Influence of ventilation systems on the spread of corona viruses
What influence do ventilation systems have on the spread of the SARS-CoV-2 virus?
Our customers increasingly ask us about risk analysis of ventilation and air conditioning systems in connection with the SARS-CoV-2 virus. They also ask what we, as a specialist company for air conditioning and cooling, can offer them in the way of preventive measures.
In the following, you will find the most relevant scientific findings and regulations together with corresponding sources.
You will also find a summary with my personal assessment of the situation at the end of this article.
In simple terms, our industry segment is primarily concerned with air in motion. The key issues we must consider in the spreading of viruses, bacteria etc. are as follows:
- The flow of air in rooms
- Options for the filtering or disinfection of air entering rooms (incoming air)
- Can the fan in a cooler, air conditioner or ventilator cope with the additional pressure loss caused by a filter without putting the primary function of the device at risk (heating, cooling, humidification or dehumidification)?
To assess this, we need basic knowledge:
- How are viruses spread in rooms?
Source: TU Berlin
When a person infected with the coronavirus coughs, speaks or sneezes, a spray consisting of droplets of different sizes and aerosols is generated, which penetrates the room air and then spreads. All of these variously sized droplets and aerosols can potentially carry viruses.
‘We are conducting a number of projects to investigate how long pathogens remain in the air under a wide range of conditions,’ reports Professor Martin Kriegel, head of the Hermann-Rietschel-Institut at the TU Berlin. His ‘Contamination Control’ team is performing its research using two clean rooms, several room airflow labs and a research operating theatre. To better understand the corona pandemic, the researchers are examining to what extent the spread of the virus is affected by the composition and size distribution of the particles in exhaled air (aerosol).
‘It seems that as far as the coronavirus is concerned, both infection via droplets and airborne transmission via aerosols apply’, Martin Kriegel explains. In the case of droplet infection, the virus particles in a droplet of saliva come into contact with the mucus membranes of another person. While in airborne transfers, the viruses – tied to the smallest fluid particles – enter our respiratory passages. Decisive for the behaviour of viruses in air are the size of the carrier aerosols as well as the indoor climate, the air exchange rate and the way in which a room is ventilated. ‘Larger particles sink faster to the ground. Smaller particles follow the airflow and can remain longer in the air’, Kriegel explains.
The spread of the mixture of particles, saliva and air takes place in two stages. Firstly, the process of coughing/speaking/sneezing produces a spray of particles that penetrates the room air, increasingly mixing with it.
‘Once the spray has fully mixed with the room air, the process of spreading begins’, explains Kriegel. ‘The smaller particles largely follow the room airflow, whilst larger particles sink successively to the ground. The fact that the very large particles are emitted only by sneezing is often overlooked. Almost all aerosols produced by normal speaking and coughing are small.’
Working in a number of projects, the research team measured the sedimentation rate (deposition time) of particles of various size categories. Almost all small particles (0.5 to 3 μm) are still present in air after a measuring period of twenty minutes. Little or no sedimentation of these particles can be detected.
More than 50 percent of medium-sized particles (3 to 10 μm) can still be detected in the air after a measuring period of twenty minutes. ‘A further study shows that, under certain conditions, even larger droplets (> >60 μm) can spread a long way through a room. This is the case, for example, if the particles are propelled from a source of warmth (for instance a human body). They rise, spread horizontally and only then start to sediment. Possible horizontal air movements strengthen the spreading effect’, says Kriegel.
- How long do SARS-CoV-2 viruses remain contagious?
On surfaces, the novel coronavirus SARS-CoV-2 remains contagious for a prolonged period of time, even at higher external temperatures. This is confirmed in laboratory experiments recently published in Journal of Infection (2020; DOI: 10.1016/j.jinf.2020.05.074).
It is probable that SARS-CoV-2 is primarily transmitted by droplets and aerosols. Transmission via objects or surfaces can, however, not be ruled out. This could be a relevant factor, above all in hospitals.
Stephanie Pfänder’s team from the Ruhr University Bochum investigated how long the viruses remain contagious on surfaces. In the course of their research, the scientists varied the surface temperature − at normal room air temperature, at refrigerator temperature (4°C) and at the high summer temperature of 30°C.
The results contradicted their expectations. The researchers had assumed (or hoped) that high summer temperatures would more rapidly kill the viruses. In fact, the opposite was the case.
With a figure of 17.9 hours, the half-life, the time in which the number of infectious particles falls by 50%, was longest when heated to a temperature of 30°C. In the case of the refrigerator temperature, the half-life was 12.9 hours. With a half-life of 9.1 hours, the decay of the viruses was fastest at room temperature.
- Filter classes in ventilation and air conditioning systems:
Categorisation of filter classes and particle sizes
Examples of use
Coarse dust filter
- Textile fibres and hairs
- Fly ash
- Cement dust
- For simple applications (e.g. insect screens for compact devices)
- Pre-filters and circulation filters for civil defence shelters
- Exhaust filters for spray painting booths, kitchens, etc.
- Dirt protection filters for air conditioners and compact devices (e.g. window air conditioners, ventilators)
- Pre-filters for filter classes M6 to F8
Fine dust filters,
1 - 10 µm
- Cement dust
- Fly ash
- Germs, bacteria
- Outside-air inlet filters for rooms with low air-purity requirements (e.g. factories, warehouses, garages)
- Pre-filters and circulation filters in central ventilation stations
- Final filters in air conditioning systems for sales rooms, department stores, offices and certain production plants
- Pre-filters for filter classes F9 to E11
- Oil fumes and agglomerated soot
- Final filters in air conditioning systems for offices, production plants, control centres, hospitals, data processing centres
< 1 µm
- Germs, bacteria, viruses
- Tobacco smoke
- Metal oxide smoke
- Final filters for rooms with high and highest purity requirements (e.g. for laboratories, for food processing plants, pharmaceutical, precision engineering, optical or electronics industries or medical facilities)
- Final filters for clean rooms Classes 100,000 or 10,000
- Oil fumes and soot in nascent state
- Radioactive suspended particulates
- Final filters for clean rooms Classes 10,000 or 100
- Final filters in civil defence shelters
- Exhaust air filters in nuclear plants
- Final filters for clean rooms, Classes 10 or 1
Classification of coarse and fine filters according to EN 779:2002
Final test pressure drop Pa
Average arrestance (Am)
ASHRAE test dust
Average efficiency (Em)
with 0.4 µm particles
50% ≤ Am < 65%
65% ≤ Am < 80%
80% ≤ Am < 90%
90% ≤ Am
40% ≤ Em < 60%
60% ≤ Em < 80%
80% ≤ Em < 90%
90% ≤ Em < 95%
95% ≤ Em
Classification of EPA und HEPA air filters according to EN 1822:2009
Note: Filters qualify for the HEPA label when they filter out at least 99.95% of a certain number of particles with a size of 0.1 to 0.3 microns (1 micron = 1/1000 mm) within a specified period of time under controlled laboratory conditions. The number required makes it clear how high the demands on modern HEPA filters are.
- How do air filters work?
Sieve effect: Smaller particles transported by the air stream through the filter are captured when they come too close to the filter fibres.
Inertial effect: Due to their inertia, larger particles strike and adhere to the filter fibres rather than being transported around them by the air stream.
Diffusion effect: Very small particles move randomly within the air stream. Due to their collisions with air molecules, they have trajectories similar to Brownian motion and therefore collide with and adhere to the filter fibres.
- Disinfection with UV light
(Source: www.baero.com – our partner for UVC disinfection)
The germicidal effect of UV radiation, in particular UVC radiation, is a well known phenomenon. As long ago as in 1877, two English scientists discovered that the growth of bacteria is stopped by exposure to sunlight. They were unable to explain this effect at that time, but, today, we know that it is due to the invisible UVC component of sunlight.
UVC radiation is light within the wavelength range of 100 to 280 nm.
However, it is the wavelength of 253.7 nm that kills bacteria, viruses, yeasts and moulds. This radiation has the strongest effect on germs or, respectively, the genetic material contained in the nucleus of cells. Thymine, one of the chemical building blocks of DNA, is particularly good at absorbing radiation of this wavelength. The process produces thymine dimers. This photochemical reaction is the reason why the cells are no longer viable or able to replicate.
Dipl.-Ing. Roland Bruch GmbH & Co. KG has successfully employed this technology in proofing cabinets, butcher’s shops and cold stores since 2012.
Central air conditioning plant with UVC module in Göteborg (Bruch GmbH & Co. KG)
Warning: direct UVC radiation is harmful to human health
(Source: Federal Office for Radiation Protection – BfS)
- • UVC radiationcan damage genetic material in a similar manner to UVB radiation, and is therefore carcinogenic to humans, as are UVA and UVB radiation. This is true even at lower intensities than those required for disinfection.
- • UVC radiationcan cause various harmful effects in the eyes and on the skin:
- Eyes: UVC radiationcan damage the surface tissues of the eyes. Studies into accidental exposureof the eye to levels exceeding threshold values generally show that ocular symptoms tend to clear up within about a week. However, higher levels of exposurecan lead to considerably more persistent eye problems.
- Skin: UVC is primarily absorbed by the stratum corneum, the outermost layer of the human epidermis. The acute effects on the skin, such as erythema (sunburn), are known to be transient phenomena.
In view of this, direct irradiation with UVC light in rooms where people congregate is not an acceptable option.
The use of UVC germicidal irradiation in air handling systems with appropriate light traps and warning devices can, however, be regarded as non-hazardous to human health.
Summary and conclusions:
All forms of air turbulence spread viruses unpredictably throughout the room and deposit them on surfaces.
This can be intensified by the use of split air conditioning systems, fan convector radiators and even conventional ventilation systems. In the case of centralised systems with an air re-circulation component, it is possible to prevent the spread to other rooms by installing HEPA filters or UVC disinfection.
Nevertheless, in the actual room, all these systems are also responsible for the unpredictable spread of the viruses.
In my personal opinion, it is especially important to protect our customers against the dubious promises of ‘white knights’ galloping to the rescue online.
In the meantime, there are quite a few mobile air purifiers with HEPA filters on the market that promise significant reductions in virus contamination.
In my view, these devices only serve the purpose of giving their users a clear conscience.
Although it is true that these devices are locally effective, the problem is that they are sold online without any regard for the dimensions of rooms and the actual conditions in which they are to be used.
A typical claim being, for example, ‘effective for rooms up to 120 m³’.
What is really needed here is situation-specific planning.
In the case of ready-to-connect solutions such as split air conditioning units and fan convector radiators etc., it is essential to check whether filters or UVC disinfection systems can be retrofitted.
Here, the advantage of UVC disinfection is that any pressure losses in the system will be almost negligible.
The ventilator fans deliver the required air flow rate as per specification (cooling or heating). Additional pressure losses reduce the air flow rate and the rated capacity is no longer guaranteed.
As only viruses entering the devices are captured, it must be determined whether such measures make good sense in the case of air circulation devices such as split air conditioning systems. Although microbiological contamination in the room air is reduced by such devices, any remaining viruses will continue to be spread by turbulence, (see also Point 1).
In contrast, I personally consider the prevention of the spreading of bacteria, viruses etc. into other rooms by measures installed in centralised ventilation systems to be a much more appropriate proposition.
The UVC hand lamps offered for the disinfection of surfaces (desks, toilets etc.) seem to be a viable alternative or additional option.
Warning: never look directly into the lamp. This can seriously damage the eyes!
It may be assumed that the current pandemic will lead to a heightened awareness of the importance of hygiene in large sections of society. Regular professional hygiene maintenance in compliance with VDI 6022, including bacterial testing of ventilation and air conditioning systems, can play an important role in this respect.
Best regards and stay healthy,
Dipl.-Ing. Roland Bruch
Kälte- und Klimatechnik GmbH & Co. KG