ISO 5371:2025

International
Standard
ISO 5371
First edition
Containment high efficiency
2025-07
filtration unit (CHEFU) in
ventilation system of biosafety
facilities
Unités de filtration à très haute efficacité de confinement
(CHEFU) dans le système de ventilation des installations de
biosécurité
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Design and construction . 3
5 Requirements . 3
5.1 Appearance .3
5.2 Resistance to air flow .3
5.3 Vacuum deformation test .4
5.4 Air tightness .4
5.5 Leakage test of the installed HEPA filter . .4
5.6 Decontamination .4
5.7 Leakage identification .4
6 Test methods . 4
6.1 Appearance .4
6.2 Resistance to air flow .5
6.3 Vacuum deformation test .5
6.4 Air tightness .5
6.5 Leakage test of the installed HEPA filter . .5
6.5.1 Test aerosol .5
6.5.2 Test apparatus .6
6.5.3 Scanning leak test.6
6.5.4 Total penetration leakage test .7
6.6 Decontamination .9
6.7 Leakage identification .9
7 Recommended test timeline . 9
7.1 General .9
7.2 Design verification test .9
7.3 Factory test .10
7.4 In situ test .10
7.5 Evaluation of the test results .10
8 Marking . .10
Annex A (normative) Air tightness test method .12
Annex B (normative) Method for aerosol concentration uniformity test .15
Annex C (informative) Method and examples for CHEFU decontamination methodology
verification . 17
Bibliography .21

iii
Foreword
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iv
Introduction
In laboratories and facilities dealing with critical or hazardous biological materials, such as biosafety
laboratories, vaccine manufacturing facilities and animal laboratory facilities, the removal of airborne bio-
aerosols in the facilities that are released to the surrounding environment is always a major concern. In
general, containment high efficiency filtration units (CHEFU) are used for such purpose. When installed in
the terminal outlet within the room, the required maintenance such as the decontamination and replacement
of filters, are performed in the containment room, e.g. the high-level biosafety lab. When installed in the
ventilation duct, all maintenance activities can be performed in the low-risk servicing area and a special
design for in situ decontamination or containment filter replacement measures needs to be put in place. This
document provides suppliers and users with basic performance requirements and test methods to validate
the performance of the devices.

v
International Standard ISO 5371:2025(en)
Containment high efficiency filtration unit (CHEFU) in
ventilation system of biosafety facilities
1 Scope
This document provides basic performance requirements and corresponding test methods for containment
high efficiency filtration units (CHEFUs). This document is applicable to the devices used to remove harmful
bio-aerosol in biosafety facilities and similar controlled environment. This document is not applicable to a
filtration unit for removing radioactive aerosol.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 9626, Stainless steel needle tubing for the manufacture of medical devices — Requirements and test methods
ISO 14644-3, Cleanrooms and associated controlled environments — Part 3: Test methods
ISO 21501-1, Determination of particle size distribution — Single particle light interaction methods — Part 1:
Light scattering aerosol spectrometer
ISO 21501-4, Determination of particle size distribution — Single particle light interaction methods — Part 4:
Light scattering airborne particle counter for clean spaces
ISO 29463-1, High efficiency filters and filter media for removing particles in air — Part 1: Classification,
performance, testing and marking
ISO 29463-2, High-efficiency filters and filter media for removing particles in air — Part 2: Aerosol production,
measuring equipment and particle-counting statistics
ISO 29463-3, High-efficiency filters and filter media for removing particles in air — Part 3: Testing flat sheet
filter media
ISO 29463-4, High-efficiency filters and filter media for removing particles in air — Part 4: Test method for
determining leakage of filter elements-Scan method
ISO 29463-5, High-efficiency filters and filter media for removing particles in air — Part 5: Test method for filter
elements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 29463-1, ISO 29463-2, ISO 29463-3,
ISO 29463-4, ISO 29463-5 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

3.1
containment high efficiency filtration unit
CHEFU
filtration unit used in specific biological risk environment to remove harmful bio-aerosol from the air
3.2
aerosol
solid and/or liquid particles suspended in a gas
Note 1 to entry: Based on European Union and US Environmental Protection Agency information, atmospheric aerosol
is divided into four size categories: the ultrafine range x < 0,1 µm, the fine range 0,1 µm ≤ x ≤ 2,5 µm, the coarse range
2,5 µm < x ≤ 10 µm, and the large coarse range x > 10 µm, whereby x is the aerodynamic diameter of the particle.
[SOURCE: ISO 29464: 2024, 3.2.1]
3.3
bioaerosol
particles of biological origin suspended in a gaseous medium
Note 1 to entry: Bioaerosol particles include viruses, bacteria, fungi, pollen, plant debris, fragments of these and their
derivatives such as endotoxins, glucans, allergens and mycotoxin.
Note 2 to entry: The size of a bioaerosol particle can be larger if it is encased within a liquid drop, for example a virus
in sputum.
[SOURCE: ISO 29464: 2024, 3.2.20]
3.4
high efficiency particulate air filter
HEPA filter
filter with performance complying with requirements of filter classes ISO 35 H to ISO 45 H as specified in
ISO 29463-1
[SOURCE: ISO 29464: 2024, 3.2.66]
3.5
rated air flow rate
flow rate through a test device, either as stated by the manufacturer for defined conditions of use or as
agreed between the interested parties for a particular installation
Note 1 to entry: The flow rate is usually expressed in volumetric units (m /h).
3.6
hourly leak rate
T
f
ratio between the hourly leakage F of the containment enclosure under normal working conditions (pressure
and temperature) and the volume V of the said containment enclosure
F
T =
f
V
-1
Note 1 to entry: It is expressed in reciprocal hours(h ).
[SOURCE: ISO 10648-2:1994, 3.2, modified — "Note 1 to entry" has been added.]
3.7
Laskin nozzle
nozzle used as part of a system to generate heterogeneous aerosol from a liquid, such as polyalphaolefins
(PAO), DiEthylHexylSebacate (DEHS), or other oil, that uses a source of compressed gas
Note 1 to entry: The mass median diameter (MMD) of an aerosol produced by Laskin nozzles is about 500 nm.

3.8
thermal generator
device that produces a liquid aerosol by means of an evaporation-condensation process in the absence of
controlled nucleation
Note 1 to entry: The MMD of an aerosol produced by a thermal generator is typically less than that of an aerosol from a
Laskin nozzle and can encroach on the most penetrating particle size of HEPA filters.
Note 2 to entry: The Aerosol size and distribution of a thermal generator varies with output.
3.9
biological indicator
BI
well-characterized preparation of a specific microorganism that has known resistance to a specific
decontamination process
Note 1 to entry: Microorganisms recognized as suitable for BIs are spore-forming bacteria; the spores of these
microorganisms are significantly more resistant than the vegetative cells that comprise the majority of bioburden in
or on materials.
3.10
decontamination effect level
DEL
effectiveness of a decontamination procedure
Note 1 to entry: For a given disinfectant, the DEL is decided by the time duration of the decontamination procedure
and the concentration of the disinfectant.
Note 2 to entry: For different target microorganisms, the DEL can be different even if the disinfectant, the concentration
of disinfectant and the time duration of the decontamination procedure are the same.
4 Design and construction
The materials of construction of the CHEFU shall be suitable for the service environment and resistant to
disinfectant corrosion. The material of the inner side of the CHEFU should be able to prevent the growth of
microorganisms. The inner part of the CHEFU shall be accessible for cleaning and decontamination.
5 Requirements
5.1 Appearance
Both the inside and outside surface of the CHEFU should be flat, smooth and free of scratches and blisters.
5.2 Resistance to air flow
The resistance to air flow of CHEFU shall be tested. The results are required for the design of the ventilation
system of the facility.
The CHEFU shall be equipped with a pressure difference measuring device which shall be capable of
monitoring the pressure drop of the filter during the service period. The pressure drop of the filter is
monitored to indicate to the user when to replace the filter so that the test range of the device should cover
at least two times the initial pressure drop of the filter and with enough resolution (e.g. a pressure gauge
with the test range of 500 Pa and the resolution of 10 Pa). The inlet line of the pressure difference measuring
device shall have measures to prevent potential contamination (e.g. installing mini in-line HEPA filters on
the inlet line).
5.3 Vacuum deformation test
The CHEFU shall be able to withstand a minimum negative pressure of - 2 500 Pa for at least 60 min. The
unit shall be free from cracks or permanent deformation (hysteresis maximum – 1,0 mm per m shorter
panel span of the test surface) after the test. After the pressure bearing test, the unit tightness shall still
meet the requirements of 5.4.
5.4 Air tightness
-1
The hourly leak rate of the CHEFU shall not be greater than 0,01 h ( 1% per hour) of the housing volume
under a minimum test pressure of 1 000 Pa.
5.5 Leakage test of the installed HEPA filter
The CHEFU shall provide the capability to Leak Test the HEPA per the requirements of ISO 14644-3. The test
can be made available via manual, semi-manual, or automated means and shall be a validated system with
evidence of such available by the manufacturer in accordance with ISO 14644-3. For the CHEFU in service,
decontamination shall be performed before the leakage test for safety reasons.
For filter leak scanning tests, a leak detected in excess of 0,01 % of the upstream concentration is deemed
to exceed the maximum allowable penetration. However, for HEPA filters of ISO 40 H and ISO 35 H, the
acceptance criterion is 0,1 %.
For the total penetration test, the permitted total penetration limitation for the particles from 0,3 μm to
0,5 μm shall be 0,01 % by using a light-scattering airborne-particle counter (LSAPC).
5.6 Decontamination
The CHEFU shall be designed to remove the hazardous airborne bioaerosols so that the decontamination of
the installed filters and exposing parts within the housing is important and shall be performed before any
maintenance to fulfil the biosafety requirements. The decontamination of CHEFU shall include methodology
verification and in situ decontamination. The recommended procedure is described in 6.6 and Annex C.
5.7 Leakage identification
If the CHEFU uses a scanning system for the leakage test, the leakage test noted in 6.5 should be used. Care
shall be taken to ensure that the size of the sampling probe is designed for the isokinetic sampling when
the CHEFU is working at the rated air flow rate. However, for an installed CHEFU, the isokinetic sampling
requirements will not always be possible for the following reasons:
— The actual air flowrate of CHEFU is lower than the rated air flowrate. In many applications, the designed
air flowrate of CHEFU is only 70 % of the rated air flowrate or even lower, which lead to that the filter
face velocity is much lower than the rated condition.
— The filter installed in the CHEFU is V-bank filters which are not accessible for ideal scanning test.
In this case, the capacity of the CHEFU to identify an artificial leakage with the penetration of approximately
0,01 % shall be tested per the procedure described in 6.7.
6 Test methods
6.1 Appearance
The CHEFU should be visually inspected to ensure that the surface of the CHEFU is nominally flat, smooth
and free from scratches and blisters when examined by unaided eye.

6.2 Resistance to air flow
The resistance to air flow of the CHEFU shall be tested in the manufacturing factory and in laboratory
conditions rather than in situ after they are installed in the servicing ventilation system. The resistance
shall be tested at the normal working conditions for the CHEFU with all the inner parts in working position
but without the filter. The normal working position means the routine settings of the CHEFU. In some
CHEFU designs, a movable baffle or other mixing measures are provided to increase the aerosol mixing
during the HEPA filter leakage test, but during actual service, the mixing measures will be moved or rotated
and parallel to air flow direction within the CHEFU to lower the resistance to air flow. Therefore, for the
resistance test, such measures shall be in the same position as during normal service. The resistance to air
flow of the CHEFU shall be tested at least 50 %, 75 %, 100 % and 125 % of the rated air flow rate of the unit.
Figure 1 gives the schematic diagram for test rig and devices for resistance to air flow.
Key
1 pressure gauge
2 CHEFU under test
3 air flow rate test section
Figure 1 — Schematic diagram for the test section for resistance to air flow test of CHEFU
6.3 Vacuum deformation test
The pressure-bearing capacity is tested for the CHEFU with its all-inner part in normal working position. All
the openings in the CHEFU shall be sealed by using airtight valve or blocking plates, and be vacuumed to at
least -2 500 Pa for at least 60 min. After the vacuum procedure, the following tests shall be conducted:
— Each surface of the CHEFU should be visually examined to confirm that there is no visual damage.
— The deflection shall be measured in the middle of a selected housing surface within an accuracy
of ±0,1 mm and compared with the length of the shorter span. The unit shall be free from cracks or
permanent deformation (hysteresis maximum – 1,0 mm per m shorter panel span of the test surface)
after the test.
— The air tightness of the CHEFU shall be retested according to the test method specified in 6.4 and shall
fulfil the requirements in 5.4.
6.4 Air tightness
The air tightness shall be tested for the CHEFU with its all inner part in normal working position. The air
tightness test of the CHEFU shall conform to requirements specified in A.1. A life cycle evaluation can be
conducted by using the alternating pressure method specified in A.2, based on the agreement between the
user and supplier. The air tightness test results of the CHEFU shall fulfil the requirements in 5.4.
6.5 Leakage test of the installed HEPA filter
6.5.1 Test aerosol
Any of the following aerosols can be used for the leak test of the installed HEPA filters:
— DEHS;
— PAO;
— other aerosols approved by the user and the supplier.
The test aerosol should be generated by either Laskin nozzle or thermal generator and introduced to the
upstream side of the HEPA filter in the CHEFU. The challenge concentration shall be uniform over the entire
upstream face of the tested filter(s) (spatial uniformity, 6.5.3.1) and remain constant throughout the entire
duration of the test (temporal uniformity, 6.5.3.2).
6.5.2 Test apparatus
The scanning leakage test can be carried out by using a photometer or a light-scattering airborne-particle
counter (LSAPC) which shall conform to the requirements in either ISO 21501-1 or ISO 21501-4. If a LSAPC
is used, the minimum detectable particle diameter shall not be greater than 0,3 μm. The particle size to be
counted should be equal or greater than 0,5 μm for scanning leak test, and approximately 0,3 μm to 0,5 μm,
for total penetration test.
6.5.3 Scanning leak test
6.5.3.1 Upstream aerosol spatial uniformity
The upstream aerosol spatial uniformity of the CHEFU should be checked as specified in B.1 by using a LSAPC.
6.5.3.2 Upstream aerosol temporal uniformity
Introduce the test aerosol at the aerosol injection port of the CHEFU under test. Sample the upstream test
aerosol for one minute using a LSAPC and determine the upstream aerosol temporal uniformity. Repeat the
measurements five times. The standard deviation of all the five test results shall not exceed 10 % of the mean
concentration. Further, the upstream aerosol concentration should be large enough to yield statistically high
confidence in the results without exceeding the maximum measuring range of the LSAPC. If the upstream
number concentration exceeds the range of the LSAPC, a dilution system shall be used between the sampling
port of the CHEFU and the LSAPC.
6.5.3.3 Procedure of test using a photometer
Where a photometer is used for the leak detection through scanning for the HEPA filter, the test procedure
shall meet the requirements of ISO 14644-3. Similar filter leakage test procedure by using a photometer can
also be found in the US recommended practice of IEST-RP-CC034 and are widely used for decades. The user
can also follow the procedure of IEST-RP-CC034 for filter leakage test using a photometer.
6.5.3.4 LSAPC test Procedure
The test procedures are as follows:
a) Introduce the test aerosol to the upstream of the installed HEPA filter in the CHEFU and, after confirming
that the upstream aerosol concentration is stable, carry out the upstream aerosol concentration c test.
u
b) Zero count rate of the LSAPC should be tested and determine the criterion N for "suspected leak" in the
p
downstream leakage detection through scanning. N can be set to 1 if the zero-count rate is less than
p
-1
1 min . During the scanning process, any sampling location that causes the LSAPC to generate non zero
readings will be marked as "suspected leak point", and further fixed sample point measurements shall
be carried out to confirm whether it is a leakage.
c) Use Formula (1) to determine the maximum scanning speed according to the width of the scanning
probe of the LSAPC used:
CP××QD×
uL SP
S ≤ (1)
r
N
P
where
S is t
...