Method of evaluating the UV dose to airborne microorganisms transiting in-duct ultraviolet germicidal irradiation devices

This document describes a method in laboratory to assess the performance of ultraviolet germicidal irradiation (UVGI) devices which will be mounted in-duct in heating, ventilating and air-conditioning (HVAC) systems. The method includes the detailed requirements for test rig, microorganisms, procedures, data calculation and result report to determine the UV dose to model microorganisms by an UVGI device at several airflow rates. By the testing results, the capacity of in-duct UVGI devices for air disinfection can be evaluated and compared reliably. If the susceptibility constant of a given microorganism is known, the inactivation rate of that micro-organism by the tested UVGI devices can be further calculated.

Méthode d'évaluation de la dose d'UV pour les microorganismes en suspension dans l'air transitant par des dispositifs d'irradiation germicide aux ultraviolets raccordés

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Published
Publication Date
11-Jul-2019
Current Stage
6060 - International Standard published
Start Date
12-Jul-2019
Completion Date
12-Jul-2019
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INTERNATIONAL ISO
STANDARD 15714
First edition
2019-07
Method of evaluating the UV dose to
airborne microorganisms transiting
in-duct ultraviolet germicidal
irradiation devices
Méthode d'évaluation de la dose d'UV pour les microorganismes
en suspension dans l'air transitant par des dispositifs d'irradiation
germicide aux ultraviolets raccordés
Reference number
ISO 15714:2019(E)
ISO 2019
---------------------- Page: 1 ----------------------
ISO 15714:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019

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Published in Switzerland
ii © ISO 2019 – All rights reserved
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ISO 15714:2019(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions, symbols and abbreviated terms ....................................................................................................... 1

3.1 Terms and definitions ....................................................................................................................................................................... 1

3.2 Symbols and abbreviated terms............................................................................................................................................... 3

3.2.1 Symbols .................................................................................................................................................................................... 3

3.2.2 Abbreviated terms .......................................................................................................................................................... 3

4 Configuration of the test rig ...................................................................................................................................................................... 4

5 Test rig qualification ......................................................................................................................................................................................... 5

6 Preparation of test microorganisms ................................................................................................................................................ 5

6.1 Test microorganisms .......................................................................................................................................................................... 5

6.1.1 Serratia marcescens ................................................................................................................................................. 5

6.1.2 Bacillus subtilis .............................................................................................................................................................. 5

6.1.3 Cladosporium sphaerospermum .................................................................................................................. 5

6.2 Preparation of microbial suspensions ................................................................................................................................ 6

6.2.1 Acquisition of pure culture of test microorganisms .......................................................................... 6

6.2.2 Cultivation and dispersion of the test microorganism .................................................................... 6

6.2.3 Dilution of the microbial suspensions ........................................................................................................... 6

7 Testing procedure for an in-duct UVGI device ....................................................................................................................... 6

7.1 Determination of airflow rate, temperature and humidity .............................................................................. 6

7.2 Production of the airborne test microorganism ........................................................................................................ 6

7.3 Measurement of the test microorganism concentration without and with UV irradiation .. 7

7.3.1 Sampling procedure ...................................................................................................................................................... 7

7.3.2 Test microorganism sampling methods .................. ..................................................................................... 7

7.3.3 Test microorganism culture and enumeration ...................................................................................... 7

7.4 Repeating the tests at other flow rates .............................................................................................................................. 7

7.5 Determination of the UV susceptibility of the test microorganism ........................................................... 7

8 Safety and environmental considerations ................................................................................................................................. 7

9 Calculation, evaluation and reporting ........................................................................................................................................... 8

9.1 Determination of the inactivation rate of the test microorganism ............................................................ 8

9.2 Determination of the UV dose of the UVGI device .................................................................................................... 8

9.3 Evaluation of the UVGI capacity ............................................................................................................................................... 8

9.4 Results reporting ................................................................................................................................................................................... 8

Annex A (informative) Recipe of culture medium for the test microorganism ........................................................9

Annex B (informative) Method for determining the UV dose-response curve and

susceptibility constant of a test microorganism in air ..............................................................................................10

Annex C (informative) Susceptibility constants of some typical microorganisms in air

by the literature ..................................................................................................................................................................................................13

Bibliography .............................................................................................................................................................................................................................16

© ISO 2019 – All rights reserved iii
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ISO 15714:2019(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso

.org/iso/foreword .html.

This document was prepared by Technical Committee ISO/TC 142, Cleaning equipment for air and

other gases.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2019 – All rights reserved
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ISO 15714:2019(E)
Introduction

Airborne microorganisms including some pathogens in indoor air may cause different types of

diseases or adverse health effects on humans. Among different air disinfection techniques, ultraviolet

germicidal irradiation (UVGI) has been used for several decades to effectively inactivate the airborne

microorganisms in indoor air and thereby prevent the transmission of a variety of airborne infections.

In-duct UVGI device is a primary form of air disinfection method by UV lamps mounted in heating,

ventilation and air-conditioning (HVAC) systems to irradiate the microorganisms in air with high

intensities. However, other than the power supply, there is no standard or index available to characterize

or understand the performance of the UVGI products made by different manufacturers. In addition,

effective parameters derived from a standard method are lacking to predict the performance of the

UVGI device on microorganism inactivation in a real HVAC system.

As microorganisms in air are irradiated by UV-C light emitted by an in-duct UVGI device, the

inactivation rate of a specific microorganism primarily depends on the UV dose given by the device

and the susceptibility of that microorganism. If the UV dose under a specific condition is known, the

inactivation capacity and disinfection performance of the UVGI devices can be compared. Furthermore,

the inactivation rate for specific microorganism can be calculated with its susceptibility data known.

Therefore, the development of a standard method to evaluate the UV dose of the in-duct UVGI device is

very useful and necessary.
© ISO 2019 – All rights reserved v
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INTERNATIONAL STANDARD ISO 15714:2019(E)
Method of evaluating the UV dose to airborne
microorganisms transiting in-duct ultraviolet germicidal
irradiation devices
1 Scope

This document describes a method in laboratory to assess the performance of ultraviolet germicidal

irradiation (UVGI) devices which will be mounted in-duct in heating, ventilating and air-conditioning

(HVAC) systems.

The method includes the detailed requirements for test rig, microorganisms, procedures, data

calculation and result report to determine the UV dose to model microorganisms by an UVGI device at

several airflow rates. By the testing results, the capacity of in-duct UVGI devices for air disinfection can

be evaluated and compared reliably.

If the susceptibility constant of a given microorganism is known, the inactivation rate of that micro-

organism by the tested UVGI devices can be further calculated.
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 15858, UV-C Devices — Safety information — Permissible human exposure
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at http: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1.1
airborne microorganism
microbial particle with an aerodynamic diameter up to 100 μm suspended in air

Note 1 to entry: Airborne microorganism includes bacterium, fungus, their spore or virus.

3.1.2
pathogen
infectious agent that causes diseases in its host

Note 1 to entry: Pathogen includes some virus, bacterium, prion, fungus, viroid, or parasite.

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ISO 15714:2019(E)
3.1.3
test microorganism
microbial surrogate representing the typical pathogen (3.1.2)

Note 1 to entry: Test microorganism is chosen to be safer than the real pathogen in order to prevent the infection

of testers or analysts.
3.1.4
air disinfection

process that can remove, inactivate or destroy the airborne microorganisms (3.1.1), especially pathogen

(3.1.2) in air
3.1.5
ultraviolet germicidal irradiation
UVGI

method for disinfection of air, water and object surfaces that uses radiation with wavelength in the

range of 240 nm to 280 nm to kill or inactivate microorganism

Note 1 to entry: UV irradiation with a wavelength of 240 nm to 280 nm can cause damage to the DNA or RNA of

the microorganisms.
[SOURCE: ISO 29464:2017, 3.6.20, modified — Note 1 to entry has been added.]
3.1.6
in-duct UVGI device

device consisting of UV lamps, ballast and other accessories, all of which could be mounted in ducts of

an HVAC system to disinfect the air or a surface

Note 1 to entry: A typical diagram of in-duct UVGI device in an HVAC system is shown in Figure 1.

Key
1 filter
2 UV lamp
3 heating or cooling coil
4 fresh air
5 conditioned air
Figure 1 — Diagram of an in-duct UVGI device in an HVAC system
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ISO 15714:2019(E)
3.1.7
UV dose

product of UV irradiance and specific exposure time on a given microorganism or surface

Note 1 to entry: UV dose is expressed in millijoules per square centimetre (mJ/cm ).

Note 2 to entry: The longer the time a microbe is exposed to UV light, the higher the UV dose it will receive. In a

UVGI air disinfection (3.1.4) device, the UV dose to every single microbe is different. For the device with evenly

distributed UV irradiation and airflow, the UV dose can be calculated based on the definition. But for most real

in-duct UVGI devices (3.1.6), it is hard to evaluate the UV dose to each microbe but the average UV dose can be

determined by the inactivation rate (3.1.9) and a known microbial susceptibility.

3.1.8
UV susceptibility

extent to which a microorganism is sensitive to UV light or how easily it can be inactivated by UV

irradiation

Note 1 to entry: UV susceptibility depends on the species and character of the microorganism. It can be described

by a constant (k) with the unit of m /J.
3.1.9
inactivation rate

reduction in active microorganism concentration expressed as N /N (%) or log(N /N), in which N is

0 0 0

the original active microorganism concentration, N is the active microorganism concentration after

disinfection
3.1.10
UV dose-response curve

quantified relationship between the inactivation rate (3.1.9) of a specific microorganism and the

average UV dose (3.1.7) it received
Note 1 to entry: In many cases, the relationship follows the equation as below:
ln(N /N) = k D (1)

in which D, k and ln(N /N) have been described in 3.1.7 to 3.1.9. In Formula (1), N/N or D can be calculated with

0 0

the other parameters known. In other cases, the relationship may not strictly follow Formula (1), but N/N or D

can also be determined according to the specific curve.
3.2 Symbols and abbreviated terms
3.2.1 Symbols
Symbol Definition
N original active microorganism concentration
N active microorganism concentration after disinfection
D UV dose
k susceptibility constant
3.2.2 Abbreviated terms
ATCC American Type Culture Collection
BMBL Biosafety in Microbiological and Biomedical Laboratories
BSL biosafety level
CDC Centres for Disease Control and Prevention of the United States
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ISO 15714:2019(E)
CFU colony-forming unit
HEPA high-efficiency particulate air
HVAC heating, ventilating and air-conditioning
PDA potato dextrose agar
UVGI ultraviolet germicidal irradiation
4 Configuration of the test rig

4.1 In order to evaluate the disinfection capacity of an in-duct UVGI device, the inactivation performance

for some specific test microorganism shall be measured through a standard test rig as shown in Figure 2.

Dimensions in millimetres
Key
1 air 7 downstream duct
2 blower 8 HEPA filter
3 damper 9 injection of test microorganisms
4 HEPA filter 10 UV irradiation
5 upstream duct 11 sampling port for microorganism detection and the
measurement of airflow, temperature and humidity
6 UVGI device mounting duct
Figure 2 — Test rig for inactivation performance of in-duct UVGI device

4.2 The test rig includes a blower (installed in front or at the end of the test rig), a damper, a HEPA

filter before the duct, an upstream duct with test microorganism injection port, a UVGI device mounting

duct, a downstream duct with sampling port and an off-gas pipe with HEPA filter.

4.3 The ducts have a square cross-section and an inner side-length of 0,6 m. Galvanized steel or

aluminium with a reflectivity of 50 % to 60 % shall be used to make the duct walls. The upstream duct

and downstream duct have a length of 2,0 m and the UVGI device mounting duct has a length of 1,0 m.

4.4 In the test rig, a damper is used to control the flow rate of the test system. A subsequent HEPA

filter shall be placed before the test microorganism injection port in order to remove the culturable

microorganisms that may exist in the air to avoid their impact on test microorganism quantification.

4.5 In the upstream duct, a test microorganism injection port of 10 mm to 15 mm in diameter is set

near the left flange of the duct (within 20 cm). The port can be set in the centre line of bottom or side wall.

4.6 In the UVGI device mounting duct, the UVGI device shall be installed following the instruction of

the manufacturer and simulating its working conditions. If a new UV lamp is used, it shall be powered on

continuously for 100 hours (called burn-in time) before it is tested.
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ISO 15714:2019(E)

4.7 In the downstream duct, a sampling port (Key 11 in Figure 2) of 15 mm to 20 mm in diameter is

placed to collect the test microorganisms and to do measurements on the airflow rate, temperature and

humidity by different sensors. The port can be set in the centre line of the bottom or side wall. Connected

with the downstream duct, another HEPA filter is recommended to be installed to minimize the test

microorganism emission to the environment.
5 Test rig qualification

Before launching the testing procedures, the test rig needs to be examined to insure that it is in a good

condition and can provide reliable results. The methods for test rig qualification shown in Section 5

of ANSI/ASHRAE 185.1 are a useful reference. Tests on the velocity uniformity and duct leakage are

recommended. All the qualification tests shall be operated at an air velocity of (2,0 ± 0,2) m/s.

6 Preparation of test microorganisms
6.1 Test microorganisms
6.1.1 Serratia marcescens

Serratia marcescens is a species of rod-shaped Gram-negative bacteria in the family Enterobacteriaceae.

They are frequently used as typical test microorganisms for biodosimetry purposes and are good

surrogates of those bacteria with high susceptibility to UV, especially many Gram-negative bacteria.

2 2

The reported susceptibility constants (k) for Serratia marcescens range in 0,1 m /J to 0,9 m /J. The

recommended culturing media are nutrient agar (solid) which are commercially available and easy to

prepare (the recipe is listed in Annex A).

Serratia marcescens is suitable for testing the UVGI device with an effective UV dose less than 25 J/m .

6.1.2 Bacillus subtilis

Bacillus subtilis is a species of rod-shaped, Gram-positive and endospore-forming bacteria in the family

Bacillaceae. They are frequently used as typical test microorganisms representing those bacteria with

low susceptibility to UV, especially the Gram-positive bacteria. The reported susceptibility constants

2 2

(k) for Bacillus subtilis (vegetative cells) range in 0,02 m /J to 0,07 m /J. The recommended culturing

media are nutrient agar (solid) which are commercially available and easy to prepare (the recipe is

listed in Annex A).
2 2

Bacillus subtilis is suitable for testing the UVGI device with an effective UV dose from 25 J/m to 120 J/m .

6.1.3 Cladosporium sphaerospermum

Cladosporium sphaerospermum is a saprobic and spore-forming fungus that inhabits a variety of

environments including the indoor and outdoor air. They are typical test microorganisms representing

those fungi with very high susceptibility to UV. The reported susceptibility constants (k) for

2 2

Cladosporium sphaerospermum spores in single-pass tests range in 0,000 8 m /J to 0,002 m /J. The

recommended culturing media are potato dextrose agar (PDA) (solid) which are commercially available

and easy to prepare (the recipe is listed in Annex A).

Cladosporium sphaerospermum is suitable for testing the UVGI device with an effective UV dose more

than 120 J/m .
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ISO 15714:2019(E)
6.2 Preparation of microbial suspensions
6.2.1 Acquisition of pure culture of test microorganisms

All the test microorganisms above are available for purchase through some national or global biological

resource centres like American Type Culture Collection (ATCC) and have a biosafety level of 1 (not

known to consistently cause disease in healthy adult humans) classified by the Centres for Disease

Control and Prevention of the United States (CDC).
6.2.2 Cultivation and dispersion of the test microorganism

Inoculate the test organism onto solid media as indicated in the previous section, incubating the culture

until mature, wiping a wetted sterile swab across the surface of the pure culture, and eluting from the

swab into sterile deionized water to obtain a 600 nm absorbance of 0,8 to 1,0 with a 1 cm cuvette.

6.2.3 Dilution of the microbial suspensions

Dilute the microbial suspension to achieve a suitable test microorganism concentration typically

4 3 8 3

ranging in 10 CFU/cm to 10 CFU/cm . The exact concentration shall meet the requirement for

producing an air stream with a desired concentration.
7 Testing procedure for an in-duct UVGI device
7.1 Determination of airflow rate, temperature and humidity

The performance of an in-duct UVGI device can be affected by airflow rate, temperature, humidity and

other factors. Among these impacting factors, airflow rate is the most important one and a reasonable

range shall be determined before the test. Adjust the airflow rate to the desired level by selecting a

suitable blower and changing the angle of the damper. Three airflow rates at (1 000 ± 100) m /h,

3 3

(2 000 ± 100) m /h and (3 000 ± 100) m /h shall be selected for the inactivation test.

Air temperature and relative humidity are less important than the airflow rate but still need to

be controlled within (25 ± 2,5) °C and 50 % ± 10 % during the testing period. The exact level of air

temperature and humidity with the UV irradiation switched on shall be noted as a reference condition

while demonstrating the testing results.

At the sampling port in downstream duct indicated in Figure 2 (Key 11), the airflow rate shall

be measured by a portable anemometer (with a relative accuracy better than ±10 %) and the air

temperature and humidity shall be measured by a digital hygrometer (with absolute accuracies better

than ±0,5 °C for temperature and ±3 % for relative humidity). The measurements of airflow rate,

temperature and humidity shall be repeated twice and the relative difference in the same index shall be

less than 10 % to ensure the testing system is stable.

All the three parameters shall be tested at the beginning and end of each test. The relative variations

shall be less than 10 %.
7.2 Production of the airborne test microorganism

The test microorganism with desired concentrations can be generated by a Collison nebulizer or

ultrasonic air humidifier with fans and shall be injected into the airstream via a tube. Refer to 4.5

for the injection position. By adjusting the test microorganism concentration in suspensions and the

setting of the generator, the microbe concentration in the air stream can be controlled. The suitable

3 3 4 3
range shall be 10 CFU/m to 10 CFU/m .
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ISO 15714:2019(E)
7.3 Measurement of the test microorganism concentration without and with UV
irradiation
7.3.1 Sampling procedure

After the test microorganism concentration is adjusted to the desired level, the airborne microorganism

shall be sampled with UVGI switched off at the sampling port in the downstream duct (Key 11 in

Figure 2). The airborne microorganism concentration with UVGI switched off functions as a control

to avoid the effect of air duct on microbe inactivation. After the sampling with UVGI switched off are

completed, turn on the UV lamp and preheat it for 15 min. Then do the test microorganism sampling

with UVGI on at the same sampling port with the same methodology.
7.3.2 Test microorganism sampling methods

The sampling of the test microorganism can be accomplished by an impaction type air sampler such

as Andersen sampler or a commercial impinger. The device and procedure for all sampling shall

be identical throughout the test. The sampling shall be repeated three times and at least two tested

concentrations shall have a relative difference less than 50 %.
7.3.3 Test microorganism culture and enumeration

After agar plates are taken out of the impaction type air sampler or the suspension sample from an

impinger is spread on agar plates, put them directly into an incubator at 32 °C for bacteria for 24 h

to 48 h and 25 °C for fungi for 72 h to 120 h after which the number of colonies is counted. Then the

airborne microorganism concentration can be calculated as the number of colonies in a certain volume

of air (CFU/m ) considering the sampling flow rate and time. All the results with less than 50 % relative

difference between any two of them shall then be averaged to do the performance calculation.

7.4 Repeating the tests at other flow rates

After completing the steps described above, the UVGI device can be turned off and a new airflow rate

can be set to repeat the testing procedure from 7.1 to 7.3. All the tests under each designated flow rate

shall be completed. If some mistakes are found or some results do not meet the requirements, some

steps or measurement shall be repeated.
7.5 Determination of the UV susceptibility of the test microorganism

The UV susceptibility (UV dose-response curve) of the test microorganism shall be tested for calculating

the UV dose of the UVGI device. The detailed method is described in Annex B. Note that the exact same

test microorganism, its generation and testing methods shall be used. Besides, all the other conditions

such as air temperature, humidity and initial microbe concentration shall also be kept at the same level.

If the UV susceptibility test is not available, the citation of UV susceptibility constant is also acceptable

(see Annex C) with the value and reference noted in the report.
8 Safety and environmental considerations

8.1 During the test, the first safety concern is the possible personal exposure to UV. The operation

of the UVGI device shall follow ISO 15
...

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