1. Particulate Air Filtration (General and HEPA Filters)
Appropriate management and specification of air filters for healthcare facilities
This module discusses air filtration with the objective to describe filtration methods, understand and apply standards and classes of filters and learn to select the correct filtration method for particular purposes
Tobias van Reenen
Tobias van Reenen

2. Contents Background Mechanism of air filtration Standards
Application of air filtration
Testing and maintenance
The topics covered include the mechanism of air filtration, standards relevant to air filtration, the application of air filtration, and lastly, testing and maintenance procedures

3. Background Types of filters (dust) Personal EN 779:2012 EN 1822:2009
Course Filters
Medium Filters
Fine Filters
Aerosol Filters
G1-G4
Filters are differentiated according to the size of the particles that they can remove from the air.
Each filter class (separated by column here) is subject to a different classification test method.
M5-M6
F7-F9
E10-E12
H13-H14
U15-U17
Average arrestance (Am) of synthetic dust %
Average efficiency (Em)  for 0.4 μm  particles %
Minimum efficiency (ME) for 0.4 μm particles %
Integral efficiency
MPPS
Integral and Local efficiency
MPPS
Personal

4. Background Application of filters (dust) Personal Course Filters
Medium Filters
Fine Filters
Aerosol Filters
Used to protect building services from Atmospheric dust
Used to keep interior spaces visibly clean
Used to keep interior spaces free of bio-aerosols and microscopic dust
Different types of dust filters are available for different applications.
Where filtration of PM10 ( µm) particles are of primary concern, such as for keeping plant rooms or ducting clean, course filters should be used. This group of filters includes G1-M5 filters with G4 being the most commonly used PRIMARY FILTER.
Where filtration of PM 2.5 particles is of greatest concern, such as for keeping interior spaces visibly clean, Fine filters should be employed. Fine filters include M6-F9 filter classes with F9 being commonly used as the SECONDARY FILTER.
Where control or reduction of bio-aerosols is a primary concern, aerosol filters should be employed. Aerosol filters commonly include EPA, HEPA and ULPA filters, with HEPA filters being the most commonly used in clinical applications. The lowest classification of aerosol filter, which is supplied with an individual EN 1822 factory test certificate, is an EN1822 H13 HEPA filter. These are recommended for use in ICUs and operating theatres. HEPA filters should be specified according to the EN1822 standard and filter test certificates retained.
Personal

5. Background HEPA filters in history Personal ~1942 1962 1982 2002 2012
The first HEPA filter (high efficiency particulate air) was designed in the 1940's by the research and development firm Arthur D. Little under the Manhattan Project.
The filter solved a critical need to control very small contaminated particles.
Considering the condensation nuclei of radioactive iodine to be most harmful, researchers focused on the ability to capture solid particles that were created through the condensation of gases and liquid aerosols into solid matter. Having identified 0.3 micron size particles as the most penetrating size particle and representative of the particle of concern, 0.3 microns was established as the particle size fraction at which to determine filter efficiency performance.
Because of its effectiveness, the filter was originally referred to as an "absolute filter." The generic acronym HEPA came into use some time following 1950 when the filter was commercialized and the original term became a registered tradename.
Over the next 50 years, HEPA filtration gradually evolved as technological breakthroughs in aerospace, pharmaceutical processing, photographic film manufacturing, data processing and microcircuitry demanded higher and higher levels of air cleanliness. If not for HEPA filtration, such milestones as the lunar landing and the introduction of the silicon chip might not have been achieved and adequate control of hazardous and toxic particulate would not be possible
The first HEPA filter (high efficiency particulate air) was designed in the 1940's by the research and development firm Arthur D. Little under a classified government contract as part of the Manhattan Project, where the first atomic bomb was developed during World War II. A major advancement in air filtration technology, the filter solved a critical need to control very small particles which had become contaminated by nuclear radioactive sources.
Personal ~

6. Mechanisms of air filtration
Principles of filtration
A HEPA (high efficiency particulate air) filter with an H13 Classification has an overall filter efficiency of 99,95 % according to EN1822.
The filter penetration should therefore not exceed 0,05%
99,95% + 0,05% = 100%
Principles of filtration:
A HEPA (high efficiency particulate air) filter with an H13 Classification has an overall filter efficiency of 99,95 % according to EN1822.
The filter penetration should therefore not exceed 0,05%
99,95% + 0,05% = 100%
Efficiency / Arrestance
= 99,95%
Penetration = 0,05%
Personal

7. Mechanisms of air filtration
Principles of filtration (continued)
Filtration mechanisms include :
Interception (Sieving) for large particles
Impaction for medium sized particles
Diffusion (Electrostatic attraction) for small particles
Personal
Camfil Farr

8. Mechanisms of air filtration
Principles of filtration (continued)
~0,4
The lowest efficiency occurs at the most penetrating particle size. The efficiency of a filter is the cumulative effect at any considered particle size.
Below 0.4 µm filtration by diffusion dominates and above 0.4 µm interception dominates.
Below 0.4 µm diffusion dominates
Above 0.4 µm interception dominates
Personal

9. Mechnaisms of air filtration
Principles of filtration (continued)
Therefore, the efficiency of a filter at filtering any considered particle size is a function of the combined filtration modes at that size. An aerosol filter therefore presents with a minimum efficiency at a most penetrating particle size. The same filter will be more efficient at filtering particles of larger and smaller sizes
Personal

10. Mechanisms of air filtration
Principles of filtration (continued)
 Group
Filter Class
EN1822
Integral Value
Local Value
 MMPS
Efficiency %
HEPA
 H13
99.95%
99.75 %
H14 %  %
The overall efficiency of a filter will be higher than efficiency of the local point with the lowest efficiency. This is important when defining the acceptance limits for filter testing.
Personal

11. Mechanisms of air filtration
Principles of filtration (continued)
MPPS = Most Penetrating Particle Size (not maximum PPS)
MPPS ≠ 0.3µm
MPPS efficiency ≠ 0.3µm efficiency
Aerosol and fine filters have their efficiency classified by the most penetrating particle size (MPPS). This is not the same as the efficiency of some maximum penetrating particle size as is commonly assumed for sieving mechanisms. Also, the MPPS is not typically the same as the 0.3 µm smoke particles used for filer testing.

12. Standards Aerosol filtration standards
The evolution of the European standards has relied somewhat on the development of the American standards. This diagram shows the historical complexity of the development with American standards in relation to the European standards. This can result in significant confusion when specifying HEPA filters in accordance with US standards and the European standards are therefore prescribed in South Africa.
Proceedings of Clima 2007: Bin Zhou, Jinming Shen

13. Standards Aerosol filtration standards USA MIL-STD-282:1995.
ANSI/UL 586:2004
ANSI N45.8
ASME N509
ASME/ANSI N510
ASME AG
MIL-STD-282
There are a number of European and American aerosol filtration standards. Here are some of the current and founding USA standards

14. Standards Aerosol filtration standards USA (continued)
IEST-RP-CC007.1:1992<Testing ULPA Filters>
IEST-RP-CC001.3:1993<HEPA and ULPA Filters>
IEST-RP-CC-006.2:1993< Testing Cleanrooms>.
IEST-RP-CC001.4 <HEPA and ULPA Filters>
IEST-RP-CC021
IEST-RPCC034.2<HEPA and ULPA Filter Leak Tests>

15. Standards Aerosol filtration standards Europe EUROVENT 4/4: 1984
ISO 26463: 2011
ISO <Continued Compliance with Part1>
ISO <Test methods>
Here are applicable European standards taking note that these Eurovent standards are no longer valid. The EN1822 and ISO are equivalent.

16. Standards EN779 Group Class EN779:2012 – Classification of filters
Final pressure drop (test)  Pa
Average arrestance (Am) of synthetic dust (%)
Average efficiency (Em) for 0.4 μm  particles (%)
Minimum efficiency ME
for 0.4 μm particles(%)
Coarse G1
250
50≤Am<65  - G2
65≤Am<80 G3
80≤Am<90 G4
90≤Am
Medium M5
450 -
40≤Em<60 M6
60≤Em<80
Fine F7
80≤Em<90 35 F8
90≤Em<95 55 F9
95≤Em 70
NOTE  The characteristics of atmospheric dust vary widely in comparison with those of the synthetic loading dust used in the tests. Because of this, the test results do not provide a basis for predicting either operational performance or service life. Loss of media charge or shedding of particles or fibres can also adversely affect efficiency.
The EN 779 standard classifies course and fine filter as shown. The EN 779 does not describe aerosol filters such as HEPA filters. The EN 779 should also not be understood to define filtration efficiency across filter classes. From the table shown it is evident that specifying filters by efficiency only can result in ambiguous selections. Consider 70% as a criteria. This could result in either a G2, M6 or F9 filter. As shown earlier, the efficiencies for these different classes are derived from very different test methods.

17. Standards EN1822 Integral Value Local Value Filter Class
Integral Value
Local Value
 Filter Class
 Collection Efficiency %
Penetration %
E10 85 15 -
E11 95 5
E12
99,5
0,5
H13
99,95
0,05
99,75
0,25
H14
99,995
0,005
99,975
0,025
U15
99,9995
0,0005
99,9975
0,0025
U16
99,99995
0,00005
99,99975
0,00025
U17
99,999995
0,000005
99,9999
0,0001
The EN1822 standard classifies the performance of aerosol filters on a log scale. As can be seen from the table, each increase in classification is a 10 fold performance improvement on the previous class.

18. SANS 1424: Filters for use in air-conditioning and general ventilation
Standards
Filtration standards in SA
SANS 1424: Filters for use in air-conditioning and general ventilation
EN 779:2012
EN 1822:2009
General purpose particulate air filters are commonly referred to as either course, medium and fine air filters and should comply with SANS 1424 locally. The SANS 1424 incorporates the EN 779:2009 already discussed.
Course Filters
Medium Filters
Fine Filters
Aerosol Filters

19. Standards Emerging standards – EU and ISO Course Filters
Medium Filters
Fine Filters
Aerosol Filters
ISO :2016 replaces EN779:2012
Complete paradigm shift in classification
Classifications not equivalent to EN779
PM10, PM2.5 and PM1 ->10, 2.5 and 1 µm
ISO : 2011 derived from EN1822
Offers equivalent and intermediate classes
The EN 779 is due to be withdrawn and will be replaced by the ISO in The ISO classifies filters according to their efficiencies in filtering PM1, PM2.5 and PM 10 particles as defined by the WHO. It may be some time before the SANS 1424 reflects this change locally.
HEPA filters shall comply with EN 1822 or ISO 26463:2011. The main EN 1822 and ISO filter grades have a linear correlation and can be directly replaced with each other. The ISO offers some intermediate filter grades which are not recognized by the EN1822

20. Application: Filter selectionG4 F9
H13
Multi-stage filter system
Considerations
Levels of Filtration
Filter Classification
Stability of efficiency
Dust holding capacity
Resistance (Pa)
Cost of ownership
Filter selection:
When deciding what type of filter to use in a particular situation the following aspects need to be considered:
Level of filtration.
Filters efficiency should be selected based on the size of airborne contaminants that need to be controlled depending on the application.
Filter classification
Filter classification should be in accordance with local standards and should be derived from the required filtration levels needed.
Stability of efficiency
Course and fine filters that rely on electrostatic attraction can experience a period of lower efficiency after some use. These performance characteristics should be declared by the manufacturer and understood by the specifier.
Dust holding capacity
Dust holding capacity determines the life of the filter. Not all filters are created equal in this regard and it can be beneficial to consider this when comparing filters of different cost and quality.
Resistance (Pa)
As with dust holding capacity, cheaper filters often have a lower surface area and therefore present higher pressure drops throughout a lower service life. Higher pressure drops result in higher fan energy consumption and higher energy bills which can easily outweigh the filter cost saving.
Cost of ownership
When selecting a filter, all the factors above can affect the cost of ownership and extend beyond the purchase price alone.
Image: Filtration Handbook, Ken Sutherland

21. Application: Filter selection
Cost of ownership G4 F9
H13
Multi-stage filter system
Cheap to buy ≠ Cheap to own
Running costs (kWh)
Initial resistance
Final resistance
Expected life
Replacement costs
Cost of ownership is vitally important to consider. Cheap to buy does not necessarily mean it is cheap to own. Running costs (in kWh) need to be considered in terms of initial resistance, final resistance and expected life.
Image: Filtration Handbook, Ken Sutherland

22. Application: Fan location
Cost of ownership - Fan location G4 F9
H13
Multi-stage filter system
Belt driven and asynchronous fans placed before fine filters
Only non shedding EC-DC Plug fans placed after fine filters
NO FANS placed after HEPA filters..
(except on exhaust)
Fan location can also influence HEPA filter life and the cost of ownership . Belt-driven and asynchronous fans should be placed before fine filters as particles shed from brushes and fan belts can clog up HEPA filters in short order. Only non-shedding EC-DC (electronically commutated Direct Current) plug fans can be placed between fine filters and HEPA filters. No fans should be placed after HEPA filters (except on exhausts) as these fans can contaminate the airstream which has just been cleaned at great cost.
Image: Filtration Handbook, Ken Sutherland

23. Filter selection Filter gaskets Type Advantages Disadvantages
Gel and Knife-Edge
Excellent seal
Reusable
Difficult installation
Degradation of fluids = low service life
One-Piece Moulded Polyurethane Gasket.
Reliable and durable seal
Seals can perish and harden.
Correct clamping force critical
Adhesive Neoprene Strips
Cheap
Poor seal, Low service life, Unreliable, Often relies on MS4 Compound (lubricant) for seal
Filter gaskets: Consider the pros and cons of the various types of gaskets available are shown in the table.

24. Testing and Maintenance
Filter testing and maintenance
Installed filter leak test:
The installed filter challenge tests (ISO :B6) should not be confused with the manufacturer’s efficiency classification tests (EN1822-4)
Filter testing and maintenance is key to confidence in filter performance and air quality.
The installed filter challenge test or installed filter leak test (ISO :B6) should not be confused with the manufacturer’s efficiency classification test (EN ). The acceptance criteria for these two tests are different as the test methods are not the same.
EN 1822:2009
Filter manufacturer’s QC test
ISO
Installed filter in-situ testing, proving absence of leaks

25. Testing and Maintenance
Installed filter leak test
ISO , B6: is based on the IEST-RP-CC034.2
B6.2: Aerosol Photometer Test (DOP)
- For filters with integral efficiency ≤ % (E10-H14)
B6.3: Discrete Particle Counter Test (DPC)
- For filters with integral efficiency ≤ %
The installed filter leak test shoud be performed on accordance with the standard ISO , and clause B6, which is based on the IEST-RP-CC034.2
Clause B6.2 should be used for an Aerosol Photometer Test (DOP) and filters with integral efficiency ≤ % (E10-H14)
Clause B6.3 describes the Discrete Particle Counter Test (DPC) and is for filters with integral efficiency ≤ %

26. Testing and Maintenace
Aerosol photometer procedure
ISO B6.2 Acceptance criteria:
Penetration >0.01% constitutes a local failure
“Different penetrations of filters and/or response times of photometers may require consideration of different designated leak criteria, refer to IEST-RP-CC034.2”
For in-situ testing of HEPA filters, the Aerosol photometer test procedure (commonly called the DOP test) should be performed in accordance with ISO B6.2 and should perform according to the recommended acceptance criteria of Penetration < 0.01%. Different penetrations of filters and/or response times of photometers may require consideration of different designated leak criteria, refer to IEST-RP-CC034.2”

27. Testing and maintenance
Aerosol photometer procedure
Ensure tests are witnessed by someone independent from filter installer/supplier
Ensure that all test equipment is currently in calibration before starting tests
Anemometer
Aerosol generator
Photometer
Confirm upstream challenge concentration frequently during tests
Confirm filter concentrations are recorded on worksheet during test
The Aerosol photometer procedure (as with all tests) must be witnessed by someone independent from the filter installer or supplier to avoid conflicts of interest. All test equipment must be in calibration during testing. This includes the anemometer, aerosol generator and photometer.
Upstream challenge concentration must be confirmed frequently during the tests, as failing aerosol generators can give false results, and filter concentrations must be recorded on a worksheet during testing.

28. Testing and Maintenance
Installed Filter Test Procedure - Discreet Particle Count Method (DPC) B6.3:
The Discrete Particle Count method is prescribed for ULPA filters only, which are not commonly used in healthcare. This method is fairly onerous and is not detailed here.

29. Resources and References
ISO (Parts 1-3) & 4
SANS 1424:2008
EN779:2012
ISO :2016
EN1822:2009
Comparison Of HEPA-ULPA Filter Test Standards Between America and Europe: Bin Zhou, Jinming Shen
Filters and Filtration Handbook 5th Ed: K Sutherland, Elsevier

30. Tobias van Reenen (tvreenen@csir.co.za)
Thank you
Tobias van Reenen