ASHRAE Journal - July 2019 - 65

COLUMN IEQ APPLICATIONS

direct estimation of infectious source strength and was
used to quantify inﬂuenza aerosol shedding rates in
the United States, Hong Kong and Singapore.5,6 One
hundred forty two cases with conﬁrmed inﬂuenza
infection were part of the study with a total of 218 samples collected.6 Each case was sampled for 30 minutes
to include different naturally occurring unrestricted
expiratory activities to better reﬂect real emissions.
Results showed that, on average, inﬂuenza infected
people emit 4 × 104 virus particles in droplets smaller
than 0.5 m, with 40 of them culturable and 4.13 × 104
virus particles in droplets larger than 0.5 m. Since the
aerosol infectious dose is one culturable virus particle,
the average amount emitted in 30 minutes can potentially infect 40 people.
With information about the source strength, it is possible to design the air supply ﬂow rate to target reduction in the propagation of secondary infection cases.
A full database of source strengths for the infectious
diseases that are primarily or only transmitted via airborne route, including tuberculosis, measles, smallpox
and chickenpox, is needed. Such a database does not
exist at the moment, but recent developments in sampling technology have removed many previous technical limitations.
Currently, airﬂow rates cited in Standard 170-2017
are based on choosing the maximum between the air
requirements for cooling load, air change rate and
makeup air. The largest of the three values will determine the supply air requirements for the air handler.
Recommended air change rates are not based on the
information about the source strength, therefore we
can determine if they are correct, too low or too high.
At the moment, evaluation of these air change rates
can be performed for one type of inﬂuenza virus.
The HVAC Design Manual2 bases the calculation of the
supply ﬂow rates on the assumption of a well-mixed
environment. This approach shows that the supply
ﬂow rate increase creates a higher level of dilution
and reduces exposure to infectious droplets. Recent
studies have shown that in some cases when the supply ﬂow rate increases the generated airﬂow patterns
will cause increased exposure to the infectious aerosols
aerosolized by breathing8 or coughing.9 Movement
of people also plays a role in the transport of infectious aerosols. The wake generated behind the moving
person or object entrains infectious droplets and can

transport them to the adjacent space.10 Complex ﬂow
interactions between expiratory activity ﬂow and the
convective boundary layer around the human body
(the background ﬂow generated by the air supply system) determines the dispersion of infectious aerosols.
It's still not fully understood if in the design process
all these complexities need to be accounted for. One
potential solution can be the use of advanced air delivery systems11 that supply clean air effectively to the
user's breathing zone.

References

1. Memarzadeh, F. 2011. "The environment of care and
health care-associated infections: An engineering perspective."
American Society of Healthcare Engineering of the American Hospital
Association. http://www.ashe.org/management_monographs/pdfs/
mg2011memarzadeh.pdf
2. Koenigshofer, D. 2013. HVAC Design Manual for Hospitals and
Clinics. Atlanta: ASHRAE.
3. Sze To, G.N., C.Y.H. Chao. 2010. Review and comparison
between the Wells-Riley and dose-response approaches to risk
assessment of infectious respiratory diseases. Indoor Air 20(1):2 - 16.
4. McDevitt, J.J., P. Koutrakis, S.T. Ferguson, J.M. Wolfson, M.P.
Fabian, M. Martins, J. Pantelic, D.K. Milton. 2013. Development
and performance evaluation of an exhaled-breath bioaerosol
collector for inﬂuenza virus. Aerosol Science and Technology
47(4):444 - 451.
5. Milton, D.K., M.P. Fabian, B.J. Cowling, M.L. Grantham, J.J.
McDevitt. 2013. Inﬂuenza virus aerosols in human exhaled breath:
particle size, culturability, and effect of surgical masks. PLoS
Pathogens 9(3):e1003205.
6. Yan, J., M. Grantham, J. Pantelic, P.J. B. de Mesquita, B. Albert,
F. Liu, S. Ehrman, D.K. Milton, EMIT Consortium. Infectious virus
in exhaled breath of symptomatic seasonal inﬂuenza cases from
a college community. Proceedings of the National Academy of Sciences
115(5):1081 - 1086.
7. Bolashikov, Z.D., A.K. Melikov, W. Kierat, Z. Popiołek, M.
Brand. 2012. Exposure of health care workers and occupants to
coughed airborne pathogens in a double-bed hospital patient
room with overhead mixing ventilation. HVAC&R Research
18(4):602 - 615.
8. Pantelic, J., K.W. Tham. 2013. Adequacy of air change rate
as the sole indicator of an air distribution system's effectiveness
to mitigate airborne infectious disease transmission caused by a
cough release in the room with overhead mixing ventilation: a case
study. HVAC&R Research 19(8):947 - 961.
9. Tang, J.W., A. Nicolle, J. Pantelic, C.A. Klettner, R. Su, P.
Kalliomaki, P. Saarinen, H. Koskela, K. Reijula, P. Mustakallio,
D.K. Cheong. 2013. Different types of door-opening motions as
contributing factors to containment failures in hospital isolation
rooms. PloS ONE 8(6):e66663.
10. Pantelic, J., K.W. Tham,D. Licina. 2015. Effectiveness of a
personalized ventilation system in reducing personal exposure
against directly released simulated cough droplets. Indoor Air
25(6):683 - 693.
J U LY 2 0 19

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ASHRAE Journal - July 2019

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