ASHRAE Journal - September 2021 - 42

TECHNICAL FEATURE
Studies have shown inactivation of viruses by photocatalysis
is initiated by their adsorption onto the catalyst's
nanoparticles followed by an attack on the protein
capsid.22 Other studies suggest the inactivation is due
to free hydroxyl radicals.22 Another study by Kozlova, et
al., found that the vaccinia virus and influenza A virus
(H3N2) were inactivated 90% to 99.8% after 30 minutes
of exposure.23 However, despite the promising results,
PCO has the potential for production of by-products like
formaldehyde due to incomplete oxidization.19,20 Also,
there is a potential reduction in catalyst efficiency over
time.19,20 These limitations should be evaluated when
implementing this technology.
Far-UV-C refers to devices that operate in the 207 nm
to 222 nm wavelength range.24 UV-C light in this range
is strongly absorbed by biological materials and doesn't
penetrate through the outer dead-cell layers (stratum
corneum) on the surface of human skin or the outer tear
layer of the eye.24 Since far-UV-C can only penetrate a
few micrometers, it cannot reach living human cells in
the skin or eyes.25 However, this light can still inactivate
bacteria and viruses with efficiencies comparable
to UV-C in the 254 nm wavelength due to the virus's
smaller cell size.24 Buonanno, et al., found that low
doses (1.2 mJ/cm2 to 1.7 mJ/cm2) of 222 nm light inactivated
99.9% of the airborne human coronavirus tested.25
Welch, et al., also found that 2 mJ/cm2 of 222 nm light
could inactivate 95% or more of aerosolized H1N1 influenza
virus.24 The threshold limit value (TLV) for 222 nm
light to which the public can be exposed is 23 mJ/cm2
per eight-hour exposure.25 Based on far-UV-C exposure
set at the regulatory limit, continuous exposure could
result in 90% viral inactivation of airborne viruses in
about eight minutes, 95% in 11 minutes, 99% in 16 minutes
and 99.9% in 25 minutes.25
Many of the emerging technologies listed above also
have the potential to produce ozone. ASHRAE Standard
62.1-201926 requires air-cleaning devices that can
produce ozone to be listed and labeled per UL 2998,
Environmental Claim Validation Procedure (ECVP) for
Zero Ozone Emissions from Air Cleaners.
Disinfectant Treatments
Some treatments being used for disinfection may be
acceptable in certain situations. These treatments are
often only used during unoccupied times, since they
are often harmful to occupants in the space. These
42
ASHRAE JOURNAL ashrae.o rg
S E PTEM B E R 2021
treatments may also require the space to be purged
before occupation.
The first ozone generator was patented by Nikola Tesla
in 1896.27 Since then ozone has been used off and on for
air and water purification. Lately, research showing the
effect of ozone on occupant health has made the use of
these devices as air purifiers unsafe in most situations.20
Ozone, even at low levels, can produce respiratory
issues in humans and actually cause other health risks
through the formation of formaldehydes and aldehydes.20
ASHRAE states that based on current science
there is " no consensus on the safe level of ozone. " 20
ASHRAE Standard 62.1-2019, Table D-126 lists the eighthour
limit at 0.07 ppm, and the EPA and other agencies
suggest avoiding the use of air cleaners that use
ozone.20,40
A few studies have shown ozone can be used for virus
inactivation. Dubuis, et al., found that " low " levels of
ozone and high relative humidity (RH)-1.23 ppm and
85% RH, respectively-could inactivate bacteriophages
two orders of magnitude after 40 minutes.28 A study by
Hudson, et al., also showed that norovirus could be inactivated
on surfaces under high levels of ozone (20 ppm to
25 ppm).29 Another study by Hiroshi, et al., showed similar
results for influenza.30 The half-life of ozone is about
20 minutes, and it quickly decays back to oxygen.29
The use of a catalytic converter can also speed up the
removal of the gas.29 Since these studies used levels of
ozone that are higher than safe levels in an occupied
space, use of ozone in an occupied space is not recommended.
Use in an unoccupied space may be acceptable.
However, other methods like ventilation make more
sense for educational occupancies.
Chemical disinfectants like hypochlorite, peroxymonosulfate,
alcohols, quaternary ammonium compounds
and hydrogen peroxide are typical for surface
disinfection of viruses.31 Vaporized hydrogen peroxide
(VHP) has also been used in engineered disinfection
systems for control of viruses.31 A study by Goyal, et
al., has showed a 4-log reduction or greater for viruses
dried on surfaces.32 VHP requires spaces to be sealed to
prevent the vapor from escaping. Also, the space must
be unoccupied since high concentrations of VHP can be
hazardous.1
Silver nanoparticles (AgNP) have been used in commercial
virus sprays for surface disinfection of viruses.
Silver has broad spectrum antimicrobial action against
http://ashrae.org
ASHRAE Journal - September 2021

Table of Contents for the Digital Edition of ASHRAE Journal - September 2021
