ASHRAE Journal - August 2020 - 27

TECHNICAL FEATURE

FIGURE 1 Test system for measuring viral filtration efficiency of HVAC filters.

Pressure Gauge

Nebulizer
HEPA
Filter
Blower

Test Filter
Mixing
Bafﬂe

Impactors
HEPA
Filter

recommended by the EPA8 for air ﬁltration tests. MS2
virus has approximately the same aerosol characteristics as a human virus and can serve as a surrogate for
viruses of similar and larger size and shape. MS2 virion
(viral particle) is about 27 nm in diameter, as determined by electron microscopy,9 much smaller than the
SARS-CoV-2 virion, which measures about 120 nm in
diameter.10,11
Another test method used for evaluating the inactivation efﬁcacy of an air cleaning device is ASHRAE
Standard 185.1-2015, Lights for Use in Air-Handling Units
or Air Ducts to Inactivate Airborne Microorganisms, which is
developed for testing UV-C light devices in air-handling
ducts for inactivation of airborne microorganisms.
ASHRAE Standard 185.1-2015 uses Mycobacterium parafortuitum and Aspergillus sydowii as test microorganisms.
In this study, MS2 bacteriophage was used as the test
microorganism because it is safe to use and has the
relevant size and shape to viruses that have signiﬁcant
public health concerns, such as the SARS-CoV-2 virus.
The tests were performed under positive pressure
with a blower pushing air through a HEPA ﬁlter bank
to provide particle-free air into the test duct airstream.
MS2 microorganisms were grown on appropriate media,
harvested and resuspended in saline to 5 × 106 plaqueforming units per milliliter (pfu/mL). Saline is commonly used in making bacterial suspension because the
cells get acclimatized and escape being ruptured. Our
experience showed that when using saline, speciﬁc size
distribution of a virus can be achieved repeatedly.
Suspension of the organisms was then aerosolized
into the test duct using a nebulizer. Nebulizers have
been widely used for generating bioaerosols. Nebulizer
generators produce droplet aerosols with mass median

Studies on Bioaerosol Removal
Eninger et al.,13 investigated the feasibility of a
novel testing protocol that allows differentiating
between the physical (total) and viable bioaerosol
penetrations through respirator ﬁlters. They found
no statistically signiﬁcant differences were observed
between penetration values obtained for physical
(total) and culturable viruses for the two control
respirators. In another work,14 a test system was
designed to determine the removal efﬁciencies of
ﬁbrous air ﬁlter media with traditional nonbiological airborne particles (dioctyl phthalate, polystyrene
latex, etc.) and bioaerosols. The study found that
efﬁciencies measured with the bioaerosols and nonbiological aerosols had similar characteristics, with
the efﬁciency for the former generally found to be a
little higher.
The biological inactivation efﬁciency by HVAC
in-duct ultraviolet (UV) light systems and ﬁlters
was investigated in another study.15 It found that
the viral ﬁltration efﬁciency and MERV rating had a
similar trend, i.e., the higher the MERV, the higher
the viral ﬁltration efﬁciency. In other words, the
ﬁlter efﬁciency measured with inert particles provided a fair indication of the ﬁlter performance
against bioaerosols.
In research16 conducted to assess if application
of nonbiological aerosols reﬂects air ﬁlters' capacity to collect particles of biological origin, the collection efﬁciency for nonbiological aerosol was
tested with the ﬁlter test system and ISO Fine Test
Dust, while bioaerosols contained four bacterial
strains of different shape and size: Micrococcus luteus,
Micrococcus varians, Pseudomonas putida and Bacillus
subtilis. Nonbiological aerosol-containing particles of
the same shape and surface characteristics (like diethyl-hexyl-sebacat [DEHS] spherical particles) did
not give representative results for all particles present in the ﬁltered air.
aerodynamic diameters of 1 µm to 3 µm.12 Mixing
bafﬂes designed per ASHRAE Standard 52.2-2017 were
mounted in the downstream of the inlet HEPA ﬁlter. The

John Zhang, Ph.D., is senior specialist, Doug Huntley is senior lab manager, Andy Fox is lead specialist and Bryan Gerhardt is an advanced specialist in the Construction and Home
Improvement Market Division of 3M Company. Al Vatine is president and John Cherne is microbiologist at LMS Technologies, Bloomington, Minn.
AU G UST 2020

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ASHRAE Journal - August 2020

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