Electric arcs generated by hot plasma

Plasma air purifier: is it safe?

Le plasma représente plus ou moins 99 % de la matière connue. Il s’agit d’un gaz composé d’un mélange de particules neutres, d’ions positifs (atomes ou molécules ayant perdu un ou plusieurs électrons) et d’électrons libres.

In nature, this ionised gas is found in the aurora borealis, lightning, solar wind, gas nebulae, etc. The plasma air purifier makes it possible to reproduce this state of matter.

How is plasma formed?

To understand the formation of plasma, we need to focus on the arrangement of atoms in matter. As a reminder, an atom is composed of a nucleus around which electrons gravitate. Depending on the state of the matter (solid, liquid, gas), the space between the atoms varies. Thus, when the matter is solid, the atoms are squeezed together, and this space increases in liquid matter and even more so in gaseous effluents.

There are two types of plasma: cold plasma and hot plasma.

Cold plasma or non-thermal plasma

Cold plasma generated by a Tesla coil (© École polytechnique - J.Barande)
(CC BY-SA 2.0) -© École polytechnique - J.Barande https://www.flickr.com/

Cold plasma can be generated by three different techniques:

  • Electron beam irradiation This technique consists of generating a beam of electrons in a very low pressure chamber, which are injected into the gas to be ionised via a thermionic cathode.
  • Microwave radiation The gas is ionised by an electromagnetic field.
  • Electrical discharges A plasma is created between two electrodes by applying a voltage.

Humidity plays an important role in the efficiency of cold plasma, for example, the degradation of formaldehyde is favoured by the presence of humidity, while the opposite is observed for the degradation of toluene.

The use of cold plasma allows the generation of active species, such as free radicals, capable of decomposing certain pollutants present in the air by oxidation. This technique is the one used for indoor air purification.

Hot plasma

Hot plasma formation occurs when a gas is subjected to intense temperatures (> 3000 °C). Under the effect of this temperature, electrons are torn from the nucleus of the atom. The ionised gas is transformed into a plasma and the electrons and nuclei in the gas are propelled at high speed.

This ionisation of the gas, however, only lasts a short time. The electrons and nuclei propelled at high speed will quickly touch the edges of the container in which they were created and then cool there. As they cool, the electrons and nuclei recombine to form atoms again: this is the end of the plasma.

It should be noted that hot plasma is not used for indoor air purification, as it requires a lot of energy. It is not very economical and not interesting enough to remove VOCs at low concentrations.

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Plasma air purifier: what is it used for?

The plasma generated by the plasma air purifier aims to improve the air quality in your home by destroying volatile organic compounds (VOCs) such as formaldehyde (found in cigarette smoke), benzene, solvent and chemical vapours, paint and glue odours, aerosols, etc. Plasma air treatment has the same objective as UV lamp air cleaners The destruction of viruses and bacteria.

In order to increase their efficiency, plasma air purifiers are often equipped with HEPA filters and activated carbon filters to enable the capture of airborne particles such as pollen, dust, pet hair, etc., and odours such as those emitted by cigarette smoke.

Is the plasma air purifier dangerous to health?

PolytechOrleans | What is plasma?

In contrast to the air purification by plantsThe plasma air purifier uses a technology where opinions differ. Some will praise its benefits on air pollution (purified air, improved air quality in the context of allergies, etc.) while others will question its health benefits (risk of respiratory problems aggravated by the emission of carcinogenic substances, due to the incomplete oxidation of VOCs suspended in the ambient air).

In principle, the decomposition of VOCs should lead only to the formation of CO2 and O2. However, in "real" practice and even in some laboratory studies, it can lead to incomplete oxidations and consequently to the formation of secondary pollutants which may be more harmful than the primary pollutants of concern. In addition, the technology can emit ozone, which in addition to its irritating properties, can also lead to the formation of ultrafine particles by reacting with terpenes present in indoor environments.

Studies currently differ on the benefits of plasma against ambient air pollution, which is why, as a precautionary principle, the use of plasma should be avoided, we do not recommend the use of a plasma air purifierThe National Agency for Food, Environmental and Occupational Health Safety (Anses) in its report : referral n° " 2012-SA-0236 ".

Advantages And Disadvantages Of The Plasma Air Purifier

Odour eliminationIncomplete mineralisation generating secondary pollutants
Removal of biological contaminantsNitrogen oxide emissions
Removal of chemical contaminantsOzone emission
Formation of secondary particles in the presence of terpenes
Cold plasma principle Mineralization of organic molecules through oxidation reactions initiated by free radicals produced in an ionizing field.