ASHRAE Journal - July 2021 - 10

LETTERS
Airborne transmission of SARSCoV-2
is brought about by virusladen
aerosols, which originate from
expiratory liquid droplets,1,2 and
humidity has an important role to
play. There are very few studies in
design guides on the evaporation
effect of droplets under different air
humidities. The life cycle of aerosol
droplets, including formation, evaporation
and time staying in air are
all important parameters that affect
the infectivity of the virus, especially
indoors.
References
1. Burkett, J. 2021. " Virus transmission
modes and mitigation strategies, part 1:
Defining viruses and droplet release. "
ASHRAE Journal (3):24 - 29.
2. Burkett, J. 2021. " Virus transmission
modes and mitigation strategies, part 2:
Airborne transmission and distribution. "
2021. ASHRAE Journal (4):10 - 16.
3. Loomans, M, A. Boerstra, F.
Franchimon, C. Wisse. 2020. " Calculating
the risk of infection. " The REHVA European
HVAC Journal 57(5)19 - 24.
4. Dai, H., B. Zhao. 2020. " Association
of the infection probability of COVID-19
with ventilation rates in confined spaces. "
Building Simulation 13:1321 -1327.
5. Kurnitski, J., M. Kiil, A. Boerstra,
O. Seppanen. 2021. " A new ventilation
criterion based on respiratory infection risk
of COVID-19. " Preprint submitted to Indoor
Air.
6. Pereira, M., R. Vilain, A. Tribess, L.
Morawska. 2015. " Risk assessment for
airborne infectious diseases between
natural ventilation and a split-system air
conditioner in a university classroom. "
Proceedings of the 23rd ABCM International
Congress of Mechanical Engineering.
7. Chow, W.K. 2000. " A comment on
studying the ventilation requirements
for buildings in the Hong Kong Special
Administrative Region. " International Journal
on Architectural Science 1(1):1 - 13.
W.K. Chow, Ph.D., Fellow ASHRAE, ,C.L. Chow, Ph.D;
C.H. Cheng, Ph.D, Hong Kong, China
The Author Responds
I would like to first thank you for
your comments and for highlighting
10
the importance of ventilation in
minimizing airborne transmission.
After source control, ventilation is
considered one of the most important
factors in infection reduction.
Ventilation is often referred
to in studies, but the term is not
always defined. ASHRAE Standard
62.1-2019 defines ventilation air as
" that portion of supply air that is
outdoor air plus any recirculated air
that has been treated for the purpose
of maintaining acceptable IAQ. "
The World Health Organization
(WHO) in their Natural Ventilation for
Infection Control in Health-Care Settings
study concludes that " lack of ventilation
or low ventilation rates are associated
with increased infection rates
or outbreaks of airborne diseases. " 1
However, research has not yet
agreed on a minimum ventilation
rate required to reduce airborne
infection.2
A study by Mousavi, et al.,3 compared
outside air ratios and filter
efficiencies and found that as filtration
efficiency increased, the
contaminant decay time was less
affected by providing more outdoor
air. Another study by Nardell, et al.,4
explored the relationship between
infections quanta generation (disease
infectiousness) and outside
air cfm. They found that as quanta
generation per hour increased, the
impact of providing additional outside
air decreased.
As you mentioned it is hard to justify
a minimum ventilation rate that
is suitable for all spaces. Since the
airborne transmission of diseases
in spaces is greatly affected by air
distribution, ventilation, filtration
and other cleaning methods, environmental
factors (such as temperature,
humidity, thermal currents
ASHRAE JOURNAL ashrae.org J U LY 2021
and occupant movement), as well as
concentration and virulence of the
pathogen, length of exposure and
occupant susceptibility, a minimum
ventilation rate that incorporates all
of these factors is difficult to calculate.
As you also mentioned, a better
approach is CFD modeling or a
simple risk analysis.
Due to the COVID-19 pandemic,
additional research on airborne
disease transmission has a renewed
focus. With this renewed focus, I
am hopeful that as we learn more
about airborne disease transmission,
improved guidance will be
forthcoming.
References
1. Atkinson, J., Y. Chartier, C.L. PessoaSilva,
P. Jensen, et al. 2009. Natural
Ventilation for Infection Control in Health-Care
Settings. World Health Organization.
2. Li, Y., G.M. Leung, J.W. Tang, X.
Yang, et al. 2007. " Role of ventilation
in airborne transmission of infectious
agents in the built environment-a
multidisciplinary systematic review. " Indoor
Air 17(1):2 - 18. https://doi.org/10.1111/j.16000668.2006.00445.x
3.
Mousavi, E.S., N. Kananizadeh, R.A.
Martinello, J.D. Sherman. 2020. " COVID-19
outbreak and hospital air quality: A
systematic review of evidence on air
filtration and recirculation. " Environmental
Science & Technology 55(7):4134 -4147. https://
doi.org/10.1021/acs.est.0c03247
4. Nardell, E.A., J. Keegan, S.A. Cheney,
S.C. Etkind. 1991. " Airborne infection:
Theoretical limits of protection achievable
by building ventilation. " American Review of
Respiratory Disease 144(2):302- 306. https://
doi.org/10.1164/ajrccm/144.2.302
Jonathan Burkett, P.E., Associate Member ASHRAE,
Columbia, S.C.
ASHRAE Journal welcomes letters
to the editor, which should
be no more than 250 words
and relate to an ASHRAE Journal
article. Send your letters to
sfoster@ashrae.org
https://doi.org/10.1111/j.1600-0668.2006.00445.x https://doi.org/10.1111/j.1600-0668.2006.00445.x https://doi.org/10.1021/acs.est.0c03247 https://doi.org/10.1021/acs.est.0c03247 https://doi.org/10.1164/ajrccm/144.2.302 https://doi.org/10.1164/ajrccm/144.2.302 http://www.ashrae.org
ASHRAE Journal - July 2021

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