ASHRAE Journal - December 2020 - 13

LETTERS

The rise to the cough maximum concentration, not
shown, would be even faster than the decay rate shown.
This would not be the case in a lower occupancy density
(OD - the spatial volume divided by the number of occupants) environment. In a house, for example, the rise
to its maximum would take much longer given its lower
OD while the maximum concentration reached might be
only a tenth that reached in an aircraft passenger cabin
before the exposed person(s) left the room with the
infector.

Inhaled Mass

some five times higher than those of passenger cabins
with a recirculation air to outside air 80/20 split versus
a cabin air 50/50 split. Thus, while aircraft ECS blowers
move their design recirculation airflows through HEPA
filters, office HVAC systems move eight times more
recirculation airflows through MERV13 filters that also
remove 0.3-micron particles. So even with their higher
0.3-micron removal efficiency, aircraft HEPA filters do
not remove more 0.3 micron aerosol particles than do
office MERV13 filters, and the net supply of virus-free
outdoor air plus virus-filtered air to office workers is two
times greater than to passengers in
wide body aircraft and four times
greater than to passengers in narrow
body aircraft (Table 1 in my paper).

A five-row CFD model (Figures 2-5
TECHNICAL FEATURE
in the Boeing letter) is insufficient
for capturing exhaled breath aerosols that are present beyond two
COVID-19 and Beyond
Comparable Airflow Rates
rows distance from a source in the
A Brief Introduction
As noted above, office air served
well-mixed far field that decays with
To Passenger Aircraft
by
MERV 13 filters will have a lower
distance. [Reference 10 in my paper,
Cabin Air Quality
viral aerosol concentration than
Bennett et al. 2013].
aircraft cabin air served by HEPA
There are several other factors to
filters for the same outdoor airflow
be considered:
rate per occupant. Further, passen* a mass reduction over time
gers within four rows of an infector
overestimates the reduction in inare inhaling more virus-containing
fectious material inhaled.
particles than are infector-aerosol* the time to reach maximum
exposed office workers in their betconcentration after an ill person
ter mixed space.
enters a space is less, and the dose
higher, the higher the OD; the high
Comparable Air Change Rates
OD of aircraft cabins causes the high air change rate
The high air change rate of conditioned air that main(ACH), not a high virus-free ventilation rate (HEPA filtains cabin pressurization is an advanced technical
tered air plus outside air).
achievement. Part of the motivation for the design of the
* the average virus age before inhalation will be
cabin ventilation system is handling the high occupant
lower, and the virus more pathogenic and likely to be
density (OD) of the cabin. However, the Boeing letter
airborne, the higher the ACH.
leaves OD out of the discussion, yet it is a fundamental
* The lower the ACH, the better the mixing; so perindoor air quality principle. In fact, ASHRAE Standard
sons nearby an infector in a high ACH setting will be
62.1 uses this very concept to specify sufficient ventilaexposed to higher viral concentrations,
* Low humidity (passenger cabin humidity at ~10% as tion on a per occupant basis.
In the case of aircraft cabins, their high ACH does not
the flight progresses is very low), weakens our immune
mean the viral exposure will be lower than in a lower
defense system and extends viral suspension time. (refACH setting. That is because the high cabin ACH is a
erence 8 in my paper, Kudo et al).
result of its high OD, and not a high virus-free airflow to
Aircraft HEPA Filters
each occupant.
In order to thermally condition their 17 to 28 times
Further, based on the USTRANSCOM glass bead dislower OD spaces than passenger aircraft cabins, buildpersion measurements, the cabin ventilation effectiveing ventilation air supplies on a per person basis are
ness for the passengers in rows nearby an infector is
BY DOUGLAS STUART WALKINSHAW, PH.D., P.ENG., FELLOW ASHRAE

The passenger aircraft industry says passenger cabin air quality is exceptionally good
compared with that of other public settings. Some airlines claim the air in aircraft
cabins is cleaner than that in offices and is on par with the air in hospitals. Another
airline says the air is particularly good because it is very dry, creating a sterile cabin
environment. Some say virus particles will only travel one or two rows. Nearly all say
the air change rate is high and recirculated air is passed through HEPA filters that
remove nearly 100% of airborne viruses.1 - 6 This article will review these claims.
Dry Air in Passenger Cabins

The air in passenger cabins is dry, with a relative humidity (RH) of 10% as the flight progresses.
Meanwhile, a portion of the cabin air with its ventilation components (very dry outdoor air plus filtered,
recirculated air) and humidity components, passes
from the cabin to behind the cabin insulation, drawn
there through liner leaks and openings by stack pressures. Some of this air is not lost as useful ventilation
air. However, all the air drawn there (perhaps 25% of the
cabin ventilation air) loses its humidity prior to recirculation, depositing its moisture as condensation on the
very cold fuselage behind the insulation. There it freezes
during flight, adding nonproductive dead weight. When

the frozen water melts when the plane is back on the
ground, this moisture causes metal corrosion, hastening
metal fatigue and creating microbial growth.7
However, in addition to air at 10% RH being uncomfortable, it has been shown to impair nasal mucociliary
clearance, innate antiviral defense and tissue repair
function in mice and is, therefore, postulated to do so
in humans.8 Additionally, RH this low rapidly turns
droplets into aerosols,9 which disperse more widely, five
rows longitudinally either way (Figure 1).10 Aerosols are
more likely to inoculate the respiratory system, where
the minimum dose requirement to inoculate is lower
and the symptoms more severe than if the inoculation
occurs in the nasal system where the larger droplets are

This peer-reviewed article does not represent official ASHRAE guidance. For more information on ASHRAE resources on COVID-19, visit ashrae.org/COVID19.

D ECEM BER 2020

ashrae.org

ASHRAE JOURNAL

13


http://www.ashrae.org/COVID19 https://www.ashrae.org/

ASHRAE Journal - December 2020

Table of Contents for the Digital Edition of ASHRAE Journal - December 2020

Contents
ASHRAE Journal - December 2020 - Intro
ASHRAE Journal - December 2020 - CT1
ASHRAE Journal - December 2020 - CT2
ASHRAE Journal - December 2020 - Cover1
ASHRAE Journal - December 2020 - Cover2
ASHRAE Journal - December 2020 - 1
ASHRAE Journal - December 2020 - Contents
ASHRAE Journal - December 2020 - 3
ASHRAE Journal - December 2020 - 4
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ASHRAE Journal - December 2020 - Cover3
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