ASHRAE Journal - July 2022 - 11

TECHNICAL FEATURE
FIGURE 1 (a) Position of distributed air nozzle inlets throughout the aircaft cabin; (b) cross sectional diagram of intended airflow design for the aircraft cabin; (c) top-down view of environmental
control system.
Air Distribution
Nozzle
(a)
(b)
(c)
1
Outside
Air
Recirculation/
HEPA Filtration
System
Return Air Grille
3
4
Mix
Manifold
6
5
Cabin Air
Distribution
2
Cooling
Packs
Outfl ow
Valve
From March 2020 to October 2020, only 44 known
cases of COVID-19 were transmitted in the air travel system
while 1.2 billion passengers traveled by air worldwide.1,2
While this transmission rate is certainly undercounted,
other studies tracking cases in the air travel
system also show very low transmission rates amongst
occupants.1 The low risk of transmission may be aided
by the multilayered approach adopted by airlines, regulators
and other organizations to keep passengers and
fl ight crews healthy.3 This approach includes physical
distancing, masking, vaccinations, washing hands,
avoiding travel for individuals feeling unwell and disinfecting
the airplane cabin and fl ight deck.
However, even with these additional precautions,
an infectious person may still be onboard. Therefore,
additional studies were undertaken to characterize the
aircraft cabin airfl ow and understand why the risk of
transmission was lower than might be expected for an
indoor commercial space (ICS).
Comparing Ventilation Systems
Ventilation that provides acceptable indoor air quality
(IAQ) and minimizes health risks is an important part of
any indoor environment. ASHRAE Standard 62.1 defi nes
ventilation air as that portion of supply air that is outdoor
air plus any recirculated air that has been treated
for the purposes of maintaining acceptable indoor air
quality,4 e.g., removal of viruses by HEPA fi lters.
Aircraft typically provide 20 to 30 air changes per hour
(ACH), with airfl ow rates of 15 cfm to 20 cfm (7 L/s to
9.4 L/s) of outdoor and recirculated air per occupant.
The recirculated component of supply air is HEPA
fi ltered (defi ned as 99.97% removal at most penetrating
particle size of 0.3 µm)5,6 and is mixed typically with
50% outdoor air. Since the outdoor air does not contain
signifi cant quantities of infectious agents, the resultant
mixed outdoor and HEPA-fi ltered supply air has no
signifi cant viral contamination. Air then fl ows in a predominantly
downward direction in the cabin and exits
through return air grilles at fl oor level where it is again
HEPA-fi ltered or expelled overboard (Figure 1). The seats
act as additional physical barriers to help isolate passengers
from each other and partially compartmentalize
the airfl ow.
When compared to an aircraft, air exchange and fi ltration
rates within an ICS are lower, typically between
2 ACH and 8 ACH. Prior to the current COVID-19 pandemic,
ICSs had a MERV of 4 to 7 (mean 10.5% to 42.2%
at 0.3 µm). Since the pandemic began, the fi ltration
effi ciency of some indoor environments has increased
to MERV 13 (mean: 85.9% at 0.3 µm).7 However, these
factors, coupled with an overall lack of directional fl ow,
allow a substantial percentage of expiratory particles to
spend a longer time in an ICS than in an airplane.
Commonly Used ICS Model
The Wells-Riley model, fi rst used by Wells8 and
Riley9 to determine the spread of measles in a school
environment, has been the basis for many retrospective
studies modeling disease transmission in ICSs.
This model assumes a space with a well-mixed air
volume that instantaneously arrives at a steady state
concentration. While necessary at the time, in hindsight,
the assumption of a well-mixed environment
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ASHRAE Journal - July 2022

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