What is the
STEREX is an electrophysical process for the disinfection of indoor air which – without chemical agents – generates hydroxyl radicals using oxygen and water vapor in the air by creating an atmospheric low-pressure plasma. These hydroxyl radicals are harmless for humans, animals, and plants, but have a very high disinfection effect against enveloped viruses, bacteria, and fungi. This achieves a targeted disinfection effect against microorganisms, while the process can be used at the same time as people being present in the room.
What is cold atmospheric plasma?
In physics, plasma is the fourth state of matter alongside solid, liquid, and gas. The term “plasma” describes a conductive gas or gas mixture through which a current flows in the conductive state. Gas molecules are physically changed by the electrical current flow: and ions are formed from the components of the gas mixture. The plasma therefore behaves simultaneously as an electron conductor. This is a characteristic of a plasma state of a gas or gas mixture.
How do we generate this plasma?
In the STEREX process, ambient air and the water vapor contained in the air are used as the gas mixture.
The reaction products in the form of hydroxyl radicals are the disinfecting electrophysical “species” (no substance correlate) as a product of the plasma reaction.
The plasma is therefore not used directly for disinfection, but rather the disinfection-relevant products of the plasma reaction in the form of hydroxyl radicals.
The plasma of the STEREX process is a low-temperature atmospheric plasma. This means that the plasma is distinct at ambient pressure and ambient temperature.
An inverter generates the necessary ignition energy between plates / grid electrodes and maintains the plasma state after ignition of the plasma (when the gas mixture between the electrodes has become conductive due to an electrical ignition pulse) by supplying a defined current flow.
How do the electrophysical reactions take place?
Firstly, the current flow and the interaction of the gas molecules with catalytically acting electrode surfaces, which are electrically polarized to the potential necessary for the formation of the plasma, split the oxygen into oxygen radicals (see equation 1). In the STEREX process, ozone formation is reduced below toxicologically relevant limits by limiting the electrical potential difference to < 3 kV on the one hand and certain catalytically acting electrode surfaces on the other. Nitrogen oxides are not formed, as this would require potential differences > 5 kV.
The previously mentioned oxygen radicals react at potential differences of < 3 kV and with corresponding catalytically acting electrode surfaces with water molecules (water vapor in the air = atmospheric humidity) to form hydroxyl radicals (see equation 2).
Hydroxyl radicals (OH°) already present continue to have a catalytic effect on the above reaction. Depending on the ambient pressure, an equilibrium is reached after a saturation phase. With the STEREX process – in a room of up to 120 m³ – this is achieved after approximately 60 minutes. Therefore, the STEREX process should always run for at least 60 minutes before rooms are used.
The hydroxyl radicals exhibit different stabilities in the context of activity periods depending on the environmental conditions. In low-dust/dust-free rooms, activity periods of up to 1 hour are observed.
Dust contamination can reduce this period. Under conditions of some background contamination (room class II), activity periods were still found to be in the range of 20 to 30 minutes.
Hydroxyl radicals can react with organic carbon compounds and thereby physically alter them. A possible sum reaction is shown in equation 3. The structural change particularly affects fatty acid molecules in phospholipids of the cell membrane (= plasma membrane).
In particular, the plasma membrane equivalents of enveloped viruses (including SARS-CoV-2) and the cell membranes of bacteria and fungi are physically altered by the action of hydroxyl radicals (for example conversion of unsaturated fatty acids into saturated fatty acids; shortening of the alkyl residues of fatty acids, etc.), so that the function of the cell membranes is no longer present and the microorganism is inactivated.
The effect takes place in the gas phase (air). Aerosols occurring in the air as liquid droplets with enclosed microorganisms react with hydroxyl radicals present in the air, resulting in the destruction of the microorganisms’ plasma membrane in accordance with equation 3.
How do these hydroxyl radicals affect humans?
The cell membrane has a different structure in the cells of higher organisms (eukaryotes, including plants and animals as well as humans).
This means that hydroxyl radicals have practically no negative effect on eukaryotic cells and that humans are not at risk from exposure to hydroxyl radicals:
For this reason, plasma states are already being used today in the field of wound treatment (for example sanitization of infected wounds caused by antibiotic-resistant bacteria).
Therefore, the STEREX method can (and should) be used during ongoing operation, for example in rooms or air-conditioning systems.