ASHRAE Journal - August 2021 - 18

TECHNICAL FEATURE
Virus Transmission Modes and Mitigation Strategies, Part 3
Ventilation, Filtration
And UVGI
BY JONATHAN BURKETT, P.E., HFDP, MEMBER ASHRAE
Parts 1 and 2 of this article in the March and April 2021 issues of ASHRAE Journal
focused on viral infection, transmission and propagation in a space. Part 3 will focus
on popular virus mitigation strategies and their application to single zone HVAC
systems.
Ventilation (Dilution)
The need for ventilation for good indoor air quality
in buildings has been recognized throughout history.
Natural ventilation has been used for centuries to
ventilate smoke, dust and odors. During the Crimean
and U.S. Civil War, physicians observed that wounded
soldiers fared better when they were housed in tents
or barns (allowing greater ventilation) rather than
crowded hospitals with poor ventilation.1 ASHRAE
Standard 62.1-2019 defi nes 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. " 2 The Wells-Riley equation (see
Part 13) shows that by increasing the ventilation air rate,
the infection risk can be reduced signifi cantly.4,5
The equation of ventilation (Figure 1) shows the basic
relationship between ventilation rate and concentration.
Like the Wells-Riley equation, the equation of ventilation
is also based on a fully mixed room and shows
that the higher the ventilation rate, the more rapid the
decay of droplet nuclei in the room.5 Research by Li,
et al., that reviewed over 40 studies concerning ventilation
and airborne infection, found " strong and suffi -
cient evidence to demonstrate the association between
ventilation and the control of airfl ow directions in
buildings and the transmission and spread of infectious
diseases such as measles, TB, chicken pox, anthrax,
infl uenza, smallpox and SARS. " 6
In their " Guidelines for Environmental Infection
Control in Health-Care Facilities, " the Centers for
Disease Control and Prevention (CDC) lists the theoretical
time for 99% and 99.9% removal of airborne contaminants
based on air changes per hour (ach) (Table 1).7
The values in Table 1 are based on the airborne contaminant
removal formula (Figure 2) and show the time to
reach the listed percent removal after the generation
of the contaminant is fi nished. Looking at an example
classroom (Figure 3), the outdoor air ventilation rate is
3.5 ach.
Using the airborne contaminant removal formula,
99% removal can be achieved in 79 minutes or 99.9%
removal in 118 minutes. This example assumes that
Jonathan Burkett, P.E., is a mechanical engineer at Buford Goff & Associates in Columbia, S.C.
This peer-reviewed article does not represent offi cial ASHRAE guidance. For more information on ASHRAE resources on COVID-19, visit ashrae.org/COVID19.
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ASHRAE Journal - August 2021

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