“Different” types of ozone
It is important to differentiate between industrial ozone, ground-level ozone and the ozone in the stratosphere.
Stratospheric ozone is formed in the atmosphere with the help of the sun as an energy source. If one talks about the hole in the ozone layer, it means that the ozone formed in the atmosphere due to solar radiation has decreased. The main reason for the thinning of the ozone layer in the stratosphere is the use of chlorofluorocarbons (CFCs) that oxidise (break down) the ozone when they reach the stratosphere.
The formation of ground-level ozone is partly a natural process. Ozone is formed during electrical charges in lightning during storms and through natural photochemical reactions (e.g. terpenes released by coniferous forests). The elevated level of ground-level ozone considered an environmental problem is formed by reactions between sunlight and gasses such as nitrogen oxides and fleeting hydrocarbon compounds. The term Nox or VOC (Volatile Organic Compounds) is often used for this.
Nitrogen oxide, hydrocarbons and ozone occur naturally in the atmosphere. The nitrogen oxide and hydrocarbon content in the air has increased due to the present-day high level of polluting emissions.
This has also caused the ozone content in the ground-level layer of air to rise. It has almost doubled in Europe since the end of the 19th century. As from the 1970s, the ozone content increased by about 1 percent per year. It is only in recent years that the increase appears to be stagnating.
Ozone is a highly oxidising substance that one should respect, but not be fearful of. Like all other oxidising substances (e.g. chlorine and hydrogen peroxide), ozone is poisonous beyond certain limit values. Ozone occurs naturally in our environment in a concentration of about 0.01 – 0.15 ppm (~ .0.3 mg/m3). In urbanised areas, the concentration can even be up to 1.0 ppm. One can say accordingly that electrically operated machines, motors and transformers generate an ozone content of 0.5 ppm.
An advantage of ozone is its specific smell. One can recognise it long before it approaches a threshold value of 0.1 ppm. Consequently, one will always be aware of entering an environment with an elevated ozone level and can avoid it accordingly.
Symptoms among people exposed to elevated ozone levels:
› 0.1-1 ppm headaches, throat dryness, eye irritation
› 1-100 ppm asthma-like symptoms, tiredness, lack of appetite
If one is exposed for a short period to an elevated level of ozone, it often results in throat irritations and coughing. However, the complaints are not chronic and disappear again a few hours later.
Ozone has been used commercially for over 100 years and there are no reports of deaths in connection with ozone exposure.
One should adhere to the regulations and laws applicable to the use of ozone. A simple way to avoid exceeding the limit values is to use an ozone monitor also called ozone controller. It can generally be set so that it warns of elevated ozone levels. It is a simple safety precaution that ensures safety at the workplace.
Hygienic limit values for ozone:
Threshold limit (8 hours) = 0.1 ppm
Upper limit value (15 minutes) = 0.3 ppm
(ppm = parts per million)
Ozone is a molecule referred to as O3, composed of three oxygen atoms, which occurs in the cycle of nature. At great altitude in the stratosphere, 10-50km above the Earth’s surface, part of the oxygen molecules (O2) in the air are converted by sunlight into ozone, creating the natural ozone layer that absorbs ultraviolet light, the so-called UVB rays (rays of a wavelength of 280-315mm). This ozone layer is a precondition for life on Earth.
Industrially manufactured ozone is produced in ozone generators with the help of UV light or corona discharge. Dry air or oxygen (O2) serve as a basis, to which energy is added.
The oxygen molecule is split into two oxygen atoms. The oxygen atoms then bind with another oxygen molecule, forming a molecule composed of three oxygen atoms – ozone.
The ozone molecule has a strong oxidation potential. This means that it reacts easily with other molecules, splitting or converting them. The ozone molecule is an unstable compound. This means that if the ozone molecule does not come into contact with any other compound capable of oxidation, the oxygen molecules in the ozone shrink. The lifespan of the ozone varies between a few minutes and an hour, depending on temperature, pressure, degree of contamination etc.
There are regulations and laws that serve the purpose of protecting the surroundings from ozone exposure. Industrial ozone, which is manufactured in ozone generators e.g. for cleansing water and air or for other processes, is not released into the environment. In industrial processes one avoids the release of the industrially produced ozone into nature.
In most systems for the industrial production of ozone, catalytic converters are used, whereby the ozone becomes oxygen again within seconds. In this process, the formation energy is returned in the form of heat. Catalysis occurs especially through catalytic conversion, with the excess ozone passing a destroyer (e.g. manganese oxide, activated carbon), or undergoing thermal decomposition (pyrolysis). The residual product is oxygen.
As ozone is an unstable molecule and disintegrates spontaneously, it has to be generated “locally”. This is an advantage, as potential problems that occur during the storage and transport of gases can be avoided.
The use of ozone has been proven to be a more environmentally friendly alternative for such uses, compared to other chemicals such as chlorine. However, although ozone is in most cases a significantly better solution than chemicals, we should not lose respect for it.
It is important to know that ozone, which is used for water disinfection, often reaches concentrations of up to 200,000 ppm. If the ozone comes into contact with water, this concentration is reduced to few ppm. When released again later into the air there is rarely ever a value of over 0.1 ppm. After a short period in the water there are no ozone remains in it anymore.
In 1997 ozone was given GRAS status (Generally Recognized As Safe) by the American FDA (Food and Drug Administration) for use in the food industry. This classification means that ozone is permitted for the cleansing and disinfection of foods, if it is used in quantities and for uses that comply with GMP (Good Manufacturing Practises). In accordance with laws and regulations for the environment, people and quality, many food and pharmaceutical manufacturers apply GMP today.
Even before ozone was granted GRAS status, it was used in the food industry. The reason for this is that ozone offers clear advantages compared to chlorine, for example. After use, ozone does not leave any hazardous by-products that can occur when using chlorine.
The use of ozone is regulated by a limit value of how much ozone may be released into the surrounding air. The hygienic limit values for ozone are 0.1 ppm as a level limit and 0.3 ppm as an upper limit. The upper limit takes account of ozone exposure during a working day, the highest limit takes account of ozone exposure within fifteen minutes.
The properties of ozone
- Chemical name: ozone
- Appearance: colourless gas at room temperature
- Smell: sharp and like chlorine
- Formula: O3
- CAS number: 10028-15-6
- Molecule weight: 48.0 g/mol
Gas: 2.144 kg/m3 (at NTP)
Liquid: 1574 kg/m3 (at -183ºC)
Solid: 1728 kg/m3
- Water solubility: 4.9 ml/l (at 0ºC)
- Boiling point: -111.9ºC (at 1 atm)
- Melting point: -192.5ºC (at 1 atm)
- Critical temperature: -12.1ºC
- Critical pressure: 5460 kPa
- Vapour formation, warmth: 681 kJ/m3 (at NTP)
Examples of ozone usage:
- Restoration after fire: removal of smoke odour
- Real estate: odour elimination in residential spaces and refuse rooms
- Fish industry: odour reduction and water treatment
- Vehicles: elimination of smoke, mould and animal odour
- Rubber and synthetics industry: odour elimination
- Hotels: elimination of smoking, perfume and body odour
- Food industry: odour reduction, elimination of bacteria
- Marine: treatment of exhaust air and black and grey water
- Paper industry: bleaching of paper mass, odour reduction
- Pump stations: odour reduction
- Restaurants: odour reduction in refuse rooms and in exhaust air treatment
- Refuse rooms: lasting odour elimination, destruction of bacteria
- Barns: Odour elimination, reduction of ammonia and sulphates
- Water damage treatment: odour and mould reduction
- Water systems: drinking water treatment