ASHRAE Journal - May 2022 - 14

TECHNICAL FEATURE
* Filtration. Recirculated indoor air passes through
a filter, which traps some fraction of the infectious particles.
*
Deactivation. Through either natural decay or active
measures (e.g., ultraviolet germicidal irradiation
[UVGI]14,15), the infectious material within the particles
is destroyed; although the particles remain in the air,
they are no longer infectious.
* Deposition. Aerosols naturally deposit onto surfaces,
which removes them from the air.
For our purposes, we use " ventilation " to refer specifically
to outdoor air, as the cleaning effect on recirculated
air provided by the in-duct filter is captured in the " filtration "
category. To compare and combine these effects,
ASHRAE16 has defined the concept of equivalent outdoor
air (EOA), which gives the volumetric flow of outdoor air
that would provide an equivalent removal rate of infectious
particles. If particle size distributions are known,
it is straightforward to determine the EOA provided by
each of the removal mechanisms.3
After the total EOA has been quantified, the model can
be used to estimate the number of disease transmissions
as
Expected Transmissions
Infectors Susceptibles Time
Equiva
∝
××
lent Outdoor Air
(1)
where the proportionality constant depends on a number
of additional factors related to occupants (such
as activity level, vocalization, immunity and whether
masks are worn) and can vary by multiple orders of
magnitude across different indoor spaces and occupancy
characteristics.4 For our purposes, it is enough
to recognize that these factors are constant regardless
of the operation of the HVAC system, and thus overall
infection risk is inversely proportional to the EOA delivery
rate.
Mitigation Strategies
Given that increased EOA delivery can reduce the risk
of airborne disease transmission, it is desirable for system
operators to do just that. Unfortunately, this cannot
always be directly accomplished. Rates of deposition and
natural deactivation are essentially determined by the
size distribution and biological response of the infectious
aerosols. These properties are influenced by relative
humidity, with higher humidity leading to larger
equilibrium particle size (e.g., a particle of radius 1 µm
14
ASHRAE JOURNAL ashrae.o rg M AY 2022
FIGURE 2 Humidity dependence of EOA provided by deposition and deactivation.
Deposition is modeled as in Bazant and Bush,4 while deactivation is modeled with
the characteristic " V " shape reported in the literature.7,17 The " Total " line shows
the sum of the two effects.
Deactivation
1.0
0.8
0.6
0.4
0.2
10
Deposition
Total
20
30
40
50
Relative Humidity (%)
at 50% humidity would have radius of 1.26 µm at 75%
humidity and a radius of 0.87 µm at 25% humidity) and
thus a faster deposition rate.4 By contrast the deactivation
rate is maximized at intermediate relative humidity
(near the efflorescence point where solid crystals begin
to form within the particles) and decreasing significantly
at both high and low humidity.7,17
The resulting impact of these effects on EOA delivery
can be modeled as shown in Figure 2, but because the net
variation is relatively weak over the 30% to 60% humidity
range encountered in typical buildings, further manipulation
of zone humidity as a direct mitigation strategy is
perhaps not well-motivated. Similarly, temperature has
at most a very weak impact on deactivation rate over the
typical range of indoor temperatures,4 and thus changes
to zone temperature setpoint are not an effective means
to mitigate transmission. By contrast, the rates of ventilation
and in-duct filtration can vary much more significantly,
but they are essentially dictated by the operation
of the HVAC system (and the in-duct filter type, in the
case of filtration). Finally, active disinfection and inzone
filtration can be provided by portable devices, but
of course these devices have to first be installed in the
space.
Based on these considerations, we identify four main
decision variables relevant to EOA delivery in typical
commercial air-handling unit/variable air volume
(AHU/VAV) systems:
* AHU Minimum Outdoor Flow Setpoint. Outdoor
air provides a direct source of infectious-particle removal,
although adjusting the minimum can have little
to no effect during economizer conditions.
* AHU Supply Temperature Setpoint. Raising the
supply temperature setpoint increases the total airflow
60
70
80
EOA Delivery (ACH)
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ASHRAE Journal - May 2022

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