ASHRAE Journal - May 2022 - 13

TECHNICAL FEATURE
* Improved fi ltration is often the
most cost-effective source of equivalent
outdoor air.
* Signifi cant reduction in infection
risk generally requires increasing the
total airfl ow supplied to zones, which
may or may not be feasible depending
on system confi guration and ambient
conditions.
* In cases where operational fl exibility
of the HVAC system is limited,
high-risk zones may need supplementary
in-zone fi ltration or other standalone
disinfection devices.
For buildings and spaces with different
HVAC confi gurations, these
observations may or may not still hold,
but similar quantitative analysis can
be performed to inform the course of
action.
To help frame our discussion, Figure 1 gives an illustration
of infectious particle dynamics and energy consumption
associated with a typical HVAC system. In
this scenario, the infector releases infectious aerosols
(shown in red) into the air when they exhale. These
particles quickly mix with the surrounding air where
they undergo a variety of removal processes (shown in
blue). In the meantime, the susceptible individual can
potentially become infected if they are exposed to a
high enough dose of the particles. To reduce this likelihood,
the HVAC system can be adjusted or augmented
to increase the removal rate of particles, reducing their
average concentration. However, these operational
measures almost always require an increase in energy
consumption and in some cases can compromise occupant
comfort. Thus, when choosing infection mitigation
strategies, it is important to consider which strategies
(or combination thereof) are most effi cient.
Exhaust
Ventilation
Outdoor Air
Filtration
Fan Cooling Heating
Gas
Infection Risk Modeling and Equivalent Outdoor Air
It has long been understood that respiratory diseases,
such as measles, tuberculosis, seasonal fl u and severe
acute respiratory syndrome (SARS), can be transmitted
through the air via exhaled aerosols (ranging from
0.1 µm to 10 µm in diameter) released into the air by
infectious individuals. Indeed, it is now widely accepted
that this is the dominant mode of transmission for
FIGURE 1 Diagram of infectious particle dynamics and HVAC energy consumption in typical buildings.
Infectious particles are shown in red, while particle removal sources are shown in blue. Energy consumption
for relevant equipment is routed to the appropriate utility.
Deactivation
Supply
HVAC
In-Zone
Air Filter
Electricity
Zone
Susceptible
Infector
Deposition
SARS-CoV-2 .4,5 Generation and distribution of respiratory
aerosols has been discussed in previous articles,6,7
so in the interest of brevity, we will not review them in
detail here.
Quantitative analysis of aerosol-based disease transmission
dates back to Wells8 and Riley, et al.,9 who proposed
that the infectious aerosols could be modeled as
a standard indoor air pollutant. This approach allows
steady-state concentrations to be estimated by fi nding
the balance point between particle generation and
removal.
A key assumption of Wells-Riley type models is that
the air within the space is well-mixed, i.e., uniform
in particle concentration, which typically results
from natural and forced turbulent convection associated
with human respiration, temperature variations
and ventilation fl ows.10,11 Although the models do
not account for short-range transmission via jet-like
respiratory fl ows that can occur when infectious and
susceptible individuals are in close proximity,12,13 the
models can also be corrected to account for the associated
elevated risk.4
Within the well-mixed model, a key observation is that
the infectious particles can be effectively removed from
the air via a variety of processes as follows:
* Ventilation. Indoor air is vented and replaced
with clean air from outdoors that is free from infectious
particles.
M AY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
13
Return

http://ashrae.org

ASHRAE Journal - May 2022

Table of Contents for the Digital Edition of ASHRAE Journal - May 2022