Air cleaner FAQ for virus filtration

Strictly speaking, electrostatic filters, or just electric filters , are not filters in the classical sense, which is why the correct terms is actually electrostatic precipitator. Such electrostatic filters are originally and primarily used for exhaust air purification in industrial processes, e.g. flue gas purification in power plants.

In these processes, the sucked-in and coarsely pre-filtered air is passed through an ionizer and recharged in an electric field under high voltage, the charged dirt particles precipitating on the oppositely charged collector side.

Corresponding industrial facilities are meticulously dimensioned for the spatial conditions and air flow rates, which is usually not the case with home devices "off the shelf".

Therefore such electrostatic filters may be generally effective, yet not with regard to the clean air volumes required, which, for design-related reasons, cannot be provided by such devices at all.

Even for a small room with an air volume of 50 m³ and the scientifically recommended minimum rate of 6 air circulations per hour, a clean air volume of 300 m³ per hour would be needed, a volume for which such home devices are not dimensioned at all in most cases. It's probably for a reason that the manufacturers of such electrostatic devices often remain silent when it comes to specify the air flow rate of their devices and prefer to omit this specification in the first place.

Another unpleasant side effect is that electrostatic filters generally cannot be cleaned or changed by the user himself at all. In order to clean these filters, a complex ultrasonic bath is required, a procedure that is costly and that can only be carried out by the manufacturer's plant service.

Air filters in the form of mechanical fibre filters are practically the state of the art for almost all filtration applications and are therefore also the standard feature of most air cleaners. In the process of mechanical filtration the air is passed through fine filter tissue, with the help of which the unwanted substances are separated. Dust filters, for example F7 prefilters, and high-efficiency particulate air filter such as the H14 filters used in the TAC XT, TAC V+ , TAC M, TAC ECO, TAC BASIC or AirgoClean^® One are distinguished. In the case of surface filtration, the medium pore size of the filter medium is smaller than the particles to be separated, which is why the sieving effect and the separation prevail at the surface.

Mechanical high-efficiency particulate air filters such as the HEPA H14 filters used in Trotec's high-performance air purifiers TAC XT, TAC V+ , TAC M, TAC ECO, TAC BASIC and AirgoClean^® One on the other hand are provided with very open fibre structures, which is why the particles to be separated penetrate the medium and can be reliably retained in the interior of the filter. This takes place due to several physical effects such as the sieving effect, inertia, interception and diffusion. In practice, all these mechanisms interact in the case of depth-filtering fibre filters and allow for a high separation efficiency even for very small particle diameters. The H14 HEPA high-performance filters integrated in Trotec air cleaners for example even filter small aerosol particles of a size from 0.1 - 0.3 µm at a percentage of 99.995 % from the room air!

In an official notification the Indoor Air Hygiene Commission of the Federal Environment Agency clearly advises against the use of air cleaners working according to the principle of photocatalysis. During operation under real-life conditions the room air quality is even deteriorated.

In the process of air purification with UV-C radiation the high-energy radiation affects the molecular bond which is thus broken. In this way, basically many viruses and germs can be inactivated. However, there is the risk that poisonous ozone will build at the same time, which is then emitted to the room, as the bond of the oxygen molecule is broken as well.  

Photocatalysis is a similar procedure. In this process, an additional medium serves as a catalyst which is stimulated by the UV light, for example a special filter coating. The process of radiation then effects an oxidative decomposition of airborne pollutants.

On the plus side of photocatalytic air purification the following can be entered: The procedure generally works, which is why the UV-C-sterilization is used successfully in industrial applications, for example in containers, storage facilities or cargo spaces.

UV-C radiation is an approved method for the sterilization of surfaces. But this method actually makes it possible to easily implement long radiation times, thus ensuring the radiation dose required.

With regard to the treatment of flowing air, it’s exactly this correlation that is the problem: You want to lead a large air volume through a small device and irradiate it there. The dwell time of the air within the device, though, would always be very short in this process, and the radiation dose required would no longer be reached. In practice now, the air volume is reduced in most cases, in order to extend the dwell time. In the end you’d have a device which can only process a very low air volume per hour and which therefore is only suited for very small rooms if the requested circulation rate of a factor of 6 is complied with.

Moreover, when using UV-C devices with a high radiation performance there is also the risk that poisonous ozone will be generated and enter the room air. During operation, secondary compounds enter the room air. This may even deteriorate the room air quality.

Not least because of this the Federal Office for Radiation Protection warns against the use of UV-C disinfection units for fighting the coronavirus.

There are also warnings from science: Current studies suggest that incomplete inactivation can lead to genetic changes in viruses and, as a result, to virus mutations. For this reason, the UV dose must be so high that at least 90% of the microorganisms are deactivated, even after a single pass.

This abbreviation stands for "Clean Air Delivery Rate" – a characteristic that has been developed by the US organization AHAM (Association of Home Appliances Manufacturers) as an indicator value for clean air so as to be able to compare the efficiency of different air purification models to each other.

The CADR value specifies how much room air is cleaned of three different types of particles within one minute, i.e. the particle groups dust, pollen and smoke. On the basis of rules defined by the AHAM a CADR value is then determined for each of these three particle sizes, which can be used to compare the filter efficiency of different air cleaners.

Clearly no. Even if this is suggested by some manufacturers, the test specification of the CADR test neither records the aerosol particle sizes from 0.1 to 0.3 µm that are relevant for virus filtration nor any statements about the fan performance or flow geometry of air cleaners and filters used.

The most convincing thing to do at this point would clearly be to quote AHAM itself about this very issue. In its document on "Frequently asked questions regarding the test of portable air cleaners" AHAM gives this reply to the following question:

"What about particles of the size of bacteria or viruses? AHAM does not provide any presentation of the efficiency for reducing the bacterial or viral load with the current CADR measurements. The AHAM procedures do not test against virus particles, and at present there is no renowned peer review for air cleaners worldwide which would make this possible."

Therefore, Trotec does not specify any CADR value for the high-performance air purifiers TAC XT, TAC V+, TAC M, TAC ECO, TAC BASIC and AirgoClean^® One since this value would have no significance or relevance whatsoever for virus filtration.

The fact that aerosols are the most frequent cause of virus transmission has become widely known in the current state of research of all scientific areas in the meantime and is quoted by the media just as frequently as the useful technical solution of the HEPA air purification. HEPA is the much-quoted measure of all things and is therefore also a big problem when it comes to finding solutions.

The abbreviation "HEPA filter" stands for "High Efficiency Particulate Air Filter" and describes a high-efficiency filter for suspended matter that offers protection against the tiniest dirt particles in the air.

The requirement with regard to the efficiency of such filters and their classification is clearly defined in different standards, however, protection of the designation HEPA unfortunately not.

Therefore a number of market players call in their marketing departments and create powerfully eloquent terms like SilentHEPA, ComfortHEPA or NanoHEPA. This all sounds like HEPA, however, it has no comparable filter efficiency!

If you select a device, therefore do not only pay attention to the catchword HEPA, but also be absolutely sure to observe the underlying filter class and standard. In contrast to HEPA, they are actually clearly and definitely classified.

HEPA is not always HEPA

If data with regard to the HEPA filter class and a test certificate that is mandatory for each filter are missing, then what you're looking at is only a "fantasy-HEPA filter", however, not a "genuine" HEPA filter. A genuine HEPA filter is exclusively certified in compliance with standard EN 1822 or ISO 29463. In addition, every standard-compliant HEPA filter must be identified with the air flow rate for which it is designed for HEPA filtration.

The specification of the standard-compliant air flow rate is essential for the filter efficiency, because of course also a considerably larger air volume can be blown through every filter. Then the filter will lose its efficiency, though and an H13 filter for example only works with the efficiency of an ineffective E11 filter.

The European filter test standard EN 1822 is the most important basis for testing absolute filters and their classification into the corresponding filter groups EPA (E), HEPA (H) and ULPA (U). Furthermore, the standard ISO 29463, which is based on this, is a global standard for EPA, HEPA and ULPA filters.

EN 1822-1 defines a process for testing the separation efficiency of the filters on the basis of particle counting procedures and allows for a uniform classification of the high-efficiency particulate air filters by their separation efficiency. Moreover, each individual filter is examined thoroughly for leakages. Only after performing a successful leakage test and determining the separation efficiency required, the filter tested is classified and equipped with a corresponding test certificate. Only filters of class H13 or H14 are certified HEPA filters complying with EN 1822!

In this respect, HEPA H13 indicates a particle separation efficiency ≥ 99.95 for particle sizes from 0.1 - 0.3 µm with a transmittance ≤ 0.05 %, and HEPA H14 even indicates a particle separation efficiency ≥ 99.995 with a transmittance ≤ 0.005 %. Illustrated by an example: the HEPA H14 filter retains 99,995 of 100,000 particles that flow through the filter, only 5 from 100,000 particles pass the filter.

Similar test procedures are taken as a basis for the standard ISO 29463. In this case, too, the filter classification is based on the MPPS-efficiency (most penetrating particle size), which has to be determined for filters of group H (HEPA). For filter class ISO 40 H (comparable to HEPA H13 in compliance with EN 1822), an overall efficiency ≥ 99.99 % with a max. transmittance ≤ 0.05 % required, and for filter class ISO 45 H (comparable to HEPA H14 in compliance with EN 1822) an overall efficiency ≥ 99.995 % with a max. transmittance ≤ 0.025 % is required.

Exclusively high-performance filter of class HEPA H14 or ISO 45 H, as they are used in the TAC XT, TAC V+, TAC M, TAC ECO, TAC BASIC or AirgoClean^® One are able to even filter even the tiniest aerosol particles carrying viruses (0.1 - 0.2 μm) from the room air, and this at a percentage of 99.995 %. Therefore, H14 filters complying with EN 1822 feature a filter performance that is ten times higher than H13 HEPA filters with 99.95 %, and even a filter performance that is 1,000 times higher than E11 EPA standard air filters with only 95 %, as they are used in most air cleaners!

Note: In accordance with EN1822, filters of classes E10, E11, E12 are only EPA filters and not HEPA filters, although they are often referred to as such in the advertising. The standard-compliant designation "HEPA" only applies to classes H13 and H14, or ISO 35 H and ISO 45 H.

When purchasing a filter, therefore always pay attention to the filter certificates that are approved in the EU. Here either the filter standard (ISO) or the filter class (EN) must be definitely indicated.

The fact that there is no alternative to an effective filter efficiency when it comes to maintaining the room air free from viruses, is not questioned by any expert, not even by competitors. One thing that you still often read, though, is that H13 filters, or even E12 or E11 filters, are sufficient for the purpose of virus filtration. But a wrong statement is not turned into a correct one just by repeating it over and over again!

The following is correct: Only H14 HEPA and ISO45H high-performance filters as they are used in the TAC XT, TAC V+, TAC M, TAC ECO, TAC BASIC or AirgoClean^® One are able to reliably filter even the tiniest aerosol particles carrying viruses (0.1 - 0.2 μm) from the room air, and this at a percentage of 99.995 %.

Therefore, H14 filters complying with EN 1822 feature a filter performance that is ten times higher than H13 HEPA filters with 99.95 %, and even a filter performance that is 1,000 times higher than E11 EPA standard air filters with only 95 %, as they are used in most air cleaners!

In a statement of the Federal Institute for Occupational Safety as a technical report on the subject "Use of HEPA filters in ventilation systems at protection/safety levels 3 and 4 – laboratories and areas where animals are kept" explicit reference is made to the fact that HEPA filtration is mandatory in the event of a corresponding hazard situation and that even if HEPA filters of class H13 may be as effective to filter out bacteria as HEPA H14 filters, however, that due to the substantial differences of the two HEPA filter classes in the MPPS area (Most Penetrating Particle Size) in the application area of virus filtration solely HEPA filters of class H14 are able to ensure the separation efficiency required!

In this context the report by the BAUA (Federal Institute for Occupational Safety) says: "The state of the art for HEPA filters and their housings is to be gathered from DIN EN 1822-1, DIN EN ISO 14644-3, DIN EN 15242, VDI 2083 Sheet 3, VDI 6022 and TRGS 522. The HEPA filters should at least comply with class H14 in compliance with DIN EN 1822-1."

Without effective virus filtration an air cleaner is already useless in the first place where the protection against an airborne infection is concerned. And yet effective filtration alone is not the overall solution, but for itself alone only shifts the problem. The viruses then may no longer be in the room air, but they are still active and infectious inside the filter. An improper filter change can then already be enough to release the viruses into the ambient air again.

In genetic engineering facilities, the legislator, for instance, therefore unmistakably requests "a HEPA filter change and filter sterilization which protects the maintenance personnel and other persons against infections (in compliance with paragraph 9.4.1. Of DIN 12980:2005)."

Unique decontamination technology

With their thermal decontamination function, Trotec's high-performance air purifiers TAC XT and TAC V+ meet even these safety requirements and in this way offer you maximum protection.

Similar to an autoclave used in medical technology for the purpose of sterilization, the TAC XT and TAC V+ heat up the fully encapsulated and heat-resistant H14 special filter "made in Germany" to approx. 100 °C at regular intervals. Explicitly for this purpose, this filter has been developed and interspersed with special, heat conducting metal fins.

Decontamination at low temperatures is not sufficient. Although most bacteria cannot survive temperatures of approx. 60 °C, there is still a large group of so-called thermophilic bacteria and fungi that even multiply particularly strongly at temperatures ranging from 60 °C to 80 °C.

For every decontamination, the golden rule is: the higher the temperature, the more bacteria and germs will be inactivated. It's not without reason that drinking water containing germs should be boiled, i.e. heated to approx. 100 °C, for at least three to five minutes before use. The same effect is achieved by thermal decontamination in the TAC XT and TAC V+.

This thermal filter decontamination technology is available exclusively at Trotec and offers you several unique benefits at the same time! By heating the filters, the proteins of the viruses filtered out, which are relevant to the infectiousness, are denatured, and the viruses are practically destroyed. Therefore infectious viruses can no longer enter the ambient air from the filter, for example by improper handling. Furthermore, bacteria, germs and other microorganisms are additionally destroyed. All this means that 100 % employee protection can be ensured during operation and also when the filter is changed!

We recommend carrying out thermal decontamination once a week. Only in hygienically sensitive areas with a low air temperature and very high humidity level should thermal decontamination be carried out once a day. Thermal decontamination of the filter carried out on a weekly basis requires only 1.0 kW of additional energy per week and will not increase the room temperature due to the limited energy input.

Another time- and money-saving additional benefit of thermal decontamination is the thermal regeneration of the filter, extending the filter lifetime and therefore the replacement intervals.

The design of classified HEPA filters compliant with EN 1822 or ISO 29463 is specified by standards. In contrast to pseudo-HEPA filters that often consist of a variety of synthetic fibres, the filter material of standard-compliant HEPA filters with the classifications H13, H14, ISO 35 H and ISO 45 H must be designed on a basis of glass, for example glass paper.

In addition, the housing of classified HEPA filters must be fully encapsulated to ensure that the filter has no leakages and can therefore successfully pass the mandatory leakage test. Fully encapsulated means that the air does not find any leakages and that it can pass at reduced filter efficiency, ensuring that the whole air is exclusively led through the filter.

Already a brief visual inspection of supposed, however, not classified pseudo filters, it's often possible to see at a glance that you're merely looking at cardboard boxes, cartridges or plastic housing with integrated filter material, which are anything other than leakage-tight. Do not be misled by such pseudo-HEPA filters, they are neither fully encapsulated nor compliant with standards, and therefore no "genuine" HEPA filters with a valid classification.

Such filter manufacturers or others then often say that their filter material is deliberately not manufactured on the basis of glass, since glass fibre parts from HEPA filters are released and can then affect the airways. This assertion is sheer nonsense! With this false statement any officially prescribed and practised clean room concepts from the fields of virus research, bio laboratories, chip production, etc. are put into question.

The following is correct: Certified high-efficiency particulate air filter from class H13 are manufactured of a specially bound glass fibre paper, which doesn't emit any fibres! Glass fibre emission by the filter medium would take the whole operating principle of the high-efficiency particulate air filter ad absurdum and these filters would neither pass the tests prescribed nor ever obtain a test certificate!

What's more, scientists request regular thermal treatment of the filter to ensure effective virus filtration, thus irreversibly denaturing the viruses separated in the filter. Such temperatures up to 100 °C would destroy ordinary synthetic filters, only filter material on a glass basis withstands these temperatures in the long term.

Clean air flow rate means that the air returned to the room again is blown out by the air cleaner as virus-free air. This can only be implemented after previous virus filtration with certified efficiency.

In this respect, the capacity of the clean air roll flow of the air cleaner is the decisive factor when it comes to reliably maintain the ambient air free from viruses. Establishing such a reliable air roll flow is of substantial importance where aerosol filtration is concerned. Infectious aerosol clouds must be – as quickly as possible within a few minutes – diluted and filtered out already while they are building. This requires an extremely high air flow rate combined with a high fan performance.

In rooms with an area of 80 m² the aerosol concentration should be halved within approx. 6 minutes. It is exactly for these requirements that high-performance air purifiers such as the TAC V+ were developed. Scientists require a minimum air circulation rate of 6 for a normal occupancy rate in rooms.

With a higher density of persons, e.g. in call centres or bars, we recommend increasing the circulation rate by at least 8 times.

In the healthcare sector and everywhere where persons actively speak, sing or move about, at least 12 air circulations are required, better still 15.

In contrast to Trotec high-performance air purifiers with FlowMatic volumetric flow control such as the TAC XT, TAC V+ or TAC M, commercially available air cleaners usually have been solely developed to continuously purify the room air from particulate matter, odours and pollen, and have not been designed to establish a strong air roll flow with a previous filtration of aerosols carrying viruses.

Neither the filters used nor the devices housing them are dimensioned for air volumes as large as those required for effective flushing by clean air.

Only a sufficient number of air circulations with filtered air minimize the risk of infection, this is scientifically proven. Where the effective treatment of the risk of infection by aerosol particles is concerned, high circulation rates for air filtration and therefore large volumes of clean air are indispensable, despite promises to the contrary made by other providers who advertise single to threefold circulation rates per hour as sufficient.

As is proven by scientific studies, effective virus filtration requires a sufficiently dimensioned volumetric flow so as to ensure that the air cleaner can provide for the number of air circulations required. If we refer to "air changes in this context, this term does not refer to a complete exchange of the air, but to the proportion of virus-filtered clean air that is supplied per hour, relating to the room volume.

In accordance with these studies we recommend room air filtration with a factor of six for normally used spaces such as offices, conference rooms, schools, daycare centres or restaurants to effectively reduce the risk of infection, i.e. 6 air circulations per hour as a minimum value. With a high density of persons or a high level of activity, 8 air circulations are recommended.

For room air virus filtration in therapy rooms, gymnastics rooms, bars, discotheques or call centres, 8 air circulations per hour are recommended, with a high density of persons or a high level of activity 9 to 10 air circulations are recommended.

And in especially sensitive areas such as medical wards, medical practices or waiting rooms, at least 12 air circulations per hour should be ensured, with a high density of persons or a high level of activity 13 to 15 air circulations.

Note: Air change is air change!

This applies irrespective of the operating principle of the air cleaner used. When it comes to promises such as "air change rates are not relevant to us" or "our technology does not work according to the principle of air change" you should take an extremely critical position and rely on the pronounced scientific opinion: Air change is air change – or, to be precise, circulation rate is circulation rate – irrespective of the operating principle or filter principle!

No, this is not what it means. Air change or air change rate is a widely used term which, however, is often misunderstood. To be technically correct, this would be the circulation rate/h.

The air change in the unit (1/h) specifies the multiple of the room volume of filtered air that is supplied to the room per hour. This number does not correspond to the actually filtered room air volume, since, to some extent, air that has already been filtered is filtered again.

With regard to virus filtration this means that room air cleaners are not able to establish completely virus-free air in the room if this room contains infected persons, but merely a mixture that still contains a certain number of viruses.

Neither can the widely recommended airing of rooms not provide for completely virus-free air, but always merely an air mixture with a strongly diluted virus concentration.

In the case of high circulation rates (air changes) of the air purification units used, however, the large filter volumes allow to establish a condition with a great deal less infectious viruses in the room air than without air purification.

Scientific studies provide guide values for this, showing how large the number of "air changes" or the filter air volume per hour should be the relation to the room volume, in order to minimize the risk of an indirect infection by aerosols (viral load).

From a scientific point of view there is substantial evidence that sufficient amounts of virus-free clean air are required to filter the room air to effectively reduce the risk of infection by aerosols carrying viruses, in order to ensure adequate dilution of the virus concentration below the infection threshold.

In normally used rooms for instance approximately a minimum of 6 air circulations per hour are considered as a recognized standard value. If a room, for example, features an air volume of 150 m³, for this case an air cleaner performance of 900 m³/h is required.

Unfortunately, in the case of some devices, merely the fan power or even the additional air swirled within the room as a result of the process is specified as the original air flow rate, however, not the classified flow rate of the filter used. It becomes clear very quickly, though, that these two things belong together.

The whole air that is drawn in and blown out with the performance of the fan must be led through the filter first, ensuring that it is cleaned to attain a virus-free condition. For this purpose, certified filters are provided with a suitability label. For the above-shown example the filter would also have to be able to filtrate 900 m³ air per hour! If the throughput performance of the filter does not correspond to the device performance, it loses its commended filter efficiency and also lets particles pass that it's actually supposed to filter.

When selecting a device, therefore better pay attention to the specified air flow rate of the filter used and stay away from devices for which only the fan performance is specified. Because these devices will be nothing more than an expensive fan, but not an air cleaner!

Yes, because even the best filter is successively clogged with coarse and fine dust, for example by traffic load, so that less air can flow through the filter due to the higher counter-pressure. As a result, less filtered air flows into the room, making it no longer possible to ensure that the air volumes and circulation rates requested are achieved.

In the high-performance air purifiers TAC XT, TAC V+ and TAC M a special control works to automatically solve this problem – the innovative FlowMatic control. Just like with the cruise control of your car, the FlowMatic sensors capture the actual air flow rate values within the entire filter ladder and consecutively adapt the system performance dynamically. In this way, the air volume target value once preset remains constant in every situation! This not only increases the filter lifetime and system efficiency, but also guarantees compliance with the circulation rate required by the corresponding hygiene concept. Air cleaners without a FlowMatic control are not able to constantly and reliably meet the requirements to comply with the air circulation rate required.

As a protective measure it is often recommended to reduce the viral load in the room by free ventilation at regular intervals. The effect of this measure, however, is overestimated: From a physical point of view, free ventilation is only effective if either there is a big temperature difference between inside and outdoor air, or if a wind is blowing outside the windows. There is often no temperature difference and if there is it is quickly reduced in the course of free ventilation, so that this mechanism in most cases is only efficient for a short time and the air exchange therefore would take correspondingly long. This has been confirmed again in recent studies by the German Aerospace Center. The wind outside the window also only strong enough in very few cases to ensure sufficient ventilation.

Since the efficiency of free ventilation depends on factors that cannot be influenced (temperature, wind, size and position of the windows), the question remains in which way ventilation is to take place if these physical mechanisms are not utilizable. Due to cold draught and the risk of catching a cold, airing often is furthermore not desirable or unreasonable in autumn and winter.

Moreover, when outdoor temperatures are winterly, free ventilation results in a huge waste of energy, which not only burdens the environment, but furthermore also results in high heating costs.

No. The CO₂ concentration is not a measurand of the risk of infection, as there is no correlation between CO₂ concentration and viral and bacterial load. There may be an infection risk even with a low CO₂ concentration, for instance if infected persons enter a freshly aired room.

Then, however, CO₂ concentrations that are considerably or permanently greater than 1,000 ppm in schools, offices and private households may the sign of an insufficient ventilation management and may indicate a potentially increased infection risk resulting therefrom. This does not only apply to window ventilation, but also to the operation of ventilation systems.

CO₂ value as an indicator of the air quality: In any case, however, CO₂ traffic lights can be used to obtain a rough orientation whether the air quality in rooms with a large number of persons is good or poor, since carbon dioxide (CO₂) is a reliable indicator of a necessary air change.

A CO₂ concentration of up to 1,000 ppm under normal conditions shows a hygienically sufficient air change. Already at a CO₂-value of 1,500 ppm, the ability to concentrate decreases noticeably, and headaches as well as fatigue or even drowsiness may be the result. With values exceeding 1,000 ppm the room should be ventilated so that the values are in the range of 400 - 500 ppm again.

CO₂ traffic lights can therefore reliably indicate whether, when, and in particular, for how long the room has to be ventilated. They do not supply any reliable information about the current infection risk in the room, though.

No, viruses cannot be directly measured, not with any commercially available portable measuring instrument. At best, hypothetical conclusions can be drawn, however, no reliable results.

In the same way as CO₂-measuring devices can be used to demonstrate an insufficient ventilation management, which may imply a resulting, potentially increased infection risk, particle measuring devices can be used to establish quantitative statements about the concentration of A-dust in the air.

A-dust refers to particles of the size PM2.5, i.e. alveolar ultra-fine particles ≤ 2.5 µm, which are so fine that they can directly penetrate the alveoli.

These ultra-fine particles can be aerosols, but they can just as well be other suspended matter of which thousands can be found in the room air. And they, in turn, may possibly be carrying viruses, but they also may not. There is simply no correlation between the particle load and viral load.

The large part of commercially available air cleaners are provided with a so-called automatic mode. In this mode, an integrated sensor system determines the air pollution in the room and for example switches the device to standby if the air pollution determined is within the preset parameters for "clean air".

in some places, now, such devices are also advertised as units suitable for virus filtration, and at the same time their practical automatic mode is highly commended – sometimes customers are even encouraged to just let the device run in automatic mode. From our point of view, however, this is fatal!

Why? Because the sensors typically integrated into such air cleaners may provide indications as to the quality of the room air, but they can never provide any statements about its infectivity! Since viruses in the room air cannot be directly measured in the device, not with any commercially available sensors.

For making the operating principle of sensor-supported air purification units and their automatic switch-off function with uncontaminated – and, as is wrongly assumed, supposedly virus-free air – comprehensible, we'll include a small excursion with regard to the technical possibilities of common air quality sensors in the following.

Typical air quality sensors in household air purifiers

If air cleaners are equipped with integrated air quality sensors, these are mostly either sensors for VOC measurement, for particle measurement, or for CO₂ measurement. Some devices come with a combination of several of these sensors.

To evaluate the general room air quality, such sensors are generally very useful:

VOC sensors

VOC are for example substances which already evaporate at low temperatures – hence "volatilize" into the gaseous state – and thus pollute the air. Which is why they are referred to as volatile organic compounds (VOC). Formaldehyde, for example, is a well-known VOC. A VOC sensor can determine and display the quantitative concentration of these substances in the room air.

Particle sensors

Particle sensors can determine and display the actual concentration of particulate matter in the room air. The term "particulate matter" (PM) refers to particles in the air that do not sink to the floor immediately, but that float in the air for some time. Such particles are for instance generated by abrasion, material evaporation, combustion or chemical processes.

All these types of particulate matter are differentiated with regard to their size. A classification of particles with a maximum diameter of 10 µm (PM10) is common – such particles reach the human nasal cavity (e.g. pollen), and particles with a maximum diameter of 2.5 µm (PM2.5) is made – such particles are alveolar, i.e. they are so tiny that they can directly enter the alveoli.

Furthermore, there are ultrafine particles with a diameter of less than 0.1 µm. For understanding: One micrometre (µm) corresponds to the thousandth part of a millimetre, meaning that it is a thousand times smaller than a millimetre. Common particle measuring devices often determine the two sizes of particulate matter PM10 and PM2.5.

CO₂ sensors

CO₂ sensors measure the carbon dioxide concentration in the room air. Carbon dioxide, for example, is generated as a result of the gas exchange in the human lung and is emitted to the room when exhaling, where, without the implementation of countermeasures, a too high CO₂ concentration can build, which then may cause fatigue, headaches or a lack of concentration. Therefore, CO₂ sensors are useful aids to indicate a required air change or an inflow of oxygen-rich fresh air. 

VOC, fine particulates and carbon dioxide are not viruses!

Therefore none of the measuring sensors represented above are able to provide actual information on the viral load in the room air, although some providers suggest this in more or less subtle way.

The technical motive: A trick with a purely theoretical background. Correlations are simply constructed here. For example by assuming that the room air doesn't contain any aerosol particles carrying viruses just because no PM2.5 particles are measurable in the air anymore. A false conclusion, because virus-carrying aerosol particles with sizes of 0.1 - 0.2 µm are often considerably smaller than PM2.5 particles!

Along the same lines, a correlation between VOC or CO₂ values and virus-carrying aerosol particles is attributed – it is implied that a lower VOC or CO₂ concentration is equivalent to a lower aerosol presence and therefore to a lower viral load. These correlations, however, are absolutely wrong, unproven, not reliable and therefore offer no protection whatsoever against an infection!

In addition, the greatest possible protection against indirect infection also requires continuous operation of the air cleaner at the necessary power level. An automatic system that reduces the power increases the hazard for people in the room.

The economic motive: It's obvious. The ability to recognize viruses is simply ascribed to the sensor system, and just like that an air cleaner against dust and pollen becomes a promotional "virus air cleaner". In our opinion, these advertising promises are misleading and grossly negligent. Do not be misled by such statements.

Knowing all this you now also know the only correct answer to our initial questions "Can air cleaners be operated in automatic mode for the purpose of virus filtration?"

The answer is – No. The integrated sensor system only uses one measurement as a basis to initiate the automatic switch-off function or standby mode at low fan stages, showing the degree of the air pollution with regard to particulate matter, VOC or carbon dioxide in the uncritical range. This has absolutely nothing to do with the viral load permanently emitted by infected persons in the room, since these sensors are not able to determine the viral load at all! There is no correlation between the viral load in the room and VOC, particles, or CO₂ values.

After all, these devices have been merely provided with parameters and means for recognizing clean air – not, however, air that is contaminated by viruses! This is technically not possible.

Think about this: If there were such sensors, we would have designed corona rapid test devices equipped with such sensors long ago, just as those for breathalyzer tests – blow, measure, done.

From today's perspective this only appears to be a wishful fantasy – just like the assumption that an air cleaner in automatic operation may be able to offer reliable protection against the risk of airborne infections!

This, by the way, is also one of the reasons why the TAC V+, for example, is neither provided with corresponding sensor elements nor switches down to an automatic or standby mode at any time. The virus could always be in the air, brought there by infected persons and still lingering after they have left the room, and it never goes to standby!

Our high-performance air purifier AirgoClean^® One and virtually all our standard air cleaners of the AirgoClean series are also equipped with VOC-, particle or CO₂ sensors. Yet we expressly point out in every instruction manual that always the fan stage requested by the respective specifications with regard to the circulation rate (air volume) must be set when the device is used for virus filtration for reducing the risk of indirect infections.

No. If air cleaners are to be actuated effectively to reduce the risk of indirect infections by aerosols, they must not run in automatic mode.

In order to effectively reduce the infection risk, in addition to filter class H14 (complying with EN 1822), it must also be possible to adjust the recommended air volume (circulation rate, also referred to as air change) in cubic metres (m³) on the device. At this fan stage, the air cleaner must then be permanently actuated during the presence of persons as well as for some time afterwards.

However, if the device is actuated in automatic mode, after a certain time or following short and intense airing, the sensor measures low concentrations of CO₂, particulate matter or VOC.  Then the fan is automatically adjusted downwards to the lowest (most silent) stages.

The devices will now work at a low-noise level which is more pleasant for the user – however, if infected persons are in the room now, there won't be any effective protection against infectious aerosols anymore, since the filter performance, due to the lower air volume of the low fan stages, has been consequently reduced to only a fractional amount of the value required.

The feeling conveyed to the users is that they're safe, while in fact they're almost completely unprotected.

Conclusion: The automatic mode of customary room air cleaners is not suitable for the purpose of virus filtration. They have their merit when they're used to regulate a good room air quality where dust, pollen, formaldehyde or ventilation control for better CO₂ values are concerned.

Professional virus filters designed for this purpose only never have an automatic mode, but work permanently at the circulation rate required for the room.

Air quality sensors as they are for example used for VOC determination are typically mixed gas sensors, measuring gases and vapours that can be oxidized, e.g. tobacco smoke or material evaporations from furniture, carpets, paint coats, adhesives, etc.

Such sensors are not suited to determine the airborne viral load, though. This would require an input measurement of aerosol particles, since they may be contaminated with viruses. Suitable measuring sensors to perform a capacitive measurement of the aerosol load, however, are not available.

The same goes for CO₂ sensors: There is no correlation between the CO₂ concentration the room air and the prevailing viral or bacterial load. Even with a lower CO₂ concentration and even when a room is freshly aired, the risk of infection can increase rapidly as soon as this room is entered by infected persons.

Most household air cleaners aim at reducing or minimizing allergy issues by keeping the room air free from pollen or other allergens. These are the tasks for which they were designed and dimensioned with regard to their performance data, although, in the meantime, they have often additionally received the promotional label of "virus filters".

However, to provide for effective aerosol filtration, a strong air roll flow is of vital importance: Infectious aerosol clouds must already be diluted and filtered out while they are building, as quickly as possible within a few minutes. This requires a sufficiently large air volume combined with a large circulation pressure capacity, two things standard devices are not designed for.

In contrast to customary household air cleaners, Trotec's high-performance air purifiers such as the TAC XT, TAC V+, TAC M, TAC ECO, TAC BASIC or AirgoClean^® One were basically designed for this task – the filtration of aerosols carrying viruses – i.e. for the purpose of infection protection.

Therefore the Trotec high-performance air purifiers TAC XT, TAC V+, TAC M, TAC ECO, TAC BASIC and AirgoClean^® One are able, even with the H14 HEPA filter that is integrated as standard, to separate all dangerous aerosol particles from the room air and, to dilute these particles, provide a virus-filtered clean air volume of 1,200 m³/h for high air circulation rates. By using the optional H14 Ultra-HighFlow filter, the virus-filtered clean air volume can even be increased to 2,000 m³/h.

The filter performance and air flow rate of the device are perfectly coordinated, furthermore models such as the TAC XT or the TAC V+ come equipped with a large variety of mandatory functions for effective virus filtration of the room air, for example thermal decontamination and thermal filter regeneration, a high fan performance and an optimum flow geometry.

On the whole, Trotec's high-performance air purifiers therefore distinguish themselves from conventional air cleaners by having been consistently designed and constructed as a solution for virus filtration, whereas this ability has been mostly just attributed to standard air cleaners later on by an advertising label.

Regular thermal treatment of the high-efficiency particulate air filter is the most reliable method to effectively inactivate the viruses and bacteria separated in the filter. In addition, heating up the H14 filter to approx. 100 °C offers further substantial user benefits.

Without such a treatment the filter would successively replenish with aerosol particles and other particulate matter very quickly, which increases the filter resistance and significantly reduces the filter lifetime, making a premature change necessary.

Moreover, the filtered aerosols mostly consists of micro-droplets, i.e. of pure water. Due to the increasing humidity in the filter, it starts to soot more quickly, and with a permanently high moisture penetration, furthermore there's a risk of filter mould or biofilm formation. Instead of eliminating a hazard, others may therefore be created. For example, a decreasing flow rate, shorter filter lifetimes and mould on the filter.

But since the filter is heated to approx. 100 °C at regular intervals, the ingress of moisture, quick sooting and filter mould are not a problem with the TAC XT or the TAC V+. As the humidity is fully discharged again in the course of the thermal regeneration cycle, not only drying the filter at regular intervals, but additionally regenerating it again via its self-cleaning function.

With this innovative procedure the filter resistance remains low and the clean air flow remains at a constantly high level for a considerably longer time. Consequently the lifetime is much longer than that of other filter systems without thermal regeneration and decontamination.

Some market players advertise their air purification technology with integrated H14 HEPA filters as being antivirally coated for virus inactivation and thus not requiring thermal inactivation, as is integrated in the TAC V+, for example.

Can such coatings be effective against virus-laden aerosols and do they perhaps even represent a low-cost alternative to thermal virus inactivation?

When considering the question of effectiveness, one thing should be kept in mind: if there was a coating with a 100 % microbial and antiviral effect, wouldn't it then immediately find its way into medical technology on a massive scale if it was also cheaper than existing methods? At present, for example, surgical and other instruments are still mostly sterilised in hot-air sterilisers – the principle of the TAC V+. No surgical instrument is antivirally coated – no – it is thermally treated after each use to inactivate any germs present.

Similarly, there are no antivirally coated cooking pots. It's no coincidence that the rule for germ-free boiling has always been: 3 to 5 minutes at 100 °C, then all microorganisms are inactivated. Heat treatment at 100 °C is effective against practically all pathogens and has been proven for centuries – which is why the TAC V+ also works according to this principle!

And what about the cost advantage of coated variants?

The most important saving factor that is often put forward is the supposedly expensive thermal decontamination of the TAC V+. We recommend carrying out thermal decontamination once a week. Only in hygienically sensitive areas with a low air temperature and very high humidity level should thermal decontamination be carried out once a day. For thermal decontamination on a weekly basis, the additional energy input amounts to only 1 kWh per week!

But the full cost situation only becomes apparent when you perform the whole calculation:

Thermal decontamination also demonstrably regenerates the filter of the TAC V+, thus increasing its lifetime. Depending on the application, the TAC V+ therefore offers a filter lifetime of 24 to 36 months!

The coated filters of competitors are designed to be permanently used for only 12 months, which means that they need to be replaced after one year of use. With this variant, the costs per month for the filter are therefore €38.18 (see calculation table below).

Here's the comparative calculation for the TAC V+: with thermal decontamination carried out once a week, the costs for virus filtration with the TAC V+ are only 29 % of the costs of the competitive model – more than three times cheaper!

Even with daily thermal decontamination, for instance due to increased hygienic requirements in medical practices, hospitals or the food industry, the filter operating costs of the TAC V+ are still only 42 % of the costs of the competitive model, meaning that the filter operating costs of the latter are still more than twice as high!

Conclusion: with the TAC V+ the filter operating costs are more than three times lower in comparison, depending on the application, and you benefit from demonstrably effective sterilisation technology by thermal decontamination! Additional benefit: no filter humidity, no formation of bacteria in the filter, no filter mould, no filter odour, and a much longer filter lifetime!

Heat sterilization is still considered as the most reliable means by far where the inactivation and elimination of microorganisms is concerned. Due to their high variance with regard to the size and structure, microorganisms are characterized by different resistances with regard to the hot-air temperature.

Whereas viruses are usually very susceptible to heat and temperature ranges from 55 to 70 °C result in their inactivation, the reliable denaturation of bacteria requires a considerably higher temperature range. Therefore the heat sterilization in the thermal decontamination process of the TAC V+ is effected at about 100 °C. At this temperature range it can be ensured that not only all viruses but also most of the bacteria are inactivated.

No. While it is true that Trotec high-performance air purifiers offer reliable protection against an indirect infection by virus-carrying aerosol particles in the room air, even they cannot prevent the risk of a direct droplet infection that is effected over a short distance by strong coughing, sneezing or loud conversations.

Optimum all-round protection is only ensured if, in addition to the use of a high-performance air purifier such as the TAC XT, TAC V+, TAC M, TAC ECO, TAC BASIC or AirgoClean^® One, the DHM formula continues to be applied, i.e. maintaining a sufficient distance to other persons (D), observing hygiene measures (H) by regularly washing and disinfecting your hands, and wearing a protective mask (M), for example mouth/nose protection. Wearing face visors as a means of protection against aerosols, however, is not effective as has been proven by several studies. As an alternative to the mask, perspex partitions can be used to obtain sufficient protection in offices and in schools. Optimum effectiveness is achieved with protective screens if they are additionally equipped with a deflecting edge all around.

The efficiency of Trotec's high-performance air purifiers such as the TAC V+ with regard to the reduction of the risk of an indirect infection by aerosols has been fully confirmed in several scientific studies by leading German institutes – for example in the often-quoted analysis by Prof. Dr. Christian Kähler at the Institute of Fluid Mechanics and Aerodynamics of the University of the German Federal Armed Forces in Munich.

Furthermore, Prof. Dr. Kähler and his team performed another study, dealing specifically with the following question: "Can safe school life during the pandemic be ensured?" This study illuminates several possible safety concepts from free ventilation, including CO₂ traffic light and protective masks to room air purification.

Conclusion of this study: Protective concepts using room air cleaners to conquer the risk of indirect infections have the advantage that viruses in the room are already separated or inactivated within a short time (1.) if the air circulation per hour is at least six times the room volume. (2.) 99.995% of the viruses are separated while passing through the device once (with a filter of class H14) and (3.) the device is silent, so that it is actually used. The Trotec air cleaner TAC V+ meets all these requirements and therefore offers reliable infection protection in classrooms and interior spaces that are used for similar purposes.

In a study, the Fraunhofer Institute for Building Physics IBP also confirms the extremely high effectiveness of the TAC high-performance air purifiers in virus inactivation using Trotec's thermal decontamination technology!

We'll be happy to provide you on request with detailed information about further studies performed on Trotec's high-performance air purifiers.

The high-performance air purifiers TAC XT, TAC V+ and TAC M meet all requirements laid down by the German Indoor Air Hygiene Commission (IRK).

First of all, the large air volume of these high-performance air purifiers in combination with the strong fan performance makes it possible to attain targeted and exact air routing in the whole room.

Secondly, the volume flow of these Trotec devices can be adjusted, and the air flow rate can be adapted individually to the local conditions. By means of optional flowstop shutters, the emission of air can be additionally blocked on one or two of the four air outlet sides of the TAC high-performance air purifier and in this way the main outlet direction can be individually adapted to the room situation.

Thirdly, all Trotec high-performance air purifiers are mobile devices and can be specifically positioned in the right places.

The Indoor Air Hygiene Commission of the Federal Environment Agency writes:

"In the view of the Indoor Air Hygiene Commission, the use of mobile air cleaners with integrated HEPA filters in classrooms is not sufficient to effectively remove suspended particles from the room air over the entire teaching time (e.g. viruses). This would require exact recording of the air routing and air flow within the room as well as targeted positioning of the mobile devices. The height of the air flow rate would also have to be adapted exactly to the room situation and occupancy rate. The use of such devices can

therefore not replace ventilation measures and should at most be used as a complementary measure in such cases, where a particularly large number of pupils (...) is in the room at the same time." [3]

Source: "The risk of a transmission of SARS-CoV-2 in interior spaces can be reduced by suitable ventilation measures", statement by the Indoor Air Hygiene Commission of the Federal Environment Agency, 12.08.2020

From the point of view of science, air filtration of at least 6 times per hour is recommended. Depending on the application situation and the need for security, the TAC XT, TAC V+ or TAC M can therefore deal with surfaces up to 80 m² when rooms have a normal height and the standard H14 HEPA filter is installed, the TAC ECO can deal with surfaces up to 66 m² and the AirgoClean^® One with surfaces up to 66 m².

Particularly with the TAC XT and TAC V+, the H14 main filter attains a very long service life due to thermal regeneration. During the operating time, the filter does not have to be checked, as the device monitors the filter load of both the F7 prefilter and the H14 main filter and automatically indicates a required change separately for both filters.

If the indication on the device does not show the necessity for changing the filter beforehand, we generally recommend replacing the H14 filter element for hygienic reasons after 12 to 8 months at the latest. The F7 prefilter should be replaced as shown by the change indicator or after 6 months at the latest.

The manual on-demand boost function provides the maximum air flow rate of the device to you for a short time. At the boost level, short and intense ventilation for quick separation with a maximum air volume in break periods or in the case of room changes can be initiated.

Please carry out the thermal decontamination immediately before changing the filter of the TAC XT or TAC V+. In this way, all viruses in the H14 filter are reliably denatured, i.e. practically destroyed. Since the filter also contains other particles and particulate matters, we recommend wearing protective gloves and a FFP2 protective mask when changing the filter of a Trotec high-performance air purifier. The H14 filter can then be disposed of in the residual waste using a waste bag, taking regional regulations into consideration.

We recommend cleaning and disinfecting all inner surfaces when the filter is changed. On request, we perform the maintenance and the filter change in the context of a maintenance contract at fixed conditions for TAC high-performance air purifiers.

There will probably never be complete safety where such virus pandemics are concerned. With our air cleaners we can significantly reduce the risk of an indirect infection via the aerosols carried in the room air. In particular against an infection via large droplets or direct contact with an infected person, air purification cannot offer any protection.

A model study demonstrates: A reduction of fine dust saves lives

Due to the lock-down measures implemented worldwide in the context of the COVID-19 pandemic, an international research team has now been able to perform a model study in the context of which the content of fine particulates in the air at more than 2,500 locations in Europe and China was analysed and its short-term effect on health was assessed.

The results of this modelling were published as a study in "The Lancet", one of the oldest and most renowned medical professional journals worldwide. The study suggests that a total of 24,000 deaths caused by air pollution in China in the period from February-March 2020 could be avoided, and a total of 2,190 deaths could be avoided in Europe in the time from mid-February to mid-May 2020.

Air cleaners can keep interior spaces free from harmful fine particulates

The study results show that the restrictions and lock-down measures during the COVID-19 pandemic bring about a substantial reduction of fine dust particles in Europe and China. The results demonstrate how important guidelines for clean air are to reduce premature deaths. And the results, in turn, also demonstrate the huge advantage offered by air cleaners that can keep interior spaces permanently free from such harmful fine dust particles.

The TAC V+ is one of the most silent devices of its class. Depending on the power level selected and the H14 filter used, only 38 to 40 dB(A) are emitted. For comparison: 40 dB(A) corresponds to a soft whisper or the background noise at night in a peaceful residential street.

In a survey, pupils and teachers who experienced the devices "in action" during lessons said that the noise of the TAC V+ in most cases was not perceived as annoying if the device was actuated up to max. level 5.

Air filtration of up to 2,100 m³/h requires a very large filter surface, as offered by the HEPA H14 filters of the TAC XT, TAC V+, TAC M, TAC ECO or TAC BASIC. We also use a fairly large fan very deliberately. Already at low speed, it can move a lot of air and achieve sufficient fan power. This is how we ensure silent operation. The overall size of the device is thus accounted for by the size of the filter and fan.

Moreover, the robust and scratch-proof housing is designed in an especially stable fashion to ensure that it can perform its service with constantly reliability in environments containing many people.

It is scientifically proven that in addition to a sufficiently dimensioned air circulation rate and effective filter technology, correct positioning of the device is a decisive factor where protection against the risk of an indirect infection is concerned. Therefore the design of the TAC V+ high-performance air purifiers is equally mobile, robust and stable.

Other providers may commend visibly smaller devices for air purification. But they are neither sufficiently dimensioned to house the technical equipment required for effective virus filtration nor are they therefore suited for this purpose.