Ozone (O₃) near the earth is formed by the reaction of oxygen, nitrogen oxides and volatile organic compounds under intense solar radiation.Even in slightly elevated concentrations, it causes severe irritation of the mucous membranes and respiratory tract.
Ozone (O₃) is a colourless to slightly blue and pungent to chlorine-like smelling gas. The odour threshold for human perception of the gas is 40 µg/m³.
It is one of the most important trace gases in the atmosphere and forms the natural ozone layer at an altitude of 20 to 30 kilometres, which protects against the sun's harmful ultraviolet radiation.
Ozone is an unstable molecule that decays to dimeric oxygen within a short time. In addition, O₃ is very reactive (oxidising) and has a toxic effect on humans. It promotes fire and can form ignitable and explosive mixtures. Ozone is heavier than air.
Due to its property to decay within a short time under normal conditions, concentration of ozone in the air should be close to zero and thus not measurable.
The EU has set various 1-hour values for O₃, which describe the concentration whose effect is developed within one hour. A value of 90 ppb (parts per billion) or 180 µg/m³ was determined as the information threshold. If this value is exceeded, the public must be informed. Furthermore, an alert threshold of 120 ppb (240 µg/m³) was decided, which must be reached before an ozone warning is issued.
According to the Federal Environment Agency, the maximum daily 8-hour value of 120 µg/m³ (60 ppb) may be exceeded on a maximum of 25 days per calendar year. In the long term, however, this value should no longer be exceeded on any day.
The former limit value (MAK value) of 100 ppb (200 µg/m³) has been replaced by the occupational exposure limit value (OEL). Since no OEL has yet been established for ozone, the previous MAC value serves as a non-binding orientation. International workplace limit values are even only 60 ppb (120 µg/m³).
Ozone is absorbed through the breath and the eyes. Due to its strong oxidising property, O₃ exposure can cause irritation and even lasting damage to the eyes and mucous membranes. Symptoms include hoarseness, cough, headache and nausea as well as nosebleeds and bronchitis.
Since ozone impairs lung function, high concentrations can lead to the formation of pulmonary oedema (water accumulation in the lungs). Pre-disposed or sensitive persons, e.g. those suffering from asthma or chronic bronchitis, are particularly susceptible to the harmful effects of ozone. Furthermore, there is a well-founded suspicion that O₃ is carcinogenic for humans.
Performance is also affected by a high O₃ content in the air, as it can lead to fatigue and poor concentration.
When lightning discharges during a thunderstorm, ozone is formed, along with other substances. It is also formed by the process of photodissociation, in which oxygen molecules in the stratosphere are split into two individual atoms under high-energy solar radiation - radicals that then react with an O₂ molecule to form O₃.
The precursor pollutants of this reaction come mainly from man-made sources such as combustion plants and overfertilised soils (nitrogen oxides). Volatile organic substances are produced by the use of solvents (including paints, varnishes, adhesives) and by fuel combustion. Natural sources of precursor pollutants include vapours from deciduous trees and conifers.
Ozone can be generated indoors by electrical devices such as printers. High voltage and UV radiation are responsible for this.
Ozone is measured by means of an electrochemical sensor. O₃ molecules that "dock" on the surface of the sensor cause a small current in the sensor. The advantage of our sensor is the individual sensitivity calibration by the manufacturer and the particularly long service life.
The sensor we use has cross-sensitivities to hydrogen sulphide (H₂S) and nitrogen dioxide (NO₂). It therefore also reacts to NO₂ and then shows a deflection even if no O₃ is present. It reacts negatively to H2S. If H₂S increases, the measured O₃ value decreases.
The air-Q, the air quality measuring device, also has a sensor that can measure ozone in the room air.