ASHRAE Journal - February 2021 - 23

COLUMN SOLVING PROBLEMS

of captured infectious particles, but probably does not
materially add to the overall effectiveness of the device,
since a HEPA ﬁlter removes nearly all entering particulate matter.
Germicidal UV installed in an air-handling unit is in
series with whatever ﬁlters the system is required to have,
so its incremental beneﬁt will decrease as ﬁlter efﬁciency
increases. For example, suppose a ﬁlter removes 60% of
the viral load in an airstream on a single pass. If a UVGI
airstream disinfection system is added that achieves
90% single pass inactivation, it is inactivating 90% of the
infectious material that was not already captured by the
ﬁlter. The removal efﬁciency of the combination is 96%,
so the UVGI system only reduces the viral load by an additional 36% beyond what was captured by the ﬁlter. This
does not make adding UVGI in such a situation a bad idea,
but it needs to be understood.
What steps can people take to avoid misapplications?
Those who are selecting UVGI systems should ﬁrst
educate themselves about the fundamentals of the
technology and understand how it is applied. The
two ASHRAE Handbook chapters on UVGI5,6 are a good
place to start. It is also very helpful to work with UVGI
system manufacturers that have a record of successful
installations.
What are your recommended best practices ?
Most important, one must deﬁne the performance goals
for a UVGI system, i.e., what dose needs to be delivered
over what period of time for the design level of microbial
susceptibility and under what ambient conditions-temperature and air speed. This deﬁnes the average ﬂuence
(the germicidal energy per unit area incident on a point
in space from all directions) or irradiance (the germicidal
energy per unit area at a point on a surface) that is needed
and can be the basis for selecting an appropriate number
and conﬁguration of lamps. System design should be supported by calculations to conﬁrm the required UV-C dose
is delivered by the system. Manufacturers should be able
to assist in this analysis.
What are some resources readers can use to learn
more about best practices for using UVGI?
In addition to the ASHRAE Handbook chapters5,6 mentioned in response to Question 3, a 2008 ASHRAE Journal
article summarizes best practices for mercury vapor

lamp systems.7 The NIOSH guide4 for upper room systems mentioned in response to Question 1 is an excellent
reference on that type of system. Kowalski's book is a
comprehensive source of information on many aspects
of germicidal ultraviolet theory and application.8
Is there anything else you think engineers need to
know about this topic?
The growing availability of germicidal LEDs that can
produce different wavelengths within the UV-C band
has the potential to greatly expand the ways in which
germicidal light can be deployed in air and surface disinfection applications. Krypton-chlorine excimer lamps
that produce 222 nm " far " UV-C are also an emerging
technology of interest. Far UV-C light with a wavelength
of approximately 220 nm and shorter is attracting a
great deal of attention because preliminary studies suggest that it is not only an effective disinfectant, but also
safe, or at least much safer for human exposure than the
current 254 nm standard.9,10

References

1. UL. 2019. " ECVP 2998, Environmental Claim Validation
Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners. " UL.
2. Wells, W.F., M.W. Wells, T.S. Wilder. 1942. " The environmental
control of epidemic contagion. I. an epidemiologic study of radiant
disinfection of air in day schools. " American Journal of Hygiene, 35(1),
pp.97 - 121.
3. Jensen, P.A., L.A. Lambert, M.F. Iademarco, R. Ridzon. 2005.
" Guidelines for Preventing the Transmission of Mycobacterium
Tuberculosis in Health-Care Settings, 2005. " " Morbidity and
Mortality Weekly Report (MMWR). " Centers for Disease Control and
Prevention. " https://tinyurl.com/y5jyqpjp
4. NIOSH. 2009. " Environmental Control for Tuberculosis:
Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines
for Healthcare Settings. " DHHS (NIOSH) Publication Number
2009-105. National Institute for Occupational Safety and Health.
https://tinyurl.com/y58n6oh5
5. 2019 ASHRAE Handbook-HVAC Applications, Chap. 62, " Ultraviolet Air and Surface Treatment. "
6. 2020 ASHRAE Handbook-HVAC Systems and Equipment, Chap. 17,
" Ultraviolet Lamp Systems. "
7. Martin, S.B., C. Dunn, J.D. Freihaut, W.P. Bahnﬂeth, et al.
2008. " Ultraviolet germicidal irradiation: current best practices. " ASHRAE Journal 50(8):28 - 36.
8. Kowalski, W. 2010. Ultraviolet Germicidal Irradiation Handbook: UVGI
for Air and Surface Disinfection. Springer Science & Business Media.
9. Welch, D., M. Buonanno, V. Grilj, I. Shuryak, et al. 2018. " FarUVC light: A new tool to control the spread of
airborne-mediated microbial diseases. " Scientiﬁc
Reports 8(1):1 - 7.
10. Woods, J.A., A. Evans, P.D. Forbes, P.J.
Coates, et al. 2015. " The effect of 222-nm UVC
phototesting on healthy volunteer skin: a pilot
https://bit.ly/3aC5wVH
study. " Photodermatology, Photoimmunology &
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Photomedicine 31(3):159 - 166.
FEBRUARY 2021

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ASHRAE Journal - February 2021

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