Preliminary remarks:
This report refers only to the virus-separating efficiency of air filters that are used in ventilation systems.
It is not addressed at any other specialist area since, at the moment, there is no definite knowledge on the type and the nature of the actual pathogens that cause swine flu (A/H1N1) and it's consequences.
We therefore expressly avoid entering into the discussion on swine flu pathogens that is spreading concern and panic, and refer to the reports that are carried out by medical research institutes (such as the Robert Koch Institute, etc.).
To summarize, in a simplified manner, the publications that are currently available, flu viruses are basically tiny capsules in which the genetic information is contained in the form of ribonucleic acid (RNA).
There are various substances anchored on the fluid surface of the viral envelope (e.g. hemagglutinin, neuraminidase, etc.) that are responsible for the production of other viruses and the transmission from person to person.
Depending on the threat to human beings, viruses are also classified with the letters “A”, “B” and “C”, viruses classified with the letter “A” represent the greatest threat to human.
This report refers only to the virus-separating efficiency of air filters that are used in ventilation systems.
It is not addressed at any other specialist area since, at the moment, there is no definite knowledge on the type and the nature of the actual pathogens that cause swine flu (A/H1N1) and it's consequences.
We therefore expressly avoid entering into the discussion on swine flu pathogens that is spreading concern and panic, and refer to the reports that are carried out by medical research institutes (such as the Robert Koch Institute, etc.).
To summarize, in a simplified manner, the publications that are currently available, flu viruses are basically tiny capsules in which the genetic information is contained in the form of ribonucleic acid (RNA).
There are various substances anchored on the fluid surface of the viral envelope (e.g. hemagglutinin, neuraminidase, etc.) that are responsible for the production of other viruses and the transmission from person to person.
Depending on the threat to human beings, viruses are also classified with the letters “A”, “B” and “C”, viruses classified with the letter “A” represent the greatest threat to human.
Precautions regarding the separation of viruses in ventilation systems:
Although the greatest risk of the A/H1N1 virus is being transferred from person to person, the following precautions are recommended for ventilation systems:
- as far as possible, ventilation systems without filters should be equiped with air filters and generally only operated with outside air.
If circulating mode cannot be switched to using only outside air, the extraction systems should have suitable filters incorporated.
- for ventilation systems with filters, it should be decided to which extent the possible concentration of viruses and the resulting problems can be reduced by an appropriate choice of filter (e.g. higher separation efficiency with lower differential pressure, etc.).
- the maintenance or replacement intervals for the filters should be reduced as much as possible and, when filters are replaced, the used filters should be packed immediately in suitable containers and/or plastic bags (that can be sealed) and disposed of straight away.
Although the greatest risk of the A/H1N1 virus is being transferred from person to person, the following precautions are recommended for ventilation systems:
- as far as possible, ventilation systems without filters should be equiped with air filters and generally only operated with outside air.
If circulating mode cannot be switched to using only outside air, the extraction systems should have suitable filters incorporated.
- for ventilation systems with filters, it should be decided to which extent the possible concentration of viruses and the resulting problems can be reduced by an appropriate choice of filter (e.g. higher separation efficiency with lower differential pressure, etc.).
- the maintenance or replacement intervals for the filters should be reduced as much as possible and, when filters are replaced, the used filters should be packed immediately in suitable containers and/or plastic bags (that can be sealed) and disposed of straight away.
Considerations on separation mechanisms:
The separation of viruses in air filters can be defined as the capacity to hold back airborne particles, this process is influenced by the most diverse physical and/or chemical factors.
Depending on the composition and design of the filter material and the corresponding operating conditions (e.g. air velocity, temperature, humidity, etc) of the ventilation system, preference is given to either one or another separation mechanism so that, as a result, the maximum or minimum separating efficiency of a filter is altered towards a quite specific particle size. see fig. 1
Viruses should be considered as tiny particles, for which a quite diverse particle size range - between 0.01 μm and 125 μm - is indicated depending on the literature source (e.g. publications by the Robert Koch Institute, patent specification for immunity boosters, etc.).
For the H1N1 virus, the sizes given, still differ depending on the corresponding publications, so that, at this point, a specific indication of size is generally avoided – although the majority of the opinion agrees that H1N1 viruses can be assigned to a particle size range between 0.06 μm and 0.13 μm.
This particle size corresponds to the range that includes the MPPS (Most Penetrating Particle Size or minimum fractional separation rate) of today’s common HEPA filters, so that all smaller and larger particles (or viruses) are separated by the filter with an even greater level of certainty than particles within that (MPPS)-range.
The particle size-range and their distribution of the viruses that are present in a ventilation system is usually not known and is difficult to determine. It therefore is accepted to evaluate the separation efficiency only in the MPPS area of the filter – since greater and smaller size particles (viruses) will be separated with a higher efficiency.
Fig.1
shows the fractional efficiency rate - Eta-F - as the sum of the individual separation effects
Eta-E : electrostatical effects
Eta-M : effects based on mass attraction
Eta-S : screening effects
Eta-T : Thermal, hydrophobic effects
shows the fractional efficiency rate - Eta-F - as the sum of the individual separation effects
Eta-E : electrostatical effects
Eta-M : effects based on mass attraction
Eta-S : screening effects
Eta-T : Thermal, hydrophobic effects
Selection of air filters for separating viruses:
The classification of HEPA filters along the EN1822, is based on the efficiency on particle sizes within the MPPS range (the Most Penetrating Particle Size).
Fine filters (F5 up to F9) that are classified along the EN779:2002, only specify the initial efficiency of the filter on 0.4 μm-size particles. Therefore these filters can hardly provide suitable protection against the considerably smaller H1N1 viruses.
The reference basis for the maximum risk, or 100% of viruses in a ventilation system, is the situation where there is no filter at all.
The proportion of viruses is then reduced depending on the separating efficiency of the filter installed and is defined by the penetration rate, that is obtained from the separating rate or efficiency, as follows:
D [%] = 100% - ETA [%]
D: Penetration rate of the filter
Eta: Separating rate or efficiency of the filter
According to the EN14175, the extent to which the threat of viruses is reduced can then be expressed by a safety factor, as follows
S = P [%] / D [%]
S: Safety factor
P: 100% of viruses occurring in a situation without filter
D: reduced percentage of viruses because of the filter – based on the penetration rate
i.e. the higher the safety factor (S) the smaller the risk of a possible transmission of viruses.
Determination of the safety factor can be explained by taking the example of an F 7 filter (initial separation rate for 0.4 μm is approximately 38%),
an H10 filter (separating rate in the MPPS range approximately 90%) and an H12 filter (separating rate in the MPPS range approx. 99.9%).
The classification of HEPA filters along the EN1822, is based on the efficiency on particle sizes within the MPPS range (the Most Penetrating Particle Size).
Fine filters (F5 up to F9) that are classified along the EN779:2002, only specify the initial efficiency of the filter on 0.4 μm-size particles. Therefore these filters can hardly provide suitable protection against the considerably smaller H1N1 viruses.
The reference basis for the maximum risk, or 100% of viruses in a ventilation system, is the situation where there is no filter at all.
The proportion of viruses is then reduced depending on the separating efficiency of the filter installed and is defined by the penetration rate, that is obtained from the separating rate or efficiency, as follows:
D [%] = 100% - ETA [%]
D: Penetration rate of the filter
Eta: Separating rate or efficiency of the filter
According to the EN14175, the extent to which the threat of viruses is reduced can then be expressed by a safety factor, as follows
S = P [%] / D [%]
S: Safety factor
P: 100% of viruses occurring in a situation without filter
D: reduced percentage of viruses because of the filter – based on the penetration rate
i.e. the higher the safety factor (S) the smaller the risk of a possible transmission of viruses.
Determination of the safety factor can be explained by taking the example of an F 7 filter (initial separation rate for 0.4 μm is approximately 38%),
an H10 filter (separating rate in the MPPS range approximately 90%) and an H12 filter (separating rate in the MPPS range approx. 99.9%).
System without filter: S = 100% / 100% = 1
For an F7 filter: S = 100% / (100% - 38%) = 1,6
For an H10 filter: S = 100% / (100% -90%) = 10
For an H12 filter: S = 100% / (100% - 99.99%) = 1.000
From these safety factors, it can be seen very expressively that when an F 7 filter is changed to an H12 HEPA filter, the risk with respect to the H1N1 viruses is reduced by almost 1.000 times.
The threat of viruses can be reduced even further by the use of higher efficiency filters (e.g. H13, H14).
Because of the modern high quality HEPA filter materials and/or the new HEPA filter designs, a filter can usually be changed even from a fine filter to a HEPA filter without any problems at all, because these filters
Therefore these new developed HEPA filters allow to increase filter efficiency (and thus the safety factor) without reducing the quantity of ventilated air.
For an F7 filter: S = 100% / (100% - 38%) = 1,6
For an H10 filter: S = 100% / (100% -90%) = 10
For an H12 filter: S = 100% / (100% - 99.99%) = 1.000
From these safety factors, it can be seen very expressively that when an F 7 filter is changed to an H12 HEPA filter, the risk with respect to the H1N1 viruses is reduced by almost 1.000 times.
The threat of viruses can be reduced even further by the use of higher efficiency filters (e.g. H13, H14).
Because of the modern high quality HEPA filter materials and/or the new HEPA filter designs, a filter can usually be changed even from a fine filter to a HEPA filter without any problems at all, because these filters
- fit in the same holding frames without requiring any conversion
- and most important, have about the same initial pressure drop!
Therefore these new developed HEPA filters allow to increase filter efficiency (and thus the safety factor) without reducing the quantity of ventilated air.